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+*** START OF THE PROJECT GUTENBERG EBOOK 43431 ***
+
+ CAMBRIDGE BIOLOGICAL SERIES.
+
+ GENERAL EDITOR:--ARTHUR E. SHIPLEY, M.A.
+
+ FELLOW AND TUTOR OF CHRIST'S COLLEGE, CAMBRIDGE.
+
+
+
+
+ THE
+
+ VERTEBRATE SKELETON.
+
+
+
+
+ London: C.J. CLAY AND SONS,
+ CAMBRIDGE UNIVERSITY PRESS WAREHOUSE,
+ AVE MARIA LANE,
+
+ AND
+
+ H.K. LEWIS,
+ 136, GOWER STREET, W.C.
+
+ [Illustration]
+
+ Glasgow: 50, WELLINGTON STREET.
+
+ Leipzig: F.A. BROCKHAUS.
+
+ New York: THE MACMILLAN COMPANY.
+
+ Bombay and Calcutta: MACMILLAN AND CO., LTD.
+
+
+ [_All Rights reserved._]
+
+
+
+
+ THE
+
+ VERTEBRATE SKELETON
+
+ BY
+
+ SIDNEY H. REYNOLDS, M.A.,
+
+ TRINITY COLLEGE, CAMBRIDGE;
+ LECTURER AND DEMONSTRATOR IN GEOLOGY AND ZOOLOGY AT UNIVERSITY
+ COLLEGE, BRISTOL.
+
+ Cambridge:
+
+ AT THE UNIVERSITY PRESS.
+
+ 1897
+
+ [_All Rights reserved._]
+
+
+
+
+ =Cambridge=:
+
+ PRINTED BY J. & C.F. CLAY,
+ AT THE UNIVERSITY PRESS.
+
+
+
+
+PREFACE.
+
+
+IN the following pages the term skeleton is used in its widest sense,
+so as to include exoskeletal or tegumentary structures, as well as
+endoskeletal structures. It was thought advisable to include some
+account of the skeleton of the lowest Chordata--animals which are not
+strictly vertebrates, but it seemed undesirable to alter the title of
+the book in consequence.
+
+The plan adopted in the treatment of each group has been to give first
+an account of the general skeletal characters of the group in question
+and of its several subdivisions; secondly to describe in detail the
+skeleton of one or more selected types; and thirdly to treat the
+skeleton as developed in the group organ by organ.
+
+A beginner is advised to commence, not with the introductory chapter,
+but with the skeleton of the Dogfish, then to pass to the skeletons of
+the Newt and Frog, and then to that of the Dog. After that he might
+pass to the introductory chapter and work straight through the book. I
+have endeavoured to make the account of each type skeleton complete in
+itself; this has necessitated a certain amount of repetition,--a
+fault that I have found it equally difficult to avoid in other parts
+of the book.
+
+Throughout the book generic names are printed in italics; and italics
+are used in the accounts of the type skeletons for the names of
+membrane bones. Clarendon type is used to emphasise certain words. In
+the classificatory table the names of extinct genera only, are printed
+in italics.
+
+In a book in which an attempt is made to cover to some extent such a
+vast field, it would be vain to hope to have avoided many errors both
+of omission and commission, and I owe it to the kindness of several
+friends that the errors are not much more numerous. I cannot however
+too emphatically say that for those which remain I alone am
+responsible. Messrs C.W. Andrews, E. Fawcett, S.F. Harmer, J. Graham
+Kerr, and B. Rogers have all been kind enough to help me by reading
+proofs or manuscript, while the assistance that I have received from
+Dr Gadow during the earlier stages and from Prof. Lloyd Morgan and Mr
+Shipley throughout the whole progress of the work has been very great.
+To all these gentlemen my best thanks are tendered.
+
+All the figures except 1, 35, 55, and 84 were drawn by Mr Edwin
+Wilson, to whose care and skill I am much indebted. The majority are
+from photographs taken by my sister Miss K.M. Reynolds or by myself in
+the British Museum and in the Cambridge University Museum of Zoology,
+and I take this opportunity of thanking Sir W.H. Flower and Mr S.F.
+Harmer for the facilities they have afforded and for permission to
+figure many objects in the museums respectively under their charge. I
+have also to thank (1) Prof. von Zittel for permission to reproduce
+figs. 27, 41, 52, 69, 70, 80, 106 A, and 107 C; (2) Sir W.H. Flower
+and Messrs A. and C. Black for figs. 1 and 84; (3) Prof. O.C. Marsh
+and Dr H. Woodward for fig. 35; (4) Dr C.H. Hurst and Messrs Smith,
+Elder, and Co. for fig. 55.
+
+A few references are given, but no attempt has been made to give
+anything like a complete list. The abbreviations of the titles of
+periodicals are those used in the _Zoological Record_.
+
+I have always referred freely to the textbooks treating of the
+subjects dealt with, and in particular I should like to mention that
+the section devoted to the skeleton of mammals is, as it could hardly
+fail to be, to a considerable extent based on Sir W.H. Flower's
+_Osteology of the Mammalia_.
+
+ SIDNEY H. REYNOLDS.
+
+ _March 10, 1897._
+
+
+
+
+ CONTENTS.
+
+
+ PAGE
+ CHAPTER I.
+
+ Introductory account of the skeleton in general 1
+
+
+ CHAPTER II.
+
+ Classification 30
+
+
+ CHAPTER III.
+
+ Skeleton of Hemichordata, Urochordata and Cephalochordata 50
+
+
+ CHAPTER IV.
+
+ Skeletal characters of the Vertebrata. The skeleton in the
+ Cyclostomata 53
+
+
+ CHAPTER V.
+
+ Skeletal characters of the Ichthyopsida. Characters of the
+ several groups of Pisces 59
+
+
+ CHAPTER VI.
+
+ The skeleton of the Dogfish (Scyllium canicula) 71
+
+
+ CHAPTER VII.
+
+ The skeleton of the Codfish (_Gadus morrhua_) and the skull of
+ the Salmon (_Salmo salar_) 83
+
+
+ CHAPTER VIII.
+
+ General account of the skeleton in Fishes 104
+
+
+ CHAPTER IX.
+
+ Characters of the several groups of Amphibia 133
+
+
+ CHAPTER X.
+
+ The skeleton of the Newt (_Molge cristata_) 138
+
+
+ CHAPTER XI.
+
+ The skeleton of the Frog (_Rana temporaria_) 151
+
+
+ CHAPTER XII.
+
+ General account of the skeleton in Amphibia 168
+
+
+ CHAPTER XIII.
+
+ Skeletal characters of the Sauropsida. Characters of the
+ several groups of Reptiles 189
+
+
+ CHAPTER XIV.
+
+ The skeleton of the Green Turtle (_Chelone midas_) 214
+
+
+ CHAPTER XV.
+
+ The skeleton of the Crocodile (_Crocodilus palustris_) 237
+
+
+ CHAPTER XVI.
+
+ General account of the skeleton in Reptiles 270
+
+
+ CHAPTER XVII.
+
+ Characters of the several groups of Birds 295
+
+
+ CHAPTER XVIII.
+
+ The skeleton of the Wild Duck (_Anas boschas_) 302
+
+
+ CHAPTER XIX.
+
+ General account of the skeleton in Birds 328
+
+
+ CHAPTER XX.
+
+ Characters of the several groups of Mammalia 343
+
+
+ CHAPTER XXI.
+
+ The skeleton of the Dog (_Canis familiaris_) 374
+
+
+ CHAPTER XXII.
+
+ General account of the skeleton in Mammalia. The exoskeleton
+ and vertebral column 416
+
+
+ CHAPTER XXIII.
+
+ General account of the skeleton in Mammalia (_continued_). The
+ skull and appendicular skeleton 455
+
+
+
+
+ LIST OF ILLUSTRATIONS.
+
+
+ FIG. PAGE
+
+ 1 Diagrammatic sections of various forms of teeth 6
+
+ 2 Cervical vertebrae of an Ox (_Bos taurus_) 15
+
+ 3 Diagram of the skeleton of _Amphioxus lanceolatus_ 51
+
+ 4 Dorsal, lateral, and ventral views of the skull of
+ _Petromyzon marinus_ 56
+
+ 5 Skull of a male _Chimaera monstrosa_ 65
+
+ 6 Lateral view of the skull of a Dogfish (_Scyllium canicula_) 75
+
+ 7 Semidorsal view of the pectoral girdle and fins of a Dogfish
+ (_Scyllium canicula_) 80
+
+ 8 Dorsal view of the pelvic girdle and fins of a male Dogfish
+ (_Scyllium canicula_) 81
+
+ 9 Dorsal and ventral views of the cranium of a Salmon (_Salmo
+ salar_) from which most of the membrane bones have been
+ removed 88
+
+ 10 Lateral view of the chondrocranium of a Salmon (_Salmo
+ salar_) 90
+
+ 11 Lateral view of the skull of a Salmon (_Salmo salar_) 92
+
+ 12 Mandibular and hyoid arches of a Cod (_Gadus morrhua_) 99
+
+ 13 Right half of the pectoral girdle and right pectoral fin of
+ a Cod (_Gadus morrhua_) 102
+
+ 14 Diagram of a section through the jaw of a Shark
+ (_Odontaspis americanus_) showing the succession of
+ teeth 107
+
+ 15 Part of the lower jaw of a Shark (_Galeus_) 108
+
+ 16 Skulls of _Notidanus_ and _Cestracion_ 118
+
+ 17 Dorsal view of the branchial arches of _Heptanchus_ 120
+
+ 18 Lateral view of the skull of a Sturgeon
+ (_Acipenser sturio_) 122
+
+ 19 Dorsal and ventral views of the cranium of
+ _Ceratodus miolepis_ 125
+
+ 20 Lateral view of the skeleton of _Ceratodus miolepis_ 128
+
+ 21 Dorsal, ventral and lateral views of the skull of a Newt
+ (_Molge cristata_) 142
+
+ 22 Ventral and lateral views of the shoulder-girdle and
+ sternum of an old male Crested Newt (_Molge cristata_) 146
+
+
+ 23 Right posterior and anterior limbs of a Newt (_Molge
+ cristata_) 148
+
+ 24 Dorsal and ventral views of the cranium of a Common Frog
+ (_Rana temporaria_) 155
+
+ 25 Dorsal and ventral views of the cranium of a Common Frog
+ (_Rana temporaria_) from which the membrane bones have
+ mostly been removed 157
+
+ 26 Lateral view of the skull and posterior view of the
+ cranium of a Common Frog (_Rana temporaria_) 159
+
+ 27 Dorsal view of the skull of a Labyrinthodont (_Capitosaurus
+ nasutus_) 176
+
+ 28 Ventral view of the cranium, and lateral view of the
+ cranium and mandible of _Siphonops annulatus_ 178
+
+ 29 Visceral arches of Amphibia: A, _Molge cristata_; B, _Rana
+ temporaria_, adult; C, Tadpole of _Rana_; D, _Siredon
+ pisciformis_ 181
+
+ 30 Shoulder-girdle and sternum of an adult male Common Frog
+ (_Rana temporaria_), and of an adult female
+ _Docidophryne gigantea_ 183
+
+ 31 A, Right antibrachium and manus of a larval Salamander
+ (_Salamandra maculosa_); B, Right tarsus and adjoining
+ bones of _Molge sp._ 186
+
+ 32 Lateral and dorsal views of the skull of an _Ichthyosaurus_ 196
+
+ 33 Lateral view and longitudinal section of the skull of a
+ Lizard (_Varanus varius_) 201
+
+ 34 Lateral view of the shoulder-girdle of a Lizard (_Varanus_) 202
+
+ 35 Restored skeleton of _Ceratosaurus nasicornis_ 206
+
+ 36 Dorsal and ventral views of the carapace of a Loggerhead
+ Turtle (_Thalassochelys caretta_) 216
+
+ 37 Plastron of a Green Turtle (_Chelone midas_) 218
+
+ 38 The skull of a Green Turtle (_Chelone midas_) 223
+
+ 39 Longitudinal vertical section through the cranium of a
+ Green Turtle (_Chelone midas_) 226
+
+ 40 Anterior limb of a young Hawksbill Turtle (_Chelone
+ imbricata_), and posterior limb of a large Green Turtle
+ (_Chelone midas_) 234
+
+ 41 The first four cervical vertebrae of a Crocodile
+ (_Crocodilus vulgaris_) 239
+
+ 42 Anterior view of a late thoracic and the first sacral
+ vertebrae of a Crocodile (_Crocodilus palustris_) 242
+
+
+ 43 Palatal aspect of the cranium and mandible of an Alligator
+ (_Caiman latirostris_) 245
+
+ 44 Lateral view of the skull of an Alligator (_Caiman
+ latirostris_) 248
+
+ 45 Longitudinal section through the skull of an Alligator
+ (_Caiman latirostris_) 253
+
+ 46 Sternum and associated membrane bones of a Crocodile
+ (_Crocodilus palustris_) 261
+
+ 47 Left half of the pectoral girdle of an Alligator
+ (_Caiman latirostris_) 262
+
+ 48 Right anterior and posterior limbs of an Alligator
+ (_Caiman latirostris_) 264
+
+ 49 Pelvis and sacrum of an Alligator (_Caiman latirostris_) 267
+
+ 50 Preparation of part of the right mandibular ramus of
+ _Crocodilus palustris_ 274
+
+ 51 Dorsal and ventral views of the skull of a Common Snake
+ (_Tropidonotus natrix_) 279
+
+ 52 Skull of Hatteria (_Sphenodon punctatus_) 282
+
+ 53 Hyoids of an Alligator (_Caiman latirostris_), and of a
+ Green Turtle (_Chelone midas_) 285
+
+ 54 Ventral view of the shoulder-girdle and sternum of
+ _Loemanctus longipes_ 287
+
+ 55 Left half of the skeleton of a Common Fowl (_Gallus
+ bankiva_) 301
+ 56 The wing of a Wild Duck (_Anas boschas_) 304
+
+ 57 Wings of a Wild Duck with the coverts removed (_Anas
+ boschas_) 305
+
+ 58 Dorsal and ventral views of the pelvis and sacrum of a Duck
+ (_Anas boschas_) 311
+
+ 59 Skull of a Duck (_Anas boschas_) 312
+
+ 60 A, Ventral view of the cranium of a Duck (_Anas boschas_);
+ B, Cranium and mandible seen from the left side 313
+
+ 61 Lateral view of the pelvis and sacrum of a Duck (_Anas
+ boschas_) 325
+ 62 Third cervical vertebra of an Ostrich (_Struthio camelus_) 331
+
+ 63 Shoulder-girdle and sternum of A, Black Vulture (_Vultur
+ cinereus_); B, Peacock (_Pavo cristatus_); C, Pelican
+ (_Pelicanus conspicillatus_) 337
+
+ 64 Bones of the right wing of A, a Penguin; B, an Ostrich
+ (_Struthio camelus_) and C, a Gannet (_Sula alba_) 339
+
+ 65 Pelvic girdle and sacrum of A, Cassowary (_Casuarius
+ galeatus_); B, Owen's Apteryx (_A. oweni_); C,
+ Broad-billed Rhea (_R. macrorhyncha_); D, Ostrich
+ (_Struthio camelus_) 340
+
+ 66 Ventral view of the shoulder-girdle and sternum of a
+ Duckbill (_Ornithorhynchus paradoxus_) 347
+
+ 67 Cervical vertebrae of a Ca'ing Whale (_Globicephalus
+ melas_) 354
+
+ 68 Dentition of a Dog (_Canis familiaris_) 375
+
+ 69 Atlas and axis vertebrae of a Dog (_Canis familiaris_) 379
+
+ 70 Second thoracic and second lumbar vertebrae of a Dog
+ (_Canis familiaris_) 382
+
+ 71 Diagram of the relations of the principal bones in the
+ Mammalian skull 385
+
+ 72 Vertical longitudinal section through skull of a Dog
+ (_Canis familiaris_) 387
+
+ 73 Dorsal view of the cranium of a Dog (_Canis familiaris_) 389
+
+ 74 Diagram of the mammalian tympanic cavity and associated
+ bones 391
+
+ 75 Ventral view of the cranium of a Dog (_Canis familiaris_) 396
+
+ 76 Sternum and sternal ribs of a Dog (_Canis familiaris_) 403
+
+ 77 Bones of the left upper arm and fore-arm of a Dog (_Canis
+ familiaris_) 407
+
+ 78 Right innominate bone, A, of a full-grown Terrier; B, of a
+ Collie Puppy 410
+
+ 79 Left leg bones of a Dog (_Canis familiaris_) 411
+
+ 80 A, Right manus; B, Right pes of a Dog (_Canis familiaris_) 413
+
+ 81 Skull of a young Indian Rhinoceros (_R. unicornis_) showing
+ the change of the dentition 421
+
+ 82 Palatal aspect of the cranium and mandible of a Donkey
+ (_Equus asinus_) 431
+
+ 83 Skull of _Procavia (Dendrohyrax) dorsalis_ 433
+
+ 84 Carnassial or sectorial teeth of Carnivora 436
+
+ 85 Mandible of Isabelline Bear (_Ursus isabellinus_) 438
+
+ 86 Left mandibular ramus of the Sea Leopard (_Ogmorhinus
+ leptonyx_) 439
+
+ 87 Cervical vertebrae of a young Fin Whale (_Balaenoptera
+ musculus_) 444
+
+ 88 Atlas and axis vertebrae of an Ox (_Bos taurus_) 445
+
+ 89 First and second thoracic vertebrae of an Ox (_Bos taurus_) 449
+
+ 90 Skulls of Tasmanian Wolf (_Thylacinus cynocephalus_) and
+ Hairy-nosed Wombat (_Phascolomys latifrons_) 456
+
+ 91 Skull of Two-fingered Sloth (_Choloepus didactylus_) 458
+
+ 92 Skull of _Rhytina stelleri_ 460
+
+
+ 93 Lateral view and longitudinal section of the skull of a
+ young Ca'ing Whale (_Globicephalus melas_) 463
+
+ 94 Cranium and mandible of a Pig (_Sus scrofa_) 466
+
+ 95 Mandible of a Hippopotamus (_Hippopotamus amphibius_) 467
+
+ 96 Skull of a young Indian Elephant (_Elephas indicus_) 474
+
+ 97 Longitudinal section of the skull of a young Indian
+ Elephant (_Elephas indicus_) 475
+
+ 98 Half-front view of the skull of a Porcupine (_Hystrix
+ cristata_) 477
+ 99 Skulls of an old and of a young Gorilla (_Gorilla savagei_) 483
+
+ 100 Malleus, stapes, and incus of Man, Dog, and Rabbit 485
+
+ 101 Skeleton of a Cape Buffalo (_Bubalus caffer_) 492
+
+ 102 Lateral and dorsal views of the shoulder-girdle and part
+ of the sternum of the Spiny Anteater (_Echidna
+ aculeata_) 494
+
+ 103 Skeleton of a Llama (_Auchenia glama_) 496
+
+ 104 Dorsal view of the sternum and right half of the shoulder
+ girdle of _Mus sylvaticus_ 498
+
+ 105 Anterior surface of the right humerus of a Wombat
+ (_Phascolomys latifrons_) 500
+
+ 106 Manus of Perissodactyles: A, Left manus of _Tapirus_; B,
+ Right manus of _Titanotherium_; C, Left manus of
+ _Chalicotherium giganteum_ 508
+
+ 107 Left manus of A, _Coryphodon hamatus_; B, _Phenacodus
+ primaevus_; C, _Procavia (Dendrohyrax) arboreus_ 510
+
+ 108 Left anterior and posterior limbs and limb girdles of
+ _Uintatherium mirabile_ 516
+
+ 109 Left femur of an Ox (_Bos taurus_) and of a Sumatran
+ Rhinoceros (_Rhinoceros sumatrensis_) 518
+
+ 110 Pes of A, a Tapir (_Tapirus americanus_); B, a Rhinoceros
+ (_Rhinoceros sumatrensis_); C, _Hipparion gracile_; D,
+ a Horse (_Equus caballus_) 524
+
+
+
+
+ERRATA.
+
+
+ p. 172, note, _for_ 14 _read_ 15.
+
+ p. 279, description of figure, _for_ Tropidinotus _read_
+ Tropidonotus.
+
+ p. 287, description of figure, _for_ shoulder-girdle of sternum
+ _read_ shoulder-girdle and sternum.
+
+ p. 393, middle of page, _for_ VIII _read_ VII.
+
+ p. 427, line 2, _for_ Grampus _read_ Killer.
+
+
+
+
+CHAPTER I.
+
+INTRODUCTORY ACCOUNT OF THE SKELETON IN GENERAL.
+
+
+BY the term =skeleton= is meant the hard structures whose function is
+to support or to protect the softer tissues of the animal body.
+
+The skeleton is divisible into
+
+A. The EXOSKELETON, which is external;
+
+B. The ENDOSKELETON, which is as a rule internal; though in some
+cases, e.g. the antlers of deer, endoskeletal structures become, as
+development proceeds, external.
+
+In Invertebrates the hard, supporting structures of the body are
+mainly =exoskeletal=, in Vertebrates they are mainly =endoskeletal=;
+but the endoskeleton includes, especially in the skull, a number of
+elements, the =dermal= or =membrane= bones, which are shown by
+development to have been originally of external origin. These membrane
+bones are so intimately related to the true endoskeleton that they
+will be described with it. The simplest and lowest types of both
+vertebrate and invertebrate animals have unsegmented skeletons; with
+the need for flexibility however segmentation arose both in the case
+of the invertebrate exoskeleton and the vertebrate endoskeleton. The
+exoskeleton in vertebrates is phylogenetically older than the
+endoskeleton, as is indicated by both palaeontology and embryology.
+Palaeontological evidence is afforded by the fact that all the lower
+groups of vertebrates--Fish, Amphibia, and Reptiles--had in former
+geological periods a greater proportion of species protected by
+well-developed dermal armour than is the case at present.
+Embryological evidence tends the same way, inasmuch as dermal
+ossifications appear much earlier in the developing animal than do the
+ossifications in the endoskeleton.
+
+Skeletal structures may be derived from each of the three germinal
+layers. Thus =hairs= and =feathers= are =epiblastic= in origin,
+=bones= are =mesoblastic=, and the =notochord= is =hypoblastic=.
+
+The different types of skeletal structures may now be considered and
+classified more fully.
+
+
+A. EXOSKELETAL STRUCTURES.
+
+I. EPIBLASTIC (epidermal).
+
+Exoskeletal structures of epiblastic origin may be developed on both
+the inner and outer surfaces of the Malpighian layer of the
+epidermis[1]. Those developed on the outer surface include =hairs=,
+=feathers=, =scales=, =nails=, =beaks= and =tortoiseshell=; and are
+specially found in vertebrates higher than fishes. Those developed on
+the inner surface of the Malpighian layer include only the =enamel= of
+teeth and some kinds of scales. With the exception of feathers, which
+are partly formed from the horny layer, all these parts are mainly
+derived from the Malpighian layer of the epidermis.
+
+=Hairs= are slender, elongated structures which arise by the
+proliferation of cells from the Malpighian layer of the epidermis.
+These cells in the case of each hair form a short papilla, which sinks
+inwards and becomes imbedded at the bottom of a follicle in the
+dermis. Each hair is normally composed of an inner cellular pithy
+portion containing much air, and an outer denser cortical portion of a
+horny nature. Sometimes, as in Deer, the hair is mainly formed of the
+pithy portion, and is then easily broken. Sometimes the horny part
+predominates, as in the bristles of Pigs. A highly vascular dermal
+papilla projects into the base of the hair.
+
+=Feathers=, like hairs, arise from epidermal papillae which become
+imbedded in pits in the dermis. But the feather germ differs from the
+hair germ, in the fact that it first grows out like a cone on the
+surface of the epidermis, and that the horny as well as the Malpighian
+layer takes part in its formation.
+
+=Nails=, =claws=, =hoofs=, and the =horns of Oxen= are also epidermal,
+as are such structures as the =scales= of reptiles, of birds' feet,
+and of _Manis_ among mammals, the =rattle= of the rattlesnake, the
+=nasal horns= of _Rhinoceros_, and the =baleen= of whales. All these
+structures will be described later.
+
+=Nails= arise in the interior of the epidermis by the thickening and
+cornification of the stratum lucidum. The outer border of the nail
+soon becomes free, and growth takes place by additions to the inner
+surface and attached end.
+
+When a nail tapers to a sharp point it is called a =claw=. In many
+cases the nails more or less surround the ends of the digits by which
+they are borne.
+
+Horny =beaks= of epidermal origin occur casing the jaw-bones in
+several widely distinct groups of animals. Thus among reptiles they
+are found in Chelonia (tortoises and turtles) as well as in some
+extinct forms; they occur in all living birds, in _Ornithorhynchus_
+among mammals, and in the larvae of many Amphibia.
+
+In a few animals, such as Lampreys and _Ornithorhynchus_, the jaws
+bear horny tooth-like structures of epidermal origin.
+
+The =enamel= of teeth and of placoid scales is also epiblastic in
+origin[2], and it may be well at this point to give some account of
+the structure of teeth, though they are partly mesoblastic in origin.
+The simplest teeth are those met with in sharks and dogfish, where
+they are merely the slightly modified scales developed in the
+integument of the mouth. They pass by quite insensible gradations into
+normal placoid scales, such as cover the general surface of the body.
+A =placoid scale=[3] is developed on a papilla of the dermis which
+projects outwards and backwards, and is covered by the columnar
+Malpighian layer of the epidermis. The outer layer of the dermal
+papilla then gradually becomes converted into dentine and bone, while
+enamel is developed on the inner side of the Malpighian layer, forming
+a cap to the scale. The Malpighian and horny layers of the epidermis
+get rubbed off the enamel cap, so that it comes to project freely on
+the surface of the body.
+
+As regards their attachment teeth may be (1) attached to the fibrous
+integument of the mouth, or (2) fixed to the jaws or other bones of
+the mouth, or (3) planted in grooves, or (4) in definite sockets in
+the jaw-bones (see p. 107).
+
+Teeth in general consist of three tissues, =enamel=, =dentine= and
+=cement=, enclosing a central pulp-cavity containing blood-vessels and
+nerves. Enamel is, however, often absent, as in all living Edentates.
+
+=Enamel= generally forms the outermost layer of the crown or visible
+part of the tooth; it is the hardest tissue occurring in the animal
+body and consists of prismatic fibres arranged at right angles to the
+surface of the tooth. It is characterised by its bluish-white
+translucent appearance.
+
+II. MESOBLASTIC (mesodermal).
+
+=Dentine= or =ivory= generally forms the main mass of a tooth. It is a
+hard, white substance allied to bone. When examined microscopically
+dentine is seen to be traversed by great numbers of nearly parallel
+branching tubules which radiate outwards from the pulp-cavity. In
+fishes as a rule, and sometimes in other animals, a variety of dentine
+containing blood-vessels occurs, this is called =vasodentine=.
+
+=Cement= or =crusta petrosa= forms the outermost layer of the root of
+the tooth. In composition and structure it is practically identical
+with bone. In the more complicated mammalian teeth, besides enveloping
+the root, it fills up the spaces between the folds of the enamel.
+
+The hard parts of a tooth commonly enclose a central pulp-cavity into
+which projects the pulp, a papilla of the dermis including
+blood-vessels and nerves. As long as growth continues the outer layers
+of this pulp become successively calcified and added to the substance
+of the dentine. In young growing teeth the pulp-cavity remains widely
+open, but in mammals the general rule is that as a tooth gets older
+and the crown becomes fully formed, the remainder of the pulp becomes
+converted into one or more tapering roots which are imbedded in the
+alveolar cavities of the jaws. The opening of the pulp-cavity is then
+reduced to a minute perforation at the base of each root. A tooth of
+this kind is called a =rooted= tooth.
+
+But it is not only in young teeth that the pulp-cavity sometimes
+remains widely open; for some teeth, such as the tusks of Elephants
+and the incisor teeth of Rodents, form no roots and continue to grow
+throughout the animal's life. Such teeth are said to be rootless or to
+have persistent pulps.
+
+An intermediate condition is seen in some teeth, such as the grinding
+teeth of Horses. These teeth grow for a very long time, their crowns
+wearing away as fast as their bases are produced; finally however
+definite roots are formed and growth ceases.
+
+[Illustration FIG. 1. DIAGRAMMATIC SECTIONS OF VARIOUS FORMS OF TEETH
+(from FLOWER).
+
+I. Incisor or tusk of elephant, with pulp-cavity persistently open at
+base. II. Human incisor during development with root imperfectly
+formed, and pulp-cavity widely open at base. III. Completely formed
+human incisor, with pulp-cavity contracted to a small aperture at the
+end of the root. IV. Human molar with broad crown and two roots. V.
+Molar of Ox, with the enamel covering the crown, deeply folded and the
+depressions filled with cement. The surface is worn by use, otherwise
+the enamel coating would be continuous at the top of the ridges. In
+all the figures the enamel is black, the pulp white, the dentine
+represented by horizontal lines, and the cement by dots.]
+
+The teeth of any animal may be =homodont=, that is, all having the
+same general character, or =heterodont=, that is, having different
+forms adapted to different functions. The dentition is heterodont in a
+few reptiles and the majority of mammals.
+
+SUCCESSION OF TEETH. In most fishes, and many amphibians and reptiles
+the teeth can be renewed indefinitely. In sharks, for example,
+numerous rows of reserve teeth are to be seen folded back behind those
+in use (see fig. 15). The majority of mammals have only two sets of
+teeth, and are said to be =diphyodont=; some have only a single series
+(=monophyodont=).
+
+DEVELOPMENT OF TEETH. A brief sketch of the method in which
+development of teeth takes place in the higher vertebrates may here be
+given. Along the surface of the jaws a thickening of the epiblastic
+epithelium takes place, giving rise to a ridge, which sinks inwards
+into the tissue of the jaw, and it is known as the primary enamel
+organ. At the points where teeth are to be developed special ingrowths
+of this primary enamel organ take place, and into each there projects
+a vascular dental papilla from the surrounding mesoblast of the jaw.
+Each ingrowth of the enamel organ forms an =enamel cap=, which
+gradually embraces the dental papilla, and at the same time appears to
+be pushed on one side, owing to the growth not being uniform. The
+external layer of the dental papilla is composed of long nucleated
+cells, the =odontoblasts=, and it is by these that the dentine is
+formed. Similarly the internal layer of the enamel organ is formed of
+columnar enamel cells, which give rise to the enamel. The mesoblastic
+cells surrounding the base of the tooth give rise to the cement.
+
+=Bone= is in many cases exoskeletal, but it will be most conveniently
+described with the endoskeleton.
+
+The =scales of fish= are wholly or in part mesoblastic in origin,
+being totally different from those of reptiles. The =cycloid= and
+=ctenoid= scales of Teleosteans (see p. 105) are thin plates coated
+with epidermis. They are sometimes bony, but as a rule are simply
+calcified. =Ganoid= scales are flat plates of bone coated with an
+enamel-like substance, and articulating together with a peg and socket
+arrangement; they are probably identical with enlarged and flattened
+placoid scales.
+
+The =armour plates= of fossil Ganoids, Labyrinthodonts, and Dinosaurs,
+and of living Crocodiles, some Lizards and Armadillos, are composed of
+bone. They are always covered by a layer of epidermis.
+
+The =antlers of deer= are also composed of bone; they will be more
+fully described in the chapter on mammals. It may perhaps be well to
+mention them here, though they really belong to the endoskeleton,
+being outgrowths from the frontal bones.
+
+
+B. ENDOSKELETAL STRUCTURES.
+
+I. HYPOBLASTIC.
+
+(_a_) The =notochord= is an elastic rod formed of large vacuolated
+cells, and is surrounded by a membranous sheath of mesoblastic origin.
+It is the primitive endoskeleton in the Chordata, all of which possess
+it at some period of their existence; while in many of the lower forms
+it persists throughout life. Even in the highest Chordata it is the
+sole representative of the axial skeleton for a considerable part of
+the early embryonic life. A simple unsegmented notochord persists
+throughout life in the Cephalochordata, Cyclostomata, and some Pisces,
+such as Sturgeons and Chimaeroids.
+
+(_b_) The enamel of the pharyngeal teeth of the Salmon and many other
+Teleosteans is hypoblastic in origin. The epiblast of the stomodaeum,
+in which the other teeth are developed, passes into the hypoblast of
+the mesenteron in which these pharyngeal teeth are formed.
+
+
+II. MESOBLASTIC.
+
+The most primitive type of a mesoblastic endoskeleton consists of a
+membranous sheath surrounding the notochord, as in _Myxine_ and its
+allies. The first stage of complication is by the development of
+cartilage in the notochordal sheath, as in _Petromyzon_. Often the
+cartilage becomes calcified in places, as in the vertebral centra of
+_Scyllium_ and other Elasmobranchs. Lastly, the formation of bone
+takes place; it generally constitutes the most important of the
+endoskeletal structures.
+
+=Bone= may be formed in two ways:--
+
+(1) by the direct ossification of pre-existing cartilage, when it is
+known as =cartilage bone= or =endochondral bone=;
+
+(2) by independent ossification in connective tissue; it is then known
+as =membrane= or =dermal= or =periosteal bone=.
+
+With the exception of the _clavicle_[4] all the bones of the trunk and
+limbs, together with a large proportion of those of the skull, are
+preformed in the embryo in cartilage, and are grouped as cartilage
+bones; while the clavicle and most of the roofing and jaw-bones of the
+skull are not preformed in cartilage, being developed simply in
+connection with a membrane. Hence it is customary to draw a very
+strong line of distinction between these two kinds of bone; in reality
+however this distinction is often exaggerated, and the two kinds pass
+into one another, and as will be shown immediately, the permanent
+osseous tissue of many of those which are generally regarded as
+typical cartilage bones, is really to a great extent of periosteal
+origin. The palatine bone, for instance, of the higher vertebrates in
+general is preceded by a cartilaginous bar, but is itself almost
+entirely a membrane bone.
+
+Before describing the development of bone it will be well to briefly
+describe the structure of adult bone and cartilage.
+
+The commonest kind of =cartilage=, and that which preforms so many of
+the bones of the embryo, is =hyaline= cartilage. It consists of oval
+nucleated cells occupying cavities (=lacunae=) in a clear
+intercellular semitransparent matrix, which is probably secreted by
+the cells. Sometimes one cell is seen in each lacuna, sometimes
+shortly after cell-division a lacuna may contain two or more cells.
+The free surface of the cartilage is invested by a fibrous membrane,
+the =perichondrium=.
+
+=Bone= consists of a series of lamellae of ossified substance between
+which are oval spaces, the =lacunae=, giving rise to numerous fine
+channels, the =canaliculi=, which radiate off in all directions. The
+lacunae are occupied by the =bone cells= which correspond to cartilage
+cells, from which if the bone is young, processes pass off into the
+canaliculi. It is obvious that the ossified substance of bone is
+intercellular in character, and corresponds to the matrix of
+cartilage.
+
+Bone may be compact, or loose and spongy in character, when it is
+known as =cancellous bone=. In compact bone many of the lamellae are
+arranged concentrically round cavities, the =Haversian canals=, which
+in life are occupied by blood-vessels. Each Haversian canal with its
+lamellae forms a =Haversian system=. In spongy bone instead of
+Haversian canals there occur large irregular spaces filled with
+marrow, which consists chiefly of blood-vessels and fatty tissue. The
+centre of a long bone is generally occupied by one large continuous
+marrow cavity. The whole bone is surrounded by a fibrous connective
+tissue membrane, =the periosteum=.
+
+THE DEVELOPMENT OF BONE.
+
+=Periosteal ossification.= An example of a bone entirely formed in
+this way is afforded by the parietal. The first trace of ossification
+is shown by the appearance, below the membrane which occupies the
+place of the bone in the early embryo, of calcareous spicules of bony
+matter, which are laid down round themselves by certain large cells,
+the =osteoblasts=. These osteoblasts gradually get surrounded by the
+matter which they secrete and become converted into bone cells, and in
+this way a mass of spongy bone is gradually produced. Meanwhile a
+definite periosteum has been formed round the developing bone, and on
+its inner side fresh osteoblasts are produced, and these with the
+others gradually render the bone larger and more and more compact.
+Finally, the middle layer of the bone becomes again hollowed out and
+rendered spongy by the absorption of part of the bony matter.
+
+=Endochondral ossification=[5]. This is best studied in the case of a
+long bone like the femur or humerus. Such a long bone consists of a
+shaft, which forms the main part, and two terminal portions, which
+form the =epiphyses=, or portions ossifying from centres distinct from
+that forming the shaft or main part of the bone.
+
+In the earliest stage the future bone consists of hyaline cartilage
+surrounded by a vascular sheath, the perichondrium.
+
+Then, starting from the centre, the cartilage becomes permeated by a
+number of channels into which pass vessels from the perichondrium and
+osteoblasts. In this way the centre of the developing shaft becomes
+converted into a mass of cavities separated by bands or trabeculae of
+cartilage. This cartilage next becomes calcified, but as yet is not
+converted into true bone. The osteoblasts in connection with the
+cavities now begin to deposit true endochondral spongy bone, and then
+after a time this becomes absorbed by certain large cells, the
+osteoclasts, and resolved into marrow or vascular tissue loaded with
+fat. So that the centre of the shaft passes from the condition of
+hyaline cartilage to that of calcified cartilage, thence to the
+condition of spongy bone, and finally to that of marrow. At the same
+time beneath the perichondrium osteoblasts are developed which also
+begin to give rise to spongy bone. The perichondrium thus becomes the
+periosteum, and the bone produced by it, is periosteal or membrane
+bone. So that while a continuous marrow cavity is gradually being
+formed in the centre of the shaft, the layer of periosteal bone round
+the margin is gradually thickening, and becoming more and more compact
+by the narrowing down of its cavities to the size of Haversian canals.
+The absorption of endochondral and formation of periosteal bone goes
+on, till in time it comes about that the whole of the shaft, except
+its terminations, is of periosteal origin. At the extremities of the
+shaft, however, and at the epiphyses, each of which is for a long time
+separated from the shaft by a pad of cartilage, the ossification is
+mainly endochondral, the periosteal bone being represented only by a
+thin layer.
+
+Until the adult condition is reached and growth ceases, the pad of
+cartilage between the epiphysis and the shaft continues to grow, its
+outer (epiphysial) half growing by the formation of fresh cartilage as
+fast as its inner half is encroached on by the growth of bone from the
+shaft. The terminal or articular surfaces of the bone remain
+throughout life covered by layers of articular cartilage.
+
+Even after the adult condition is reached the bone is subject to
+continual change, processes of absorption and fresh formation going on
+for a time and tending to render the bone more compact.
+
+METHODS IN WHICH BONES ARE UNITED TO ONE ANOTHER.
+
+The various bones composing the endoskeleton are united to one another
+either by =sutures= or by movable =joints=.
+
+When two bones are suturally united, their edges fit closely together
+and often interlock, being also bound together by the periosteum.
+
+In many cases this sutural union passes into fusion or =ankylosis=,
+ossification extending completely from one bone to the other with the
+obliteration of the intervening suture. This feature is especially
+well marked in the cranium of most birds.
+
+The various kinds of joints or articulations[6] may be subdivided into
+imperfect joints and perfect joints.
+
+In =imperfect joints=, such as the intervertebral joints of mammals,
+the two contiguous surfaces are united by a mass of fibrous tissue
+which allows only a limited amount of motion.
+
+In =perfect joints= the contiguous articular surfaces are covered with
+cartilage, and between them lies a synovial membrane which secretes a
+viscid lubricating fluid.
+
+The amount of motion possible varies according to the nature of the
+articular surfaces; these include--
+
+_a._ =ball and socket joints=, like the hip and shoulder, in which the
+end of one bone works in a cup provided by another, and movements can
+take place in a variety of planes.
+
+_b._ =hinge joints=, like the elbow and knee, in which as in
+ball-and-socket joints one bone works in a cup provided by another,
+but movements can take place in one plane only.
+
+
+THE ENDOSKELETON.
+
+The endoskeleton is divisible into =axial= and =appendicular= parts;
+and the =axial= skeleton into--
+
+ 1. the spinal column,
+
+ 2. the skull {_a._ the cranium,
+ {_b._ the jaws and visceral skeleton,
+
+ 3. the ribs and sternum[7].
+
+
+I. THE AXIAL SKELETON.
+
+1. THE SPINAL COLUMN.
+
+The spinal column in the simplest cases consists of an unsegmented
+rod, the notochord, surrounded by the =skeletogenous layer=, a sheath
+of mesoblastic origin, which also envelops the nerve cord. Several
+intermediate stages connect this simple spinal column with the
+vertebral column characteristic of higher vertebrates. A typical
+vertebral column may be said to consist of (1) a series of
+cartilaginous or bony blocks, the vertebral =centra=, which arise in
+the sheath surrounding the notochord. They cause the notochord to
+become constricted and to atrophy to a varying extent, though a
+remnant of it persists, either permanently or for a long period,
+within each centrum or between successive centra. (2) From the dorsal
+surface of each centrum arise a pair of processes which grow round the
+spinal cord and unite above it, forming a =dorsal= or =neural arch=.
+(3) A similar pair of processes arising from the ventral surface of
+the centrum form the =ventral= or =haemal arch=. To the ventral arch
+the ribs strictly belong, and it tends to surround the ventral
+blood-vessels and the body cavity with the alimentary canal and other
+viscera.
+
+A =neural spine= or spinous process commonly projects upwards from the
+dorsal surface of the neural arch, and a pair of =transverse
+processes= project outwards from its sides. When, as is commonly the
+case, the two halves of the haemal arch do not meet, the ventral
+surface of the centrum often bears a downwardly-projecting
+=hypapophysis=.
+
+The character of the surfaces by which vertebral centra articulate
+with one another varies much. Sometimes both surfaces are concave, and
+the vertebra is then said to be =amphicoelous=; sometimes a centrum is
+convex in front and concave behind, the vertebra is then
+=opisthocoelous=, sometimes concave in front and convex behind, when
+the vertebra is =procoelous=. Again, in many vertebrae both faces of
+the centra are flat, while in others they are saddle-shaped, as in the
+neck vertebrae of living birds, or biconvex, as in the case of the
+first caudal vertebra of crocodiles.
+
+In the higher vertebrates pads of fibrocartilage--the =intervertebral
+discs=--are commonly interposed between successive centra, these or
+parts of them often ossify, especially in the trunk and tail, and are
+then known as =inter centra=.
+
+[Illustration FIG. 2. CERVICAL VERTEBRAE OF AN OX (_Bos taurus_).
+
+A, is the fifth; B, the fourth; C, the third. x 1/4 (Camb. Mus.)
+
+ 1. neural spine.
+ 2. transverse process.
+ 3. hypapophysis.
+ 4. convex anterior face of the centrum.
+ 5. concave posterior face of the centrum.
+ 6. prezygapophysis.
+ 7. postzygapophysis.
+ 8. vertebrarterial canal.
+ 9. neural canal.
+ 10. inferior lamella of transverse process.]
+
+The vertebrae of the higher forms can generally be arranged in the
+following five groups, each marked by certain special characteristics:
+
+1. The =cervical= or =neck vertebrae=. These connect the skull with
+the thorax, and are characterised by relatively great freedom of
+movement. They often bear small ribs, but are distinguished from the
+succeeding thoracic vertebrae by the fact that their ribs do not reach
+the sternum. The first cervical vertebra which articulates with the
+skull is called the =atlas=, but a study of the nerve exits shows that
+the first vertebra is not serially homologous throughout the
+Ichthyopsida, so that it is best to reserve the term atlas for the
+first vertebra in Sauropsida and Mammalia.
+
+2. The =thoracic vertebrae= (often called dorsal) bear movably
+articulated ribs which unite ventrally with the sternum.
+
+3. The =lumbar vertebrae= are generally large, and are often more
+movable on one another than are the thoracic vertebrae. They bear no
+ribs.
+
+4. The =sacral vertebrae= are characterised by the fact that they are
+firmly fused together, and are united with the pelvic girdle by means
+of their transverse processes and rudimentary ribs.
+
+5. The =caudal= or =tail vertebrae= succeed the sacral. The anterior
+ones are often fused with one another and with the sacrals, but they
+differ from true sacral vertebrae in that there are no rudimentary
+ribs between their transverse processes and the pelvic girdle. They
+often bear V-shaped =chevron bones=.
+
+In fish and snakes the vertebral column is divisible into only two
+regions, an anterior trunk region, whose vertebrae bear ribs, and a
+posterior tail region, whose vertebrae are ribless.
+
+
+2. THE SKULL.
+
+Before giving a general account of the adult skull it will be well to
+briefly describe its development.
+
+
+GENERAL DEVELOPMENT OF THE CRANIUM[8].
+
+Shortly after its appearance, the central nervous system becomes
+surrounded by a membranous mesodermal investment which in the region
+of the spinal cord is called the =skeletogenous layer= or =perichordal
+sheath=, while in the region of the brain it is called the
+=membranous cranium=. Ventral to the central nervous system is the
+notochord, which extends far into the region of the future cranium,
+and like the nervous system, is enclosed by the skeletogenous layer.
+The primitive cartilaginous cranium is formed by histological
+differentiation within the substance of the membranous cranium and
+always consists of the following parts:
+
+(_a_) the =parachordals=. These are a pair of flat curved plates of
+cartilage, each of which has its inner edge grooved where it comes in
+contact with the notochord. The parachordals, together with the
+notochord, form a continuous plate, which is known as the =basilar
+plate=. The basilar plate is the primitive floor below the hind- and
+mid-brain. In front the parachordals abut upon another pair of
+cartilaginous bars, the trabeculae, the two pairs of structures being
+sometimes continuous with one another from the first;
+
+(_b_) the =trabeculae= which meet behind and embrace the front end of
+the notochord. Further forwards they at first diverge from one
+another, and then converge again, enclosing a space, the =pituitary
+space=. After a time they generally fuse with one another in the
+middle line, and, with the parachordals behind, form an almost
+continuous basal plate. The trabeculae generally appear before the
+parachordals. They form the primitive floor below the fore-brain;
+
+(_c_) the cartilaginous =capsules= of the three pairs of =sense
+organs=. At a very early stage of development involutions of the
+surface epiblast give rise to the three pairs of special sense
+organs--the olfactory or nasal organs in front, the optic in the
+middle, and the auditory behind. The olfactory and auditory organs
+always become enclosed in definite cartilaginous capsules, the eyes
+often as in the Salmon, become enclosed in cartilaginous sclerotic
+capsules, while sometimes, as in mammals, their protecting capsules
+are fibrous.
+
+Each pair of sense capsules comes into relation with part of the
+primitive cranium, and greatly modifies it. Thus the auditory or
+periotic capsules press on the parachordals till they come to be more
+or less imbedded in them. Perhaps owing to the pressure of the nasal
+capsules the trabeculae fuse in front, and then grow out into an
+anterior pair of processes, the =cornua trabeculae=, and a posterior
+pair, the =antorbital processes=, which together almost completely
+surround the nasal capsules. The sclerotic capsules of the eyes
+greatly modify the cranium, although they never become completely
+united with it.
+
+The cartilaginous cranium formed of the basal plate, together with the
+sense capsules, does not long remain merely as a floor. Its sides grow
+vertically upwards, forming the =exoccipital= region of the cranium
+behind, and the =alisphenoidal= and =orbitosphenoidal= regions further
+forwards. In many forms, such as Elasmobranchs, all these upgrowths
+meet round the brain, roofing it in and forming an almost complete
+cartilaginous cranium. But in most vertebrata, while in the occipital
+region, the cartilaginous cranium is completed dorsally, in the
+alisphenoidal and orbitosphenoidal regions the cartilage merely forms
+the lateral walls of the cranium, the greater part of the brain having
+dorsal to it a wide space, closed by merely membranous tissue in
+connection with which the large frontal and parietal bones are
+subsequently formed.
+
+
+The SKULL includes
+
+_a._ the cranium,
+
+_b._ the jaws and visceral skeleton.
+
+The =cranium= can be further subdivided into
+
+(1) an axial portion, the =cranium proper= or =brain case=;
+
+(2) =the sense capsules.= The capsules of the auditory and olfactory
+sense organs are always present, and as has been already mentioned,
+in many animals the eye likewise is included in a cartilaginous
+capsule.
+
+
+(1) THE CRANIUM PROPER OR BRAIN CASE.
+
+The cranium varies much in form and structure. In lower vertebrates,
+such as Sharks and Lampreys, it remains entirely cartilaginous and
+membranous, retaining throughout life much of the character of the
+embryonic rudiment of the cranium of higher forms. The dogfish's
+cranium, described on pp. 73 to 76, is a good instance of a cranium of
+this type. But in the majority of vertebrates the cartilage becomes
+more or less replaced by cartilage bone, while membrane bones are also
+largely developed and supplant the cartilage.
+
+The cranium of most vertebrates includes a very large number of bones
+whose arrangement varies much, but one can distinguish a definite
+=basicranial axis= formed of the basi-occipital, basisphenoid, and
+presphenoid bones, which is a continuation forwards of the axis of the
+vertebral column. From the basicranial axis a wide arch arises,
+composed of a number of bones, which form the sides and roof of the
+brain-case These bones are arranged in such a manner that if both
+cartilage and membrane bones are included they can be divided into
+three rings or segments. The hinder one of these segments is the
+occipital, the middle the parietal, and the anterior one the frontal.
+
+The occipital segment is formed of four cartilage bones, the
+=basi-occipital= below, two =exoccipitals= at the sides, and the
+=supra-occipital= above. The parietal segment is formed of the
+=basisphenoid= below, two =alisphenoids= at the sides and two membrane
+bones, the _parietals_ above, and the frontal segment in like manner
+consists of the =presphenoid= below, the two =orbitosphenoids= at the
+sides, and two membrane bones, the _frontals_, above. The parietals
+and frontals, being membrane bones, are not comparable to the
+supra-occipital, in the way that the presphenoid and basisphenoid are
+to the basi-occipital.
+
+The cartilage bones of the occipital segments are derived from the
+parachordals of the embryonic skull, those of the parietal and frontal
+segments from the trabeculae.
+
+In front of the presphenoid the basicranial axis is continued by the
+=mesethmoid=.
+
+
+(2) THE SENSE CAPSULES.
+
+These enclose and protect the special sense organs.
+
+(_a_) =Auditory capsule.=
+
+The basisphenoid is always continuous with the basi-occipital, but the
+alisphenoid is not continuous with the exoccipital as the =periotic=
+or =auditory capsule= is interposed between them. Each periotic
+capsule has three principal ossifications; an anterior bone, the
+=pro-otic=, a posterior bone, the =opisthotic=, and a superior bone,
+the =epi-otic=.
+
+These bones may severally unite, or instead of uniting with one
+another they may unite with the neighbouring bones. Thus the epi-otic
+often unites with the supra-occipital, and the opisthotic with the
+exoccipital.
+
+Two other bones developed in the walls of the auditory capsule are
+sometimes added, as in Teleosteans; these are the =pterotic= and
+=sphenotic=.
+
+(_b_) =Optic capsule.=
+
+The eye is frequently enclosed in a cartilaginous sclerotic capsule,
+and in this a number of scale-like bones are often developed.
+
+Several membrane bones are commonly formed around the orbit or cavity
+for the eye. The most constant of these is the _lachrymal_ which lies
+in the anterior corner; frequently too, as in Teleosteans, there is a
+_supra-orbital_ lying in the upper part of the orbit, or as in many
+Reptiles, a _postorbital_ lying in the posterior part of the orbit.
+
+(_c_) =Nasal capsule.=
+
+In relation to the nasal capsules various bones occur.
+
+The basicranial axis in front of the presphenoid is ossified, as the
+=mesethmoid=, dorsal to which there sometimes, as in Teleosteans,
+occur a _median ethmoid_ and a pair of =lateral ethmoids=[9]. Two
+pairs of membrane bones very commonly occur in this region, viz. the
+_nasals_ which lie dorsal to the mesethmoid, and the _vomers_
+(sometimes there is only one) which lie ventral to it.
+
+The part of the skull lying immediately in front of the cranial cavity
+and in relation to the nasal capsules constitutes the =ethmoidal
+region=.
+
+There remain certain other membrane bones which are often found
+connected with the cranium. Of these, one of the largest is the
+_parasphenoid_ which, in Ichthyopsids, is found underlying the
+basicranial axis. _Prefrontals_ often, as in most reptiles, occur
+lying partly at the sides and partly in front of the frontal, and
+_postfrontals_ similarly occur behind the orbit lying partly behind
+the frontals and partly at their sides. Lastly a _squamosal bone_ is,
+as in Mammals, very commonly developed, and lies external and partly
+dorsal to the auditory capsules.
+
+THE JAWS AND VISCERAL SKELETON.
+
+In the most primitive fish these consist of a series of cartilaginous
+rings or arches placed one behind another and encircling the anterior
+end of the alimentary canal. Originally they are mainly concerned with
+branchial respiration.
+
+The first or =maxillo-mandibular= arch forms the upper jaw and the
+lower jaw or mandible.
+
+The second or =hyoid= arch bears gills and often assists in attaching
+the jaws to the cranium. The remaining arches may bear gills, though
+the last is commonly without them.
+
+The above condition is only found in fishes, in higher animals the
+visceral skeleton is greatly reduced and modified.
+
+The first or maxillo-mandibular arch is divisible into a dorsal
+portion, the =palato-pterygo-quadrate bar=, which forms the primitive
+upper jaw and enters into very close relations with the cranium, and a
+ventral portion, =Meckel's cartilage=, which forms the primitive lower
+jaw. The cartilaginous rudiments of both these portions disappear to a
+greater or less extent and become partly ossified, partly replaced by
+or enveloped in membrane bone.
+
+The posterior part of the palato-pterygo-quadrate bar becomes ossified
+to form the =quadrate=, the anterior part to form the palatine and
+pterygoid, or the two latter may be formed partially or entirely of
+periosteal bone, developed round the cartilaginous bar. Two pairs of
+important membrane bones, the _premaxillae_ and _maxillae_ form the
+anterior part of the upper jaw, and behind the maxilla lies another
+membrane bone, the _jugal_ or _malar_, which is connected with the
+quadrate by a _quadratojugal_. The premaxillae have a large share in
+bounding the external nasal openings or anterior nares.
+
+In lower vertebrates the nasal passage leads directly into the front
+part of the mouth cavity and opens by the posterior nares. In some
+higher vertebrates, such as mammals and crocodiles, processes arise
+from the premaxillae and palatines, and sometimes from the pterygoids,
+which meet their fellows in the middle line and form the palate,
+shutting off the nasal passage from the mouth cavity and causing the
+posterior nares to open far back.
+
+The cartilage of the lower jaw is in all animals with ossified
+skeletons, except the Mammalia, partly replaced by cartilage bone
+forming the =articular=, partly overlain by a series of membrane bones
+the _dentary_, _splenial_, _angular_, _supra-angular_ and _coronoid_.
+In many sharks large paired accessory cartilages occur at the sides of
+the jaws; and in a few reptiles and some Amphibia, such as the Frog,
+the ossified representative of the anterior of these structures occurs
+forming the =mento-meckelian= bone. In mammals the lower jaw includes
+but a single bone.
+
+The quadrate in all animals with ossified skeletons, except the
+Mammalia, forms the suspensorium of the mandible or the skeletal link
+between the jaw and the cranium; in the Mammalia, however, the
+mandible articulates with the squamosal, while the quadrate is greatly
+reduced, and is now generally considered to be represented by the
+tympanic ring of the ear.
+
+The second visceral or hyoid arch in fishes consists of two pieces of
+cartilage, a proximal[1] piece the =hyomandibular=, and a distal[10]
+piece the =cerato-hyal=. The cerato-hyals of the two sides are commonly
+united by a median ventral plate, the =basi-hyal=. The hyoid arch
+bears gills on its posterior border, but its most important function
+in most fishes is to act as the suspensorium. In higher vertebrates
+the representative of the hyomandibular is much reduced in size, and
+comes into relation with the ear forming the =auditory ossicles=; the
+cerato-hyal looses its attachment to the hyomandibular and becomes
+directly attached to the cranium, forming a large part of the hyoid
+apparatus of most higher vertebrates.
+
+Behind the hyoid arch come the branchial arches. They are best
+developed in fishes, in which they are commonly five in number and
+bear gills. Their ventral ends are united in pairs by median pieces,
+the =copulae=.
+
+In higher vertebrates they become greatly reduced, and all except the
+first and second completely disappear. In the highest vertebrates, the
+mammals, the second has disappeared, but in birds and many reptiles it
+is comparatively well developed.
+
+3. THE RIBS AND STERNUM.
+
+The =ribs= are a series of segmentally arranged cartilaginous or bony
+rods, attached to the vertebrae; they tend to surround the body
+cavity, and to protect the organs contained within it. Ribs are very
+frequently found attached to the transverse processes of the
+vertebrae, but a study of their origin in fish shows that they are
+really the cut off terminations of the ventral arch, not of the
+transverse processes which are outgrowths from the dorsal arch. In the
+tail their function is to surround and protect structures like the
+ventral blood-vessels which do not vary much in size, consequently
+they meet one another, and form a series of complete ventral or haemal
+arches. But the trunk contains organs like the lungs and stomach which
+are liable to vary much in size at different times, consequently the
+halves of the haemal arch do not meet ventrally, and then the ribs
+become detached from the rest of the haemal arch. Having once become
+detached, they are able to shift about and unite themselves to various
+points of the vertebra. They frequently, as has been already
+mentioned, become entirely attached to the transverse process, or they
+may be attached to the transverse process by a dorsal or =tubercular=
+portion and to the centrum or to the ventral arch by a ventral or
+=capitular= portion.
+
+In all animals above fishes the distal ends of the thoracic ribs unite
+with a median breast bone or sternum which generally has the form of a
+segmented rod. The =sternum= is really formed by the fusion of the
+distal ends of a series of ribs. In many animals elements of the
+shoulder girdle enter into close relation with the rib elements of the
+sternum.
+
+II. THE APPENDICULAR SKELETON.
+
+This consists of the skeleton of the anterior or =pectoral=, and the
+posterior or =pelvic= limbs, and their girdles. In every case (except
+in Chelonia) the parts of the appendicular skeleton lie external to
+the ribs.
+
+1. THE LIMB GIRDLES.
+
+=The Pectoral girdle=[11]. In the simplest case the pectoral or
+shoulder girdle consists of a hoop of cartilage incomplete dorsally.
+It is attached by muscle to the vertebral column, and is divided on
+either side into dorsal and ventral portions by a cavity, the =glenoid
+cavity=, at the point where the anterior limb articulates. In higher
+fishes this hoop is distinctly divided into right and left halves; it
+becomes more or less ossified, and a pair of important bones, the
+clavicles, are developed in connection with its ventral portion.
+
+In higher vertebrates ossification sets up in the cartilage and gives
+rise on each side to a dorsal bone, the =scapula=, and frequently to
+an anterior ventral bone, the =precoracoid=, and a posterior ventral
+bone, the =coracoid=. The precoracoid is often not ossified, and upon
+it is developed the clavicle which more or less replaces it. In some
+forms a =T= shaped _interclavicle_ occurs, in others =epicoracoids= are
+found in front of the coracoids. In all vertebrata above fish, except
+the great majority of mammals, the coracoids are large and articulate
+with the sternum. But in mammals the coracoids are nearly always quite
+vestigial, and the pectoral girdle is attached to the axial skeleton
+by the clavicle or sometimes by muscles and ligaments only.
+
+The =Pelvic girdle=[12] like the pectoral consists primitively of a
+simple rod or hoop of cartilage, which in vertebrata above fishes is
+divided into dorsal and ventral portions, by a cavity, the
+=acetabulum=, with which the posterior limb articulates. In the pelvic
+girdle as in the pectoral one dorsal, and (commonly) two ventral
+ossifications take place. The dorsal bone is the =ilium= and
+corresponds to the scapula. The posterior ventral bone is the
+=ischium= corresponding to the coracoid. The anterior ventral bone is
+the =pubis= and is generally compared to the precoracoid, but in some
+cases a fourth pelvic element, the =acetabular= or =cotyloid= bone is
+found, and this may correspond to the precoracoid.
+
+The pelvic girdle differs from the pectoral in the fact that the
+dorsal bones--the ilia--are nearly always firmly united to transverse
+processes of the sacral vertebrae, by means of rudimentary ribs. The
+pubes and ischia generally meet in ventral symphyses.
+
+2. THE LIMBS.
+
+It will be most convenient to defer a discussion of the limbs of
+fishes to chap. VIII.
+
+All vertebrates above fishes have the limbs divisible into three main
+segments:--
+
+ =Anterior or Fore limb.= =Posterior or Hind limb.=
+
+ Proximal segment. upper arm or _brachium_. thigh.
+ Middle segment. fore-arm or _antibrachium_. shin or _crus_.
+ Distal segment. _manus_. _pes_.
+
+The proximal segments each contain one bone, the =humerus= in the case
+of the upper arm, and the =femur= in the case of the thigh. The middle
+segments each contain two bones, the =radius= and =ulna= in the case
+of the fore-arm, and the =tibia= and =fibula= in the case of the shin.
+
+ * * * * *
+
+The manus and pes are further subdivided into
+
+(_a_) two or three proximal rows of bones forming the wrist or
+=carpus= in the case of the manus, and the ankle or =tarsus= in the
+case of the pes.
+
+(_b_) a middle row called respectively the =metacarpus= and
+=metatarsus=.
+
+(_c_) a number of distal bones called the =phalanges= which form the
+skeleton of the fingers and toes, or =digits=.
+
+Typically the manus and pes both have five digits (pentedactylate).
+The first digit of the manus is commonly called the =pollex=, and the
+first digit of the pes the =hallux=.
+
+In a very simple =carpus= such as that of _Chelydra_, there are nine
+bones. They are arranged in a proximal row of three, the radiale,
+intermedium, and ulnare,--the first being on the radial side of the
+limb, and a distal row of five called respectively carpale 1, 2, 3, 4,
+5, beginning on the radial side. Between these two rows is a single
+bone the centrale, or there may be two.
+
+Similarly there are nine bones in a simple =tarsus= such as that of
+_Salamandra_. They form a proximal row of three, the tibiale,
+intermedium and fibulare, and a distal row of five, called
+respectively tarsale 1, 2, 3, 4, 5, beginning on the tibial side.
+Between the two rows there is a centrale as in the carpus, or there
+may be two.
+
+ * * * * *
+
+The following names derived from human anatomy are commonly applied to
+the various carpal and tarsal bones:
+
+ =Carpus.=
+
+ radiale = scaphoid
+ intermedium = lunar
+ ulnare = cuneiform
+ centrale = central
+ carpale 1 = trapezium
+ " 2 = trapezoid
+ " 3 = magnum
+ " 4 } = unciform
+ " 5 }
+
+ =Tarsus.=
+
+ tibiale }
+ intermedium } astragalus
+ fibulare = calcaneum
+ centrale = navicular
+ tarsale 1 = internal cuneiform
+ " 2 = middle "
+ " 3 = external "
+ " 4 } = cuboid
+ " 5 }
+
+ NOTE. The above is the view commonly accepted concerning the
+ homology of the carpal and tarsal bones. But with regard to the
+ proximal row of tarsal bones there is difference of opinion.
+ All anatomists are agreed that the calcaneum is the fibulare
+ and that the intermedium is contained in the astragalus, but
+ while the majority regard the astragalus as the fused tibiale
+ and intermedium, Baur considers that a small bone found on the
+ tibial side of the tarsus in _Procavia_, many Rodents,
+ Insectivores, and the male _Ornithorhynchus_, is the vestigial
+ tibiale, and regards the astragalus as the intermedium
+ alone[13]. He also considers that the mammalian scaphoid
+ represents a centrale.
+
+
+MODIFICATIONS IN THE POSITIONS OF THE LIMBS[14].
+
+In their primitive position the limbs are straight and are extended
+parallel to one another at right angles to the axis of the trunk. Each
+limb then has a dorsal surface, a ventral surface, an anterior or
+=pre-axial= edge, and a posterior or =postaxial= edge.
+
+In the anterior limb the radius and the pollex are pre-axial, the ulna
+and the fifth finger are postaxial. In the posterior limb the tibia
+and the hallux are pre-axial, the fibula and the fifth toe are
+postaxial. The Cetacea and various extinct reptiles, such as
+_Ichthyosaurus_ and _Plesiosaurus_, have their limbs in practically
+this primitive position.
+
+The first modification from it is produced by the bending ventrally of
+the middle segments of both limbs upon the proximal segments, while
+the distal segment is bent in the opposite direction on the middle
+segment. Then the ventral surfaces of the antibrachium and crus come
+to look inwards, and their dorsal surfaces to look outwards. The
+brachium and manus, thigh and pes still have their dorsal surfaces
+facing upwards and their ventral surfaces facing downwards as before,
+and the relations of their pre-and postaxial borders remain as they
+were. Many Amphibians and Reptiles, such as tortoises, carry their
+limbs in this position.
+
+In all higher vertebrates, however, a further change takes place, each
+limb is rotated as a whole from its proximal end, the rotation taking
+place in opposite directions in the fore and hind limbs respectively.
+The anterior limb is rotated backwards from the shoulder, so that the
+brachium lies nearly parallel to the body, and the elbow points
+backwards, the antibrachium downwards, and the manus backwards; the
+pre-axial surface of the whole limb with the radius and pollex now
+faces outwards, and the postaxial surface with the ulna and fifth
+finger now faces inwards. In the Walrus and, to a certain extent, in
+the Sea lions the anterior limb remains throughout life in this
+position. The posterior limb is also rotated, but the rotation in this
+case takes place forwards, so that the thigh lies nearly parallel to
+the body, the knee-joint pointing forwards; the crus downwards and the
+pes forwards. The pre-axial surface of the whole limb with the tibia
+and hallux looks towards the middle of the body, the postaxial surface
+with the fibula and fifth toe looks outwards. This is the position in
+which the hind limb is carried in nearly all mammals.
+
+In nearly all mammals a further change takes place in the position of
+the anterior limb. The radius and ulna have hitherto been parallel to
+one another, but now the lower end of the radius, carrying with it the
+manus, comes to be rotated forwards round the ulna, so that the manus,
+as well as the pes, comes to be forwardly-directed, and its pre-axial
+surface faces inwards.
+
+In the majority of mammals the radius and ulna are permanently fixed
+in this, which is known as the =prone= position, but in man and some
+other mammals the manus can be pronated or turned into this position
+at will. When the radius and ulna are parallel throughout their whole
+length the manus is said to be in the =supine= position.
+
+The =extensor= side of a limb is that to which the muscles which
+straighten it are attached, the =flexor= side is that to which the
+muscles which bend it are attached.
+
+
+FOOTNOTES:
+
+[1] The skin consists of an outer layer of epiblastic origin, the
+epidermis, and an inner layer of mesoblastic origin, the dermis. The
+epidermis is divided into two principal layers, an outer one, the
+horny layer or _stratum corneum_, and an inner one, the _stratum
+Malpighii_. The innermost part of the stratum corneum is distinguished
+as the _stratum lucidum_, and the outermost part of the stratum
+Malpighii as the _stratum granulosum_.
+
+[2] The enamel of the pharyngeal teeth of some Teleosteans is
+hypoblastic in origin.
+
+[3] See also p. 71.
+
+[4] It is usual to regard the clavicle as a membrane bone, but
+Kölliker has shown that in rabbit embryos of about the 17th day it is
+cartilaginous.
+
+[5] In compiling these paragraphs on Histology, free use has been made
+of Klein and Noble Smith's _Atlas of Histology_, the small Histologies
+of Klein and Schäfer, Huxley's _Elementary Physiology_, and Lloyd
+Morgan's _Animal Biology_.
+
+[6] See Huxley's _Elementary Physiology_, Revised edition, London,
+1886, p. 180.
+
+[7] Strictly speaking the jaws, visceral skeleton, ribs and sternum do
+not form part of the axis, but it is convenient to group them as parts
+of the axial skeleton.
+
+[8] F.M. Balfour, _Comparative Embryology_, vol. II., London, 1881, p.
+465. W.K. Parker and G.T. Bettany, _The Morphology of the Skull_,
+London, 1877.
+
+[9] Sometimes also called ectethmoids or parethmoids.
+
+[10] The _proximal_ end of anything is the one nearest the point of
+origin or attachment, the _distal_ end is the one furthest from the
+point of origin or attachment.
+
+[11] W.K. Parker, _A Monograph of the Shoulder Girdle and Sternum_,
+Ray Soc. London, 1868.
+
+[12] See R. Wiedersheim, _Zeitschr. wiss. Zool._ vol. LIII. suppl. p.
+43, 1892.
+
+[13] G. Baur, _Beiträge zur Morphogenie des Carpus und Tarsus der
+Vertebraten_, Theil 1. Batrachia. Jena, 1888, and _Amer. Natural._,
+vol. XIX. 1885 (several papers).
+
+[14] This account is based on Chapter XX. of Flower's _Osteology of
+the Mammalia_. London 1876.
+
+
+
+
+CHAPTER II.
+
+CLASSIFICATION.
+
+
+THE following classification includes _only the forms mentioned in the
+succeeding pages_. The relative value of some of the terms employed in
+classification is not identical throughout the book. This remark
+applies specially to the term _group_, which is a convenient one,
+owing to its not having such a hard and fast zoological meaning as has
+the term _family_, for instance. The term _group_ is applied in this
+book to divisions of the animal kingdom of very different
+classificatory importance.
+
+ PHYLUM CHORDATA.
+
+ SUBPHYLUM A. HEMICHORDATA.
+
+ Balanoglossus.
+ Cephalodiscus.
+ Rhabdopleura.
+ ? Phoronis.
+ (? Actinotrocha--larval Phoronis).
+
+ SUBPHYLUM B. UROCHORDATA (TUNICATA).
+
+ Group LARVACEA and others.
+
+ SUBPHYLUM C. CEPHALOCHORDATA.
+
+ Amphioxus--lancelet.
+
+ NOTE. In this chapter all the generic names printed in italics
+ are those of extinct animals.
+
+ SUBPHYLUM D. VERTEBRATA.
+
+ DIVISION (I). CYCLOSTOMATA.
+
+ Order 1. MARSIPOBRANCHII.
+
+ Family =Myxinoidei=. Myxine--hag-fish.
+ Bdellostoma.
+
+ Family =Petromyzontidae=. Petromyzon--lamprey.
+ (Ammocoetes--larval lamprey.)
+
+ Family =Palaeospondylidae=. _Palaeospondylus._
+
+ Order 2. OSTRACODERMI.
+
+ Suborder 1. HETEROSTRACI.
+
+ Family =Pteraspidae=. _Pteraspis._
+
+ Suborder 2. OSTEOSTRACI.
+
+ Family =Cephalaspidae=. _Cephalaspis._
+
+ Suborder 3. ANTIARCHA.
+
+ Family =Asterolepidae=. _Pterichthys._
+ _Asterolepis._
+
+
+ DIVISION (II). GNATHOSTOMATA.
+
+ A. ICHTHYOPSIDA.
+
+ CLASS I. PISCES.
+
+ Order 1. ELASMOBRANCHII.
+
+ Suborder (1). ICHTHYOTOMI.
+
+ Family =Pleuracanthidae=. _Xenacanthus._
+
+ Suborder (2). PLEUROPTERYGII.
+ _Cladoselache._
+
+ Suborder (3). SELACHII.
+
+ Group SQUALIDAE.
+
+ Family =Notidanidae=. Heptanchus.
+ Hexanchus.
+ Chlamydoselache--frill-gilled shark.
+
+ Family =Cochliodontidae=. _Cochliodus._
+
+ Family =Cestraciontidae=. Cestracion--Port Jackson shark.
+ _Acrodus._
+
+ Family =Scylliidae=. Scyllium--spotted dogfish.
+
+ Family =Lamnidae=. Odontaspis.
+
+ Family =Carcharidae=. Galeus--tope.
+
+ Family =Spinacidae=. Acanthias--spiny dogfish.
+ Scymnus.
+
+ Family =Squatinidae=. Squatina (Rhina)--angel fish.
+
+ Group BATOIDEI.
+
+ Family =Pristidae=. Pristis--saw-fish.
+
+ Family =Raiidae=. Raia--skate.
+
+ Family =Myliobatidae=. Myliobatis--eagle ray.
+
+ Family =Trygonidae=. Trygon--sting ray.
+
+ Family =Torpedinidae=. Torpedo--electric ray.
+
+ Suborder (4). ACANTHODII.
+
+ Family =Acanthodidae=. _Acanthodes._
+
+ Family =Diplacanthidae=. _Diplacanthus._
+
+ Order 2. HOLOCEPHALI.
+
+ Family =Chimaeridae=. Chimaera--rabbit fish.
+ Harriotta.
+ Callorhynchus.
+ _Ischyodus._
+
+ Order 3. GANOIDEI.
+
+ Suborder (1). CHONDROSTEI.
+
+ Family =Palaeoniscidae=. _Palaeoniscus._
+ _Trissolepis._
+
+ Family =Acipenseridae=. Acipenser--sturgeon.
+ Scaphirhynchus.
+
+ Family =Polyodontidae=. Polyodon (Spatularia)--spoon-beaked sturgeon.
+ Psephurus--slender-beaked sturgeon.
+
+ Suborder (2). CROSSOPTERYGII.
+
+ Family =Holoptychiidae=. _Holoptychius._
+
+ Family =Rhizodontidae=. _Rhizodus._
+
+ Family =Osteolepidae=. _Osteolepis._
+
+ Family =Polypteridae=. Polypterus--bichir.
+ Calamoichthys--reed-fish.
+
+ Suborder (3). HOLOSTEI.
+
+ Family =Lepidosteidae=. Lepidosteus--gar pike.
+
+ Family =Semionotidae=. _Lepidotus._
+
+ Family =Amiidae=. Amia--bow-fin.
+
+ Order 4. TELEOSTEI.
+
+ Suborder (1). PLECTOGNATHI.
+
+ Family =Balistidae=. Balistes--file-fish.
+
+ Family =Gymnodontidae=. Diodon--globe-fish.
+
+ Family =Ostracionidae=. Ostracion--coffer-fish.
+
+ Suborder (2). PHYSOSTOMI.
+
+ Family =Siluridae=.--cat-fishes.
+
+ Family =Cyprinidae=. Cyprinus--carp.
+
+ Family =Esocidae=. Esox--pike.
+
+ Family =Salmonidae=. Salmo--salmon.
+
+ Family =Clupeidae=. Clupeus--herring.
+ Exocaetus--'flying fish'.
+
+ Family =Muraenidae=. Anguilla--eel.
+
+ Suborder (3). ANACANTHINI.
+
+ Family =Gadidae=. Gadus--cod, haddock, whiting.
+
+ Family =Pleuronectidae=. Solea--sole.
+
+ Suborder (4). PHARYNGOGNATHI.
+
+ Family =Labridae=. Labrus--wrasse.
+ Scarus--parrot fish.
+
+ Suborder (5). ACANTHOPTERYGII.
+
+ Family =Cataphracti=. Dactylopterus--flying gurnard.
+
+ Family =Percidae=. Perca--perch.
+
+ Order 5. DIPNOI.
+
+ Suborder (1). SIRENOIDEI.
+
+ Family =Dipteridae=. _Dipterus._
+
+ Family =Monopneumona=. Ceratodus--barramunda.
+
+ Family =Dipneumona=. Protopterus--African mud-fish.
+ Lepidosiren.
+
+ Suborder (2). ARTHRODIRA.
+
+ Family =Coccosteidae=. _Coccosteus._
+ _Dinichthys._
+
+ NOTE. Palaeontological research has disclosed the existence of
+ a great number of forms which seem to connect with one another
+ almost all the orders of fishes as usually recognised. Forms
+ connecting the living Ganoids with the Teleosteans have been
+ especially numerous, so that these terms Ganoid and Teleostean
+ can hardly be any longer used in a precise and scientific
+ sense. This has rendered the subject of the classification of
+ fishes a very difficult one. Though unsuitable for adoption in
+ a work like the present, by far the most natural classification
+ hitherto proposed seems to be that of Smith Woodward[15]. He
+ considers that the course of development of fishes has followed
+ two distinct lines, the autostylic and hyostylic (see p. 119),
+ and groups the various forms as follows:
+
+ HYOSTYLIC. AUTOSTYLIC.
+ Subclass 1. ELASMOBRANCHII. Subclass 3. HOLOCEPHALI.
+ 1. Ichthyotomi. 1. (unknown).
+ 2. Selachii. 2. Chimaeroidei.
+ 3. Acanthodii. 3. (unknown).
+
+ Subclass 2. TELEOSTOMI. Subclass 4. DIPNOI.
+ 1. Crossopterygii (Palaeozoic 1. Sirenoidei.
+ and Mesozoic).
+ 2. Crossopterygii (Cainozoic). 2. (unknown).
+ 3. Actinopterygii. 3. Arthrodira.
+
+ The primitive forms in each of these four subclasses have the
+ fins archipterygia (see p. 127).
+
+ CLASS II. AMPHIBIA.
+
+ Order 1. URODELA.
+
+ Suborder (1). ICHTHYOIDEA.
+
+ Group A. PERENNIBRANCHIATA.
+
+ Family =Menobranchidae=. Menobranchus.
+
+ Family =Proteidae=. Proteus--olm.
+
+ Family =Sirenidae=. Siren.
+
+ Group B. DEROTREMATA.
+
+ Family =Amphiumidae=. Megalobatrachus.
+ Cryptobranchus (Menopoma).
+ Amphiuma.
+
+ Suborder (2). SALAMANDRINA.
+
+ Family =Salamandridae=. Salamandra--salamander.
+ Molge--newt.
+ Onychodactylus.
+ Amblystoma.
+ (Siredon--axolotl, larval Amblystoma).
+ Batrachoseps.
+ Spelerpes (Gyrinophilus).
+
+ Order 2. LABYRINTHODONTIA.
+
+ Group =Lepospondyli=. _Branchiosaurus._
+
+ Group =Temnospondyli=. _Archegosaurus._
+ _Nyrania._
+ _Euchirosaurus._
+
+ Group =Stereospondyli=. _Capitosaurus._
+ _Mastodonsaurus._
+
+ Order 3. GYMNOPHIONA.
+
+ Family =Caeciliidae=. Siphonops.
+ Epicrium.
+
+ Order 4. ANURA.
+
+ Suborder (1). AGLOSSA.
+
+ Family =Xenopidae=. Xenopus.
+
+ Family =Pipidae=. Pipa--Surinam toad.
+
+ Suborder (2). PHANEROGLOSSA.
+
+ Group ARCIFERA.
+
+ Family =Discoglossidae=. Discoglossus--painted frog.
+ Bombinator--fire-bellied frog.
+ Alytes--midwife frog.
+
+ Family =Pelobatidae=. Pelobates--toad frog.
+
+ Family =Hylidae=. Hyla--green tree-frog.
+
+ Family =Bufonidae=. Bufo--toad.
+ Docidophryne.
+
+ Family =Cystignathidae=. Ceratophrys--horned frog.
+
+ Group FIRMISTERNIA.
+
+ Family =Ranidae=. Rana--common and edible frogs.
+
+ Family =Engystomatidae=. Brachycephalus.
+
+ B. SAUROPSIDA.
+
+ CLASS I. REPTILIA[16].
+
+ Order 1. THEROMORPHA.
+
+ Group =Anomodontia=. _Dicynodon._
+ _Udenodon._
+
+ Group =Placodontia=. _Placodus._
+
+ Group =Pariasauria=. _Pariasaurus._
+ _Elginia._
+
+ Group =Theriodontia=. _Dimetrodon._
+ _Galesaurus._
+ _Cynognathus._
+
+ Order 2. SAUROPTERYGIA.
+
+ Family =Mesosauridae=. _Mesosaurus._
+
+ Family =Nothosauridae=. _Nothosaurus._
+
+ Family =Plesiosauridae=. _Plesiosaurus._
+ _Pliosaurus._
+
+ Order 3. CHELONIA.
+
+ Suborder (1). TRIONYCHIA.
+
+ Family =Trionychidae=. Trionyx--snapping turtle.
+
+ Suborder (2). CRYPTODIRA.
+
+ Family =Dermochelydidae=. Dermochelys (Sphargis)--leathery
+ turtle.
+
+ Family =Chelonidae=. Chelone--green turtle.
+
+ Family =Chelydridae=. Chelydra--terrapin.
+
+ Family =Chersidae=. Testudo--tortoise.
+
+ Suborder (3). PLEURODIRA.
+
+ Family =Chelydae=. Chelys.
+
+ Order 4. ICHTHYOSAURIA.
+
+ Family =Ichthyosauridae=. _Ichthyosaurus._
+
+ Order 5. RHYNCHOCEPHALIA.
+
+ Suborder (1). RHYNCHOCEPHALIA VERA.
+
+ Family =Sphenodontidae=. Sphenodon (Hatteria).
+
+ Family =Rhynchosauridae=. _Hyperodapedon._
+
+ Suborder (2). PROGANOSAURIA.
+
+ Family =Proterosauridae=. _Proterosaurus._
+
+ Order 6. SQUAMATA.
+
+ Suborder (1). LACERTILIA.
+
+ Group =Lacertilia vera=.
+
+ Family =Geckonidae=. Gecko.
+
+ Family =Pygopodidae=. Lialis--scale-foot.
+
+ Family =Agamidae=. Draco--flying lizard.
+ Agama.
+
+ Family =Iguanidae=. Iguana.
+
+ Family =Anguidae=. Ophisaurus (Bipes, Pseudopus).
+ Anguis--blindworm.
+
+ Family =Varanidae=. Varanus--monitor.
+
+ Family =Amphisbaenidae=. Chirotes.
+ Amphisbaena.
+
+ Family =Scincidae=. Tiliqua (Cyclodus).
+ Scincus--skink.
+ Chalcides (Seps).
+
+
+ Group =Rhiptoglossa=.
+
+ Family =Chamaeleonidae=. Chamaeleon.
+
+ Suborder (2). OPHIDIA.
+
+ Family =Typhlopidae=. Typhlops--blind snake.
+
+ Family =Boidae=. Python.
+
+ Family =Colubridae=. Tropidonotus--ringed snake.
+
+ Family =Hydrophidae=--sea snakes.
+
+ Family =Crotalidae=. Crotalus--rattlesnake.
+
+ Suborder (3). PYTHONOMORPHA.
+
+ Family =Mosasauridae=. _Mosasaurus._
+
+ Order 7. DINOSAURIA.
+
+ Suborder (1). SAUROPODA.
+
+ Family =Atlantosauridae=. _Brontosaurus._
+
+ Family =Cetiosauridae=. _Morosaurus._
+
+ Suborder (2). THEROPODA.
+
+ Family =Megalosauridae=. _Megalosaurus_ (_Ceratosaurus_).
+
+ Family =Compsognathidae=. _Compsognathus._
+
+ Suborder (3). ORTHOPODA.
+
+ Section (_a_). STEGOSAURIA.
+
+ Family =Scelidosauridae=. _Polacanthus._
+
+ Family =Stegosauridae=. _Stegosaurus._
+
+ Section (_b_). CERATOPSIA.
+
+ Family =Ceratopsidae=. _Polyonax_ (_Ceratops_).
+
+ Section (_c_). ORNITHOPODA.
+
+ Family =Camptosauridae=. _Hypsilophodon._
+
+ Family =Iguanodontidae=. _Iguanodon._
+
+ Family =Hadrosauridae=. _Hadrosaurus._
+
+ Order 8. CROCODILIA.
+
+ Suborder (1). PARASUCHIA.
+
+ Family =Phytosauridae=. _Phytosaurus_ (_Belodon_).
+
+ Suborder (2). EUSUCHIA.
+
+ Family =Teleosauridae=. _Teleosaurus._
+ _Metriorhynchus._
+
+ Family =Goniopholidae=. _Goniopholis._
+
+ Family =Alligatoridae=. Alligator.
+ Caiman.
+ Jacare.
+
+ Family =Crocodilidae=. Crocodilus.
+
+ Family =Garialidae=. Garialis (Gavialis).
+
+ Order 9. PTEROSAURIA.
+
+ Family =Pterodactylidae=. _Pterodactylus._
+
+ Family =Rhamphorhynchidae=. _Rhamphorhynchus._
+
+ Family =Pteranodontidae=. _Pteranodon._
+
+
+FOOTNOTES:
+
+[15] A. Smith Woodward, _Catalogue of Fossil Fishes in the British
+Museum_, Part II., Introduction, p. xii.
+
+[16] This classification of reptiles is mainly based on that of
+Lydekker (_Catalogue of Fossil Reptiles in the British Museum_) but in
+some respects that of von Zittel has been followed.
+
+
+
+
+ CLASS II. AVES[17].
+
+
+ Subclass (I). ARCHAEORNITHES.
+ _Archaeopteryx._
+
+ Subclass (II). NEORNITHES.
+
+ Order 1. RATITAE.
+
+ Group =Æpyornithes=. _Æpyornis._
+
+ Group =Apteryges=. Apteryx--kiwi.
+
+ Group =Dinornithes=. Moas.
+
+ Group =Megistanes=. Casuarius--cassowary.
+ Dromaeus--emeu.
+
+ Group =Rheornithes=. Rhea--American ostrich.
+
+ Group =Struthiornithes=. Struthio--ostrich.
+
+ Order 2. ODONTOLCAE.
+ _Hesperornis._
+
+ Order 3. CARINATAE.
+
+ Group =Ichthyornithiformes=.
+ _Ichthyornis._
+ _Apatornis._
+ _Odontopteryx._
+
+ Group =Colymbiformes=.
+ Subgroup Colymbi--divers.
+
+ Group =Sphenisciformes=.
+ Subgroup Sphenisci--penguins.
+
+
+ Group =Ciconiiformes=.
+
+ Subgroup Steganopodes. Sula--gannet.
+ Pelicanus--pelican.
+ Phaëthon--frigate bird.
+ Phalacrocorax--cormorant.
+
+ Subgroup Ardeae. Ardea--heron
+
+ Subgroup Ciconiae. Leptoptilus--adjutant.
+ Ciconia--white stork.
+
+ Group =Anseriformes=.
+
+ Subgroup Palamedeae. Palamedea }
+ } screamers.
+ Chauna }
+
+ Subgroup Anseres. Anas--wild duck.
+ Anser--goose.
+ Plectropterus--spur-winged goose.
+ Cygnus--swan.
+ Mergus--merganser.
+
+ Group =Falconiformes=.
+
+ Subgroup Cathartae. Cathartes--American vulture.
+
+ Subgroup Accipitres. Falco--falcon.
+ Vultur--vulture.
+ Harpagus.
+ Gypogeranus--secretary bird.
+
+ Group =Tinamiformes=.
+
+ Subgroup Tinami. Tinamus.
+
+ Group =Galliformes=.
+
+ Subgroup Galli. Gallus--fowl.
+ Pavo--peacock.
+
+ Subgroup Opisthocomi. Opisthocomus--hoatzin.
+
+ Group =Gruiformes=.
+ Gruidae--cranes.
+
+ Group =Stereornithes=. _Phororhacos._
+
+ Group =Charadriiformes=.
+
+ Subgroup Limicolae. Charadriidae--plovers.
+ Parra--jacana.
+
+ Subgroup Lari. Laridae--gulls.
+ Alcidae--auks.
+
+ Subgroup Pteroclidae. Pterocles--sandgrouse.
+
+ Subgroup Columbidae. Columbae--pigeons.
+ _Didus_--dodo.
+ _Pezophaps_--solitaire.
+
+ Group =Cuculiformes=.
+
+ Subgroup Cuculi. Scythrops.
+
+ Subgroup Psittaci. Stringops--owl-parrot.
+
+ Group =Coraciiformes=.
+
+ Subgroup Coraciae. Coracias--roller.
+ Buceros--hornbill.
+ Upupa--hoopoe.
+
+ Subgroup Striges. Owls.
+
+ Subgroup Cypseli. Cypselidae--swifts.
+ Trochilidae--humming-birds.
+
+ Subgroup Trogonidae. Trogons.
+
+ Subgroup Pici. Rhamphastos--toucan.
+ Picus--woodpecker.
+
+ Group =Passeriformes=. Crows, finches, larks, warblers,
+ and many others.
+
+C. MAMMALIA[18].
+
+Class MAMMALIA.
+
+Subclass (I). ORNITHODELPHIA or PROTOTHERIA.
+
+Order. MONOTREMATA.
+
+ Family =Ornithorhynchidae=. Ornithorhynchus--duck-bill.
+
+ Family =Echidnidae=. Echidna--spiny ant-eater.
+
+ Group =Multituberculata=. _Tritylodon._
+
+ Subclass (II). DIDELPHIA or METATHERIA.
+
+ Order. MARSUPIALIA.
+
+ Suborder (1). POLYPROTODONTIA.
+
+ Family =Amphitheriidae=. _Phascolotherium._
+
+ Family =Didelphyidae=. Didelphys--opossum.
+
+ Family =Dasyuridae=. Thylacinus--Tasmanian wolf.
+ Sarcophilus--Tasmanian devil.
+ Dasyurus.
+
+ Family =Peramelidae=. Perameles--bandicoot.
+ Choeropus.
+
+ Family =Notoryctidae=. Notoryctes--marsupial mole.
+
+ Suborder (2). DIPROTODONTIA.
+
+ Family =Phascolomyidae=. Phascolomys--wombat.
+
+ Family Phalangeridae. Tarsipes.
+ Phalanger--cuscus.
+ Phascolarctus--koala.
+ _Thylacoleo._
+
+ Family =Diprotodontidae=. _Diprotodon._
+
+ Family =Nototheriidae=. _Nototherium._
+
+ Family =Macropodidae=. Macropus--kangaroo.
+
+ Family =Epanorthidae=. Coenolestes.
+
+ Subclass (III). MONODELPHIA or EUTHERIA.
+
+ Order 1. EDENTATA.
+
+ Family =Bradypodidae=. Bradypus }
+ }--sloths.
+ Choloepus }
+
+ Family =Megatheriidae=. _Megatherium_--ground sloth.
+
+ Family =Myrmecophagidae=. Myrmecophaga--great ant-eater.
+ Cycloturus--two-toed ant-eater.
+
+ Family =Dasypodidae=. Chlamydophorus }
+ Dasypus }--armadillos.
+ Priodon }
+ Tatusia }
+
+ Family =Glyptodontidae=. _Glyptodon._
+
+ Family =Manidae=. Manis--pangolin.
+
+ Family =Orycteropodidae=. Orycteropus--aard-vark.
+
+ Order 2. SIRENIA.
+
+ Family =Manatidae=. Manatus--manatee.
+
+ Family =Rhytinidae=. _Rhytina_--Steller's sea-cow.
+
+ Family =Halicoridae=. Halicore--dugong.
+
+ Family =Halitheriidae=. _Halitherium._
+
+ Order 3. CETACEA.
+
+ Suborder (1). ARCHAEOCETI.
+
+ Family =Zeuglodontidae=. _Zeuglodon._
+
+ Suborder (2). MYSTACOCETI or BALAENOIDEA.
+
+ Family =Balaenidae=. Balaena--right whale.
+ Megaptera--humpbacked whale.
+ Balaenoptera--rorqual.
+
+ Suborder (3). ODONTOCETI.
+
+ Family =Physeteridae=. Physeter--sperm whale.
+ Hyperoödon--bottlenose.
+ Ziphius.
+ Mesoplodon.
+
+ Family =Physodontidae=. _Physodon._
+
+ Family =Squalodontidae=. _Squalodon._
+
+ Family =Platanistidae=. Platanista--Gangetic dolphin.
+ Inia.
+ Pontoporia.
+
+ Family =Delphinidae=. Monodon--narwhal.
+ Phocaena--porpoise.
+ Orca--killer.
+ Globicephalus--Ca'ing whale.
+ Grampus.
+ Lagenorhynchus.
+ Delphinus--dolphin.
+ Tursiops.
+ Prodelphinus.
+
+ Order 4. UNGULATA.
+
+ Division A. UNGULATA VERA.
+
+ Suborder (1). ARTIODACTYLA.
+
+ Section (_a_). SUINA.
+
+ Family =Hippopotamidae=. Hippopotamus.
+
+ Family =Suidae=. Sus--pig.
+ Babirussa.
+ Phacochaerus--wart hog.
+ _Hyotherium._
+
+ Family =Cotylopidae=. _Cotylops (Oreodon)._
+ _Cyclopidius._
+
+ Family =Agriochoeridae=. _Agriochoerus._
+
+ Family =Anoplotheriidae=. _Anoplotherium._
+
+ Section (_b_). TYLOPODA.
+
+ Family =Camelidae=. Camelus--camel.
+ Auchenia--llama.
+
+ Section (_c_). TRAGULINA.
+
+ Family =Tragulidae=. Dorcatherium (Hyomoschus)--chevrotain.
+
+ Section (_d_). RUMINANTIA or PECORA.
+
+ Family =Cervidae=. Moschus--musk deer.
+ Cervus--deer.
+ Cervulus--muntjac.
+ Hydropotes--Chinese water deer.
+
+ Family =Giraffidae=. Giraffa--giraffe.
+ _Sivatherium._
+
+ Family =Antilocapridae=. Antilocapra--prongbuck.
+
+ Family =Bovidae=. Tetraceros--four-horned antelope.
+ Gazella--gazelle.
+ Bos--ox.
+ Bison.
+ Bubalus--buffalo.
+
+ Suborder (2). PERISSODACTYLA.
+
+ Family =Tapiridae=. Tapirus--tapir.
+
+ Family =Lophiodontidae=. _Lophiodon._
+ _Hyracotherium._
+
+ Family =Palaeotheriidae=. _Palaeotherium._
+
+ Family =Equidae=. _Hipparion._
+ Equus--horse.
+
+ Family =Rhinocerotidae=. Rhinoceros.
+ _Elasmotherium._
+
+ Family =Titanotheriidae=. _Titanotherium (Brontops)._
+ _Palaeosyops._
+
+ Family =Chalicotheriidae=. _Chalicotherium._
+
+ Family =Macraucheniidae=. _Macrauchenia._
+
+ Division B. SUBUNGULATA.
+
+ Suborder (1). TOXODONTIA.
+
+ Family =Astrapotheriidae=. _Astrapotherium._
+
+ Family =Nesodontidae=. _Nesodon._
+
+ Family =Toxodontidae=. _Toxodon._
+
+ Family =Typotheriidae=. _Typotherium._
+
+
+ Suborder (2). CONDYLARTHRA.
+
+ Family =Phenacodontidae=. _Phenacodus._
+
+ Suborder (3). HYRACOIDEA.
+
+ Family =Hyracidae=. Procavia (Hyrax).
+
+ Suborder (4). AMBLYPODA.
+
+ Family =Coryphodontidae=. _Coryphodon._
+
+ Family =Uintatheriidae=. _Uintatherium_ (_Dinoceras_).
+
+ Suborder (5). PROBOSCIDEA.
+
+ Family =Dinotheriidae=. _Dinotherium._
+
+ Family =Elephantidae=. _Mastodon._
+ Elephas--elephant.
+
+ Group Tillodontia.
+
+ Order 5. RODENTIA.
+
+ Suborder (1). SIMPLICIDENTATA.
+
+ Section SCIUROMORPHA.
+
+ Family =Castoridae=. Castor--beaver.
+
+ Section MYOMORPHA.
+
+ Family =Lophiomyidae=. Lophiomys.
+
+ Family =Muridae=. Hydromys.
+ Acanthomys--spiny mouse.
+ Mus--mouse.
+
+ Family =Spalacidae=. Bathyergus.
+
+ Family =Dipodidae=. Dipus--jerboa.
+ Pedetes--Cape jumping-hare.
+
+ Section HYSTRICOMORPHA.
+
+ Family =Hystricidae=. Hystrix--porcupine.
+
+ Family =Chinchillidae=. Chinchilla.
+ Lagostomus--viscacha.
+
+ Family =Dasyproctidae=. Coelogenys--paca.
+ Dasyprocta--agouti.
+
+ Family =Caviidae=. Cavia--guinea-pig.
+ Hydrochaerus--capybara.
+
+ Suborder (2). DUPLICIDENTATA.
+
+ Family =Leporidae=. Lepus--hare and rabbit.
+
+ Order 6. CARNIVORA.
+
+ Suborder (1). CREODONTA.
+
+ Family =Hyaenodontidae=. _Hyaenodon._
+
+ Suborder (2). CARNIVORA VERA or FISSIPEDIA.
+
+ Section ÆLUROIDEA.
+
+ Family =Felidae=. Felis--cat, lion, tiger.
+ _Machaerodus_--sabre-toothed lion.
+
+ Family =Viverridae=. Viverra--civet.
+ Paradoxurus--palm civet.
+
+ Family =Protelidae=. Proteles--aard wolf.
+
+ Family =Hyaenidae=. Hyaena.
+
+ Section CYNOIDEA.
+
+ Family =Canidae=. Canis--dog, wolf, fox.
+
+ Section ARCTOIDEA.
+
+ Family =Ursidae=. Ursus--bear.
+
+ Family =Mustelidae=. Latax--sea otter.
+
+ Suborder (3). PINNIPEDIA.
+
+ Family =Otariidae=. Otaria--sea lion.
+
+ Family =Trichechidae=. Trichechus--walrus.
+
+ Family =Phocidae=. Ogmorhinus--sea leopard.
+
+ Order 7. INSECTIVORA.
+
+ Suborder (1). DERMOPTERA.
+
+ Family =Galeopithecidae=. Galeopithecus--'flying lemur'.
+
+ Suborder (2). INSECTIVORA VERA.
+
+ Family =Macroscelidae=. Macroscelides--jumping shrew.
+
+ Family =Erinaceidae=. Erinaceus--hedgehog.
+ Gymnura.
+
+ Family =Soricidae=. Sorex--shrew.
+
+ Family =Talpidae=. Talpa--mole.
+
+ Family =Potamogalidae=. Potamogale.
+
+ Family =Solenodontidae=. Solenodon.
+
+ Family =Centetidae=. Microgale.
+ Centetes--tenrec.
+
+ Family =Chrysochloridae=. Chrysochloris--golden mole.
+
+ Order 8. CHIROPTERA.
+
+ Suborder (1). MEGACHIROPTERA.
+
+ Family =Pteropidae=. Pteropus--flying fox.
+
+ Suborder (2). MICROCHIROPTERA.
+
+ Family =Rhinolophidae=. Horseshoe bats.
+
+ Family =Phyllostomatidae=. Desmodus--vampire.
+
+ Order 9. PRIMATES.
+
+ Suborder (1). LEMUROIDEA.
+
+ Family =Tarsiidae=. Tarsius--tarsier.
+
+ Family =Chiromyidae=. Chiromys--aye aye.
+
+ Suborder (2). ANTHROPOIDEA.
+
+ Family =Hapalidae=. Hapale--marmoset.
+
+ Family =Cebidae=. Mycetes--howling monkey.
+ Ateles--spider monkey.
+
+ Family =Cercopithecidae=. Cynocephalus--baboon.
+ Macacus.
+ Colobus.
+
+ Family =Simiidae=. Hylobates--gibbon.
+ Simia--orang.
+ Gorilla.
+ Anthropopithecus--chimpanzee.
+
+ Family =Hominidae=. Homo--man.
+
+
+FOOTNOTES:
+
+[17] This classification of birds is essentially that of Gadow and
+Selenka in Bronn's _Classen und Ordnungen des Thierreichs_, Band VI.,
+Abth. IV., Vögel. Leipzig, 1891.
+
+[18] The classification adopted is almost entirely that given in
+Flower and Lydekker's _Mammals Living and Extinct_. London, 1891.
+
+
+
+
+CHAPTER III.
+
+SKELETON OF HEMICHORDATA, UROCHORDATA, AND CEPHALOCHORDATA.
+
+
+SUBPHYLUM A. HEMICHORDATA.
+
+THE subphylum includes three genera, _Balanoglossus_[19],
+_Cephalodiscus_ and _Rhabdopleura_; and perhaps a fourth, _Phoronis_.
+
+The skeletal structures found in _Balanoglossus_[20] are all
+endoskeletal. They include:
+
+(1) The =notochord=. This arises as a diverticulum from the alimentary
+canal which grows forwards into the proboscis and extends beyond the
+front end of the central nervous system. It is hypoblastic in origin
+and arises in the same way as does the notochord of _Amphioxus_. Its
+cells become highly vacuolated and take on the typical notochordal
+structure[21]. The cavity of the primitive diverticulum becomes
+obliterated in front, but behind it opens throughout life into the
+alimentary canal.
+
+(2) The =axial skeletal rods=. These are a pair of chitinous rods
+which lie ventral to the notochord and in the collar region unite to
+form a single mass.
+
+(3) The =branchial skeleton=. The gill bars separating the gill slits
+from one another are strengthened by chitinous rods in a way closely
+similar to that in _Amphioxus_. But between one primary forked rod and
+the next there are two secondary unforked rods--not one, as in
+_Amphioxus_.
+
+(4) The =chondroid tissue=. This is of mesoblastic origin and may be
+regarded as an imperfect sheath for the notochord.
+
+In _Cephalodiscus_ and _Rhabdopleura_ as in _Balanoglossus_ the
+notochord forms a small diverticulum growing forwards from the
+alimentary canal into the proboscis stalk.
+
+Recent researches on _Phoronis_[22] show the existence in the collar
+region of the larva (_Actinotrocha_) of a paired organ, which is
+regarded by its discoverer as representing a double notochord.
+
+
+SUBPHYLUM B. UROCHORDATA (TUNICATA).
+
+Skeletal structures of epiblastic and hypoblastic origin occur in the
+Urochordata. Most Tunicates are invested by a thick gelatinous test
+which often contains calcareous spicules, and serves as a supporting
+organ for the soft body. The cells of this test are mesodermal in
+origin.
+
+In larval Tunicata and in adults of the group Larvacea the tail is
+supported by a typical notochord, which is confined to the tail. In
+all Tunicata except Larvacea all trace of the notochord is lost in the
+adult.
+
+
+SUBPHYLUM C. CEPHALOCHORDATA.
+
+[Illustration FIG. 3. DIAGRAM OF THE SKELETON OF _Amphioxus
+lanceolatus_ × 3 (after a drawing in the Index collection at the Brit.
+Mus.).
+
+ 1. skeleton of dorsal fin.
+ 2. notochord.
+ 3. neural tube.
+ 4. buccal skeleton.
+ 5. branchial skeleton.
+ 6. septa separating the myotomes.
+ 7. skeleton of ventral fin.]
+
+This subphylum includes the well-known genus _Amphioxus_[23]. In
+_Amphioxus_ the skeleton is very simple. It contains no trace of
+cartilage or bone and remains throughout life in a condition
+corresponding to a very early stage in Vertebrata. The skeleton of
+_Amphioxus_ is partly hypoblastic, partly mesoblastic in origin.
+
+(_a_) =Hypoblastic skeleton.=
+
+The =notochord= (fig. 3, 2) is an elastic rod extending along the
+whole length of the body past the anterior end of the nerve cord. It
+lies ventral to the nerve cord, and shows no trace of segmentation. It
+is chiefly made up of greatly vacuolated cells containing lymph, but
+near the dorsal and ventral surfaces the cells are less vacuolated.
+The notochord is immediately surrounded by a structureless cuticular
+layer, the _chordal sheath_, and outside this comes the mesoblastic
+_skeletogenous layer_, which also surrounds the nerve cord.
+
+The =branchial skeleton=. This consists of a series of chitinous
+elastic rods which strengthen the gill bars and are alternately forked
+and unforked ventrally. The forked rods are primary, and are U-shaped
+in section, the unforked rods are secondary, and are circular in
+section. All these rods are united at intervals by transverse rods.
+
+(_b_) =Mesoblastic skeleton.=
+
+The =buccal skeleton=. On each side of the mouth there is a curved bar
+resembling the notochord in structure. The bars are segmented, and
+each segment bears a smaller rod which supports a tentacle, the whole
+forming the buccal skeleton (fig. 3, 4).
+
+The notochord is enclosed in a thick =sheath= of connective tissue
+continuous with a thinner sheath round the nerve cord. The sheaths of
+the notochord and nerve cord together form the skeletogenous layer,
+and prolongations of it form the myomeres or septa between the
+myotomes or segments of the great lateral muscles of the body.
+
+The =skeleton of each median fin= consists of small cubical masses of
+a gelatinous substance arranged in rows (fig. 3, 1 and 7), and serving
+to strengthen the fins.
+
+
+FOOTNOTES:
+
+[19] The name _Balanoglossus_ is used here in its widest sense to
+include all the Enteropneusta.
+
+[20] See W. Bateson, _Quart. J. Micr. Sci._ n. s. vol. XXIV. 1884, p.
+208 and later; also E.W. Macbride, _Ibid._ vol. XXXVI. 1894, p. 385.
+
+[21] See p. 52.
+
+[22] A.T. Masterman, _P.R. Soc. Edinb._ 1895-96, p. 59; and _Anat.
+Anz._ 1896, p. 266.
+
+[23] See E. Ray Lankester, _Quart. J. Micr. Sci._ vol. XXIX. n. s.
+1889, p. 365. W.B. Benham, _Ibid._ vol. XXXV. n. s. 1893, p. 97. J.W.
+Kirkaldy, _Ibid_. vol. XXXVII. n. s. 1895, p. 303. The last-named
+writer divides the genus into three subgenera.
+
+
+
+
+CHAPTER IV.
+
+SUBPHYLUM D. VERTEBRATA.
+
+
+THE animals included in this great group all possess an internal axial
+skeleton forming the vertebral column or back-bone; and a dorsal
+spinal cord. The vertebral column is developed from the skeletogenous
+layer, which surrounds the spinal cord together with the notochord and
+its sheath; and in the great majority of cases the notochord becomes
+more or less modified and reduced in the adult. In some cases the
+notochord remains unmodified and the skeletogenous layer surrounding
+it is not segmented to form vertebrae, but in every case the neural
+arches which protect the spinal cord are segmented. The notochord
+never extends further forwards than the mid-brain.
+
+All true vertebrates possess a cranium or skeletal box enclosing the
+brain.
+
+
+(I.) CYCLOSTOMATA.
+
+The mouth in living forms is suctorial and is not supported by jaws.
+In some fossil forms the character of the mouth is unknown.
+
+
+_Order I._ MARSIPOBRANCHII[24].
+
+In these animals limbs and limb girdles are always completely absent.
+They have no exoskeleton except horny teeth.
+
+The endoskeleton, excluding the notochord, is entirely cartilaginous
+or membranous. The axial skeleton consists of a cartilaginous cranium
+without jaws, succeeded by a thick persistent notochord enveloped in
+a sheath. The notochord in living forms is unsegmented, but segmented
+cartilaginous neural arches are present in some cases. A complicated
+series of cartilaginous elements occurs in relation to the mouth,
+gills, and sense organs. The median fins are supported by
+cartilaginous pieces, the radiale. The order includes the Lampreys and
+Hags.
+
+
+_Order II._ OSTRACODERMI[25].
+
+The forms included in this group have long been extinct, being known
+only from beds of Upper Silurian and Lower Devonian age. They differ
+much from all other known animals. The exoskeleton is always greatly
+developed and includes (1) large bony plates covering the anterior
+region; (2) scales covering the posterior region. The plates are
+deeply marked by canals belonging to dermal sense organs. Jaws are
+unknown, and arches for the support of the appendicular skeleton are
+rudimentary or absent. The tail is heterocercal (see p. 60).
+
+
+_Suborder_ (1). HETEROSTRACI.
+
+The exoskeleton consists principally of calcifications forming dorsal
+and ventral shields which cover the head and abdominal region; the
+dorsal shield is formed of a few plates firmly united, the ventral
+shield of a single plate. The shields are composed of three layers,
+the middle layer being traversed by canals belonging to the dermal
+sense organs which open to the exterior by a series of pores. The tail
+is sometimes covered by scales. The orbits are widely separated and
+laterally placed. Paired appendages are absent. These curious forms
+are found in beds of Upper Silurian and Lower Devonian age. One of the
+best known genera is _Pteraspis_.
+
+
+_Suborder_ (2). OSTEOSTRACI.
+
+The exoskeleton as in the Heterostraci consists of shields and scales,
+the shields being divisible into three layers. The anterior part of
+the body is covered dorsally by a single large shield which differs
+from those of the Heterostraci in having the inner layer ossified. The
+middle layer contains canals for the passage of blood vessels, but the
+exoskeleton shows no impressions of dermal sense organs. The posterior
+part of the body is covered by large quadrangular scales. Paired
+appendages are absent, but median dorsal and caudal fins occur
+supported by scales, not fin-rays. _Cephalaspis_, the best known of
+these animals, occurs in beds of Lower Devonian age.
+
+
+_Suborder_ (3). ANTIARCHA.
+
+The exoskeleton is formed of bony plates, the dorsal and ventral
+shields each consisting of several symmetrically arranged pieces. The
+tail may be covered with small scales or may be naked. The head is
+articulated with the trunk, and its angles are drawn out into a pair
+of segmented paddle-like appendages, covered with dermal plates. The
+orbits are close together. A dorsal fin and traces of mouth parts
+occur in _Pterichthys_, but the endoskeleton is unknown. The best
+known forms _Pterichthys_[26] and _Asterolepis_ occur in beds of Lower
+Devonian age.
+
+
+GENERAL ACCOUNT OF THE SKELETON OF MARSIPOBRANCHII.
+
+The Marsipobranchii are worm-like animals. The living forms include
+two families, the Myxinoidei (Hags)--genera _Myxine_ and
+_Bdellostoma_--and the Petromyzontidae (Lampreys).
+
+Three species of _Petromyzon_ are known, _P. fluviatilis_, _P.
+marinus_ and _P. planeri_. The larval forms were for a long time
+thought to belong to a separate genus and were called _Ammocoetes_.
+
+The Myxinoids, although very highly specialised in their own way, are
+at distinctly a lower stage of development than the adult Lamprey, and
+come nearer to the larval Lamprey or Ammocoete.
+
+SPINAL COLUMN.
+
+[Illustration FIG. 4. A, DORSAL; B, LATERAL AND C, VENTRAL VIEW OF THE
+SKULL OF _Petromyzon marinus_ × 1 (after PARKER).
+
+ 1. horny teeth.
+ 2. labial cartilage.
+ 3. anterior dorsal cartilage.
+ 4. posterior dorsal cartilage.
+ 5. nasal capsule.
+ 6. auditory capsule.
+ 7. dorsal portion of trabeculae.
+ 8. lateral distal mandibular.
+ 9. lingual cartilage.
+ 10. branchial basket.
+ 11. cartilaginous cup supporting pericardium.
+ 12. sheath of notochord.
+ 13. neural plate.]
+
+In Myxinoids and larval lampreys, the notochord is enclosed in a thick
+chordal sheath, in connection with which in the tail region there
+occur cartilaginous pieces forming neural arch elements. In the trunk
+region, however, no cartilage occurs in connection with the spinal
+column, the only cartilage present being that forming the radiale of
+the dorsal fin. On the other hand in most species of lamprey
+(_Petromyzon_) cartilaginous pieces forming imperfect neural arches
+(fig. 4, B, 13) are found lying in the tough skeletogenous layer
+dorsal to the notochord, and extending throughout the whole length of
+the trunk and tail. Two of these pieces, which are probably homologous
+with the neural plates (see p. 72) of Elasmobranchs, occur to each
+_neuromere_, or segment as determined by the spinal nerves. The
+dorsal and caudal fins are supported by paired cartilaginous radiale
+which are connected proximally with the skeletogenous layer.
+
+THE SKULL.
+
+In Myxinoids the cranium is a mere cartilaginous floor without side
+walls or roof, and the trabeculae[27] end without growing forwards
+into cornua. In Lampreys the trabeculae grow forwards and send up
+plates of cartilage which meet above (fig. 4, 7) and form side walls
+and a roof for part of the brain case. In Lampreys a labial suctorial
+apparatus is well developed, including a large ring-like piece of
+cartilage (fig. 4, 2) which supports the oral funnel and bears a large
+armament of horny teeth. In Myxinoids on the other hand the labial
+skeleton is small and consists merely of barbels round the mouth.
+
+The olfactory organ of Myxinoids has a very curious skeleton. It is
+covered with a kind of grating of cartilage which is prolonged in
+front into a tube composed of a series of imperfect cartilaginous
+rings. In Lampreys the olfactory organ opens merely by a short
+membranous passage. In correlation with the small development of the
+labial suctorial apparatus in Myxinoids the lingual apparatus is very
+greatly developed. The tongue in _Myxine_ has been said to 'dominate
+the whole body' (Parker). It is supported by a great median
+cartilaginous bar which when followed forwards first becomes bifid and
+still further forwards becomes four-cleft.
+
+The horny teeth in Myxinoids are chiefly borne on the very large
+supralingual apparatus. They form a double series arranged in the form
+of an arch. In _Myxine_ there are seven large teeth and nine small
+ones on each side. In _Bdellostoma_ the teeth of the two rows are more
+equal in size. In _Bdellostoma_ and _Myxine_ it has been shown that
+imperfect calcified teeth occur below the horny teeth.
+
+In Lampreys the lingual apparatus (fig. 4, C, 9) is well developed,
+but not excessively so. It consists of a long median cartilaginous bar
+which ends in front with a semicircular piece of cartilage supporting
+the median part of the tongue.
+
+In both Myxinoids and Lampreys there is a complicated branchial basket
+apparatus, but while in Myxinoids the basket apparatus is
+interbranchial, formed deep within the head near the hypoblastic
+lining of the throat, in Lampreys it is extra-branchial and formed
+outside the head cavities (fig. 4, 10). The two sides of the basket
+apparatus in _Myxine_ are not symmetrical. In the interbranchial
+basket apparatus of Myxinoids the hyoid and first and second branchial
+arches can be recognised. Traces of the interbranchial skeleton of
+Myxinoids can be detected in Lampreys, and similarly in Myxinoids,
+there are indications of the extra-branchial skeleton of _Petromyzon_.
+The branchial basket in Lampreys forms at its posterior end a kind of
+cup which supports the pericardium (fig. 4, 11).
+
+A remarkable Cyclostome named _Palaeospondylus_[28] has recently been
+described from the Scottish Old Red Sandstone. It differs however from
+all living Cyclostomes, in having a spinal column formed of distinct
+vertebrae with well-developed neural arches. The caudal fin is well
+developed and the dorsal radiale are forked as in lampreys. The skull
+is well calcified and the auditory capsules are specially large. The
+mouth is very similar to that of lampreys, being circular and without
+jaws; it is provided with barbels or cirri. There is no trace of limbs
+and the average length is only about 1-1-1/2 inches.
+
+
+FOOTNOTES:
+
+[24] See W.K. Parker On the skeleton of the Marsipobranch fishes,
+_Phil. Trans._ 1883, London.
+
+[25] See A. Smith Woodward, _Catalogue of Fossil Fish in the British
+Museum_, Part II., 1891. A. Smith Woodward, _Nat. Sci._ vol. I. 1892,
+p. 596.
+
+[26] See R.H. Traquair, _Ann. Nat. Hist._, ser. 6, vol. II. 1888, p.
+485.
+
+[27] See p. 17.
+
+[28] R.H. Traquair, _Ann. Nat. Hist._ vol. VI. 1890, p. 485; _P. Phys.
+Soc. Edinb._ vol. XII. 1892-93, pp. 87-94, and 312-320. A. Smith
+Woodward, _Nat. Sci._ vol. III. p. 128, 1893.
+
+
+
+
+CHAPTER V.
+
+(II.) GNATHOSTOMATA.
+
+
+THE mouth is supported by definite jaws.
+
+
+ICHTHYOPSIDA.
+
+The epiblastic exoskeleton is generally unimportant, the mesoblastic
+exoskeleton is usually well developed.
+
+The notochord with its membranous sheath (1) may remain unmodified, or
+(2) may be replaced by bone or cartilage derived from the
+skeletogenous layer, or (3) may be calcified to a varying extent.
+
+The first vertebra is not homologous throughout the whole series and
+so is not strictly comparable to the atlas of Sauropsids and Mammals.
+
+The centra of the vertebrae have no epiphyses. The skull may be (_a_)
+incomplete and membranous, or (_b_) more or less cartilaginous, or
+(_c_) bony. Membrane bones are not included in the cranial walls, and
+there are large unossified tracts in the skull. When membrane bones
+are developed in connection with the skull, a large parasphenoid
+occurs. The basisphenoid is always small or absent. The skull may be
+immovably fixed to the vertebral column, or may articulate with it by
+a single or double occipital condyle. When the occipital condyle is
+double, it is formed by the exoccipitals, and the basi-occipital is
+small or unossified. The mandible may be (_a_) cartilaginous, (_b_)
+partially ossified, or (_c_) membrane bones may be developed in
+connection with it,--if so, there is usually more than one membrane
+bone developed in connection with each half.
+
+There are at least four pairs of branchial arches present during
+development. The sternum, if present, is not costal in origin.
+
+
+CLASS I. PISCES.
+
+The exoskeleton is in the form of scales, which may be entirely
+mesoblastic or dermal in origin (e.g. _cycloid_ and _ctenoid_ scales),
+or may be formed of both mesoblast and epiblast (e.g. _placoid_ and
+_ganoid_ scales). Large bony plates may be derived from both these
+types of scale. In general fish with a greatly developed dermal armour
+have the endoskeleton poorly developed; and the converse also holds
+good.
+
+The integument of the dorsal and ventral surfaces is commonly
+prolonged into longitudinal unpaired fins, supported by an internal
+skeleton. These fins are distinguished according to their position as
+dorsal, caudal and anal fins. The dorsal and anal fins are used
+chiefly as directing organs, the caudal fin is however a most
+important organ of propulsion.
+
+Three types of tail are found in fishes, viz.:--
+
+1. The =diphycercal=, in which the axis is straight and the tail is
+one-bladed and symmetrical, an equal proportion of radiale[29] being
+attached to the upper and lower surfaces of the axis.
+
+2. The =heterocercal=, in which the tail is asymmetrical and the axis
+is bent upwards, the proportion of radiale or of fin-rays attached to
+its upper surface being much smaller than that attached to its lower
+surface.
+
+3. The =homocercal=, in which the tail though externally symmetrical,
+so far resembling the diphycercal type, is internally really
+heterocercal, the great majority of the radiale or of the fin-rays
+being attached to the lower surface of the axis.
+
+The cranium in the simplest cases (e.g. Selachii) forms a
+cartilaginous box enclosing the brain and sense organs; in bony fishes
+it is greatly complicated. When palatine or pterygoid bones are
+present they are formed by the ossification of cartilage; in
+Sauropsida and Mammalia they are laid down as membrane bones. There is
+no tympanic cavity or auditory ossicle in relation to the ear.
+
+There are two principal types of suspensorium by means of which the
+jaws are attached to the cranium:--
+
+(1) The =Autostylic=. This is the primitive condition in which the
+mandibular arch articulates with the base of the cranium in front of
+the hyoid and in a similar manner.
+
+(2) The =Hyostylic=. In this case the mandibular arch becomes
+connected with the hyomandibular and supported by the hyoid arch.
+These terms are more fully discussed in Chapter VIII.
+
+There is always an internal framework supporting the gills; it usually
+consists of the hyoid arch and five, rarely six or seven, pairs of
+branchial arches. The limbs are represented by two pairs of fins, the
+pectoral and the pelvic; they are not divided into proximal, middle
+and distal portions. The ribs do not unite with a median ventral
+sternum, or meet in the mid-ventral line in any other way in the trunk
+region.
+
+
+_Order I._ ELASMOBRANCHII.
+
+The exoskeleton is in the form of placoid scales which are sometimes
+so numerous as to give the whole skin a rough surface forming
+shagreen. In some cases the placoid scales are enlarged to form plates
+or spines capped or coated with enamel. These spines may be imbedded
+in the flesh in front of the paired or unpaired fins, or may be
+attached to the tail. They are specially characteristic of the
+suborder Acanthodii. The endoskeleton is cartilaginous and true bone
+is never found. Much of the skeleton, especially of the vertebral
+column, is however often calcified, this being especially well seen in
+the anterior part of the vertebral column of Rays (Raiidae). In living
+forms cartilaginous biconcave vertebrae are always well developed, but
+in some extinct forms the notochord persists unconstricted. Neural and
+haemal arches are however always developed; they sometimes remain
+separate, sometimes fuse with the centra. Ribs are often wanting and
+when present are often not separated off from the vertebrae. The
+cranium is a simple cartilaginous box whose most prominent parts are
+the capsules which enclose the sense organs. The skull is sometimes
+immovably fixed to the vertebral column, sometimes articulates with it
+by means of two condyles. There is no operculum and no representative
+of the maxilla or premaxillae. The teeth are very variable. Large
+pectoral and pelvic fins always occur.
+
+The Elasmobranchii may be divided into four suborders:--
+
+ (1) Ichthyotomi.
+ (2) Pleuropterygii.
+ (3) Selachii.
+ (4) Acanthodii.
+
+
+_Suborder_ (1). ICHTHYOTOMI[30].
+
+The members of this suborder range from the Devonian to the Permian
+and so have long been extinct.
+
+The endoskeletal cartilage has granular calcifications evenly
+distributed throughout it. The notochord is unconstricted, but the
+neural and haemal arches are well-developed, and the neural spines are
+long and slender. There is a continuous dorsal fin with separate
+basalia and radiale. The tail is diphycercal, and the pectoral fins
+are typical archipterygia[31]. The pelvic fins of the male are
+prolonged to form claspers.
+
+The best known of these primitive Elasmobranchs are the
+Pleuracanthidae.
+
+
+_Suborder_ (2). PLEUROPTERYGII.
+
+This suborder was formed for the reception of _Cladoselache_, an
+Elasmobranch found in the Lower Carboniferous of Ohio[32].
+
+The exoskeleton is in the form of small, thickly-studded dermal
+denticles. The vertebral centra are unossified, and the tail is
+strongly heterocercal. There were certainly five, perhaps seven gill
+slits, and the suspensorium is apparently hyostylic. The paired fins
+are, according to the view which derives them by concentration from
+continuous lateral folds, the most primitive known (see p. 129) and
+claspers are absent.
+
+
+_Suborder_ (3). SELACHII.
+
+Cartilaginous or partially calcified biconcave vertebrae are always
+well developed; they constrict the notochord intervertebrally. The
+neural and haemal arches and spines are stout and intercalary
+cartilages (interdorsalia) are present. The tail is heterocercal, but
+in some cases (_Squatina_) approaches the diphycercal condition. In
+most cases the suspensorium is hyostylic, the jaws being attached to
+the cranium by means of the hyomandibular, and the palato-pterygo
+quadrate bar not being fused to the cranium. There are generally five
+pairs of branchial arches, and gill rays are borne on the posterior
+surface of the hyoid arch, and on both the anterior and posterior
+surfaces of the first four branchial arches. The Notidanidae differ
+from most Selachians in two respects, first as regards the
+suspensorium,--Meckel's cartilage articulating directly with the
+palato-pterygo-quadrate bar, and not being connected with the hyoid
+arch; and secondly as regards the number of branchial arches,--six
+pairs occurring in _Hexanchus_ and seven in _Heptanchus_.
+
+The pectoral fins are without the segmented axis of the
+archipterygium. In most cases they are sharply marked off from the
+body and lie almost at right angles to it; but in the Rays they have
+the form of lateral expansions in the same plane as the body, from
+which they are not sharply marked off. The pelvic fins in the male
+bear long grooved cartilaginous rods which are accessory copulatory
+organs or claspers.
+
+There are two principal groups of Selachii, the Squalidae or Sharks
+and Dogfish, and the Batoidei or Skates and Rays. The Squalidae have
+the shape of ordinary fish, the pectoral fins are vertically placed
+and the body ends in a powerful heterocercal tail. The Batoidei have
+flattened bodies owing to the great size and horizontal position of
+the pectoral fins. The tail is long and thin and is often armed with
+spines. The teeth in Selachii differ much in character in the
+different forms, and are always arranged in numerous rows. They are
+generally pointed and triangular or conical in the Squalidae, while in
+the Batoidei they are often broad and flattened.
+
+
+_Suborder_ (4). ACANTHODII.
+
+The fishes included in this group are all extinct and in some respects
+are intermediate between Elasmobranchii and Ganoidei. The body is
+elongated and closely covered with small scales consisting of dentine
+enamelled at the surface. The notochord is persistent and the
+calcification of the endoskeletal cartilage is only superficial. The
+tail is heterocercal. The jaws bear small conical teeth, or in some
+cases are toothless. The skeleton of all the fins differs from that of
+modern Elasmobranchs in having the cartilaginous radiale much reduced,
+and the fins are nearly always each provided with an anterior spine,
+which except in the case of the pectoral fins is merely inserted
+between the muscles. These spines are really enormous dermal fin-rays;
+the pectoral fin-spine is articulated to the pectoral girdle.
+
+The suborder includes many well-known extinct forms like _Acanthodes_
+and _Diplacanthus_; it ranges from the Devonian to the Permian.
+
+
+PISCES, HOLOCEPHALI.
+
+_Order II._ HOLOCEPHALI.
+
+This order includes a single suborder only.
+
+
+_Suborder._ CHIMAEROIDEI.
+
+[Illustration FIG. 5. SKULL OF A MALE _Chimaera monstrosa_ (after
+HUBRECHT).
+
+ 1. nasal capsule.
+ 2. cartilaginous appendage to the fronto-nasal region.
+ 3. erectile appendage.
+ 4. foramen by which the ophthalmic nerves leave the orbit.
+ 5. foramen by which the ophthalmic branch of the Vth nerve enters
+ the orbit.
+ 6. auditory capsule.
+ 7. interorbital septum.
+ 8. mandible articulating with an outgrowth from the posterior part
+ of the palato-pterygo-quadrate.
+ 9. teeth.
+ 10. labial cartilage.
+ II. III. V. VII. IX. X. foramina for the passage of cranial nerves.]
+
+These singular fish have the skin smooth and in living forms almost or
+quite scaleless. The palato-pterygo-quadrate bar and hyomandibular are
+fused to the cranium, and Meckel's cartilage articulates directly with
+the part corresponding to the quadrate. The skull is distinctly
+articulated with the spinal column, the notochord is persistent and
+unconstricted, and the skeletogenous layer shows no trace of metameric
+segmentation, though in the neural arches this segmentation is
+readily traceable. The neural arches of the first few vertebrae are
+fused together and completely surround the notochord, while they do
+not in other parts of the body. The tail is diphycercal. Of the living
+genera, in _Callorhynchus_ there is no trace of calcification in the
+skeletogenous layer, while in _Chimaera_ rings of calcification are
+found, there being three to five for each vertebra as indicated by the
+foramina for the exit of the spinal nerves. The pelvic fins are
+produced into claspers. Besides the living genera _Chimaera_,
+_Harriotta_ and _Callorhynchus_ a fair number of fossil forms are
+known, e.g. _Ischyodus_.
+
+
+_Order_ III. GANOIDEI.
+
+The fishes included under the term Ganoidei form a very heterogeneous
+group, some of which closely approach the Dipnoi, others the
+Elasmobranchii, others the Teleostei. The great majority of them are
+extinct, only eight living genera being known; these are all
+inhabitants of the northern hemisphere, and with the exception of
+_Acipenser_, which is both fluviatile and marine, are entirely
+confined to fresh water.
+
+The following is a list of the living genera of Ganoids with their
+respective habitats:--
+
+ _Acipenser._ Rivers and seas of the northern hemisphere.
+
+ _Scaphirhynchus._ Mississippi and rivers of Central Asia.
+
+ _Polyodon_ (_Spatularia_). Mississippi.
+
+ _Psephurus._ Yan-tse-kiang, and Hoangho.
+
+ _Polypterus._ Rivers of tropical Africa.
+
+ _Calamoichthys._ Some rivers of West Africa.
+
+ _Lepidosteus._ Freshwaters of Central and North America and
+ Cuba.
+
+ _Amia._ Rivers of Carolina.
+
+The exoskeleton is very variable, thus the body may be:--
+
+(_a_) Naked or with minute stellate ossifications as in the
+Polyodontidae. (_b_) Partially covered with large detached bony
+plates as in _Scaphirhynchus_ and _Acipenser_. (_c_) Entirely covered
+with rhomboidal ganoid scales as in _Lepidosteus_, _Polypterus_,
+_Palaeoniscus_ and many extinct forms. (_d_) Covered with rounded
+scales shaped like the cycloid scales of Teleosteans as in _Amia_.
+(_e_) Having the trunk and part of the tail covered with rhomboidal
+scales, and the remainder of the tail with rounded scales as in
+_Trissolepis_.
+
+The teeth also are very variable. The endoskeleton shows every stage
+of transition from an almost entirely cartilaginous state as in
+_Acipenser_ to a purely bony state as in _Lepidosteus_. Sometimes, as
+in _Acipenser_, the notochord persists, and its sheath is unsegmented;
+sometimes, as in _Lepidosteus_, there are fully formed vertebrae. The
+tail may be heterocercal, as in _Acipenser_, or diphycercal as in
+_Polypterus_. The cartilaginous cranium is always covered with
+external membrane bone to a greater or less extent, and the
+suspensorium is markedly hyostylic. The pectoral girdle is formed of
+two parts, one endoskeletal and cartilaginous, corresponding with the
+pectoral girdle of Elasmobranchs, and one exoskeletal and formed of
+membrane bones, corresponding with the clavicular bones of
+Teleosteans. The pelvic fins are always abdominal. The fins often, as
+in _Polypterus_, have spines (fulcra) attached to their anterior
+borders.
+
+The order Ganoidei may be divided into three suborders:
+
+ (1) CHONDROSTEI. Living genera _Acipenser_, _Scaphirhynchus_,
+ _Polyodon_ and _Psephurus_.
+
+ (2) CROSSOPTERYGII. Living genera _Polypterus_ and
+ _Calamoichthys_.
+
+ (3) HOLOSTEI. Living genera _Lepidosteus_ and _Amia_.
+
+
+_Suborder_ (1). CHONDROSTEI.
+
+The skull is immovably fixed to the vertebral column. By far the
+greater part of the skeleton is cartilaginous. The notochord is
+persistent and unconstricted, its sheath is membranous, but
+cartilaginous neural and haemal arches are developed. Intercalary
+pieces (interdorsalia) occur between the neural arches, and similar
+pieces (interventralia) between the haemal arches. The cranium is
+covered with membrane bone, and teeth are but slightly developed. The
+tail is heterocercal. Gill rays occur on the hyoid arch, and the gills
+are protected by a bony operculum attached to the hyomandibular. The
+skin (1) may be almost or quite naked, (2) may carry bony plates
+arranged in rows, or may be covered (3) with rhomboidal scales, or (4)
+partly with rhomboidal, partly with cycloidal scales.
+
+
+_Suborder_ (2). CROSSOPTERYGII.
+
+The exoskeleton has the form of cycloidal or rhomboidal scales. The
+condition of the vertebral column differs in the different genera.
+Sometimes, as in _Polypterus_, there are well-developed ossified
+vertebrae; sometimes, as in many extinct forms, the notochord persists
+and is unconstricted. The tail may be diphycercal or heterocercal. The
+pectoral and sometimes the pelvic fins consist of an endoskeletal axis
+bearing a fringe of dermal rays.
+
+
+_Suborder_ (3). HOLOSTEI.
+
+The exoskeleton has the form of cycloidal or rhomboidal scales. The
+notochord is constricted and its sheath is segmented and ossified,
+forming distinct vertebrae, which are generally biconcave, sometimes
+opisthocoelous (_Lepidosteus_). The cartilaginous cranium is largely
+replaced by bone, and in connection with it we find not only membrane
+bone, but cartilage bone, as the basi-occipital, exoccipitals, and
+pro-otic are ossified. The tail is heterocercal. The suspensorium
+resembles that of Teleosteans, consisting of a proximal ossification,
+the hyomandibular, which is movably articulated to the skull and a
+distal ossification, the symplectic. The two are separated by some
+unossified cartilage. The cartilaginous upper and lower jaws are to a
+great extent surrounded and replaced by a series of membrane bones.
+
+
+_Order_ IV. TELEOSTEI.
+
+The exoskeleton is sometimes absent but generally consists of
+overlapping cycloid or ctenoid scales. Bony plates are sometimes
+present, as in the Siluridae, or the body may be encased in a complete
+armour of calcified plates, as in _Ostracion_. Enamel is however never
+present, and the plates are entirely mesodermal. The skeleton is bony,
+but in the skull much cartilage generally remains. The vertebral
+centra are usually deeply biconcave, and the tail is of the masked
+heterocercal type distinguished as _homocercal_. In the skull the
+occipital region is always completely ossified, while the sphenoidal
+region is generally less ossified. The skull has usually a very large
+number of membrane bones developed in connection with it. The teeth
+vary much in character in the different members of the order, but are
+as a rule numerous and pointed, and are ankylosed to the bone. The
+suspensorium is hyostylic and the jaws have much the same arrangement
+as in the Holostei. There are five pairs of branchial arches, of which
+all except the last bear gill rays. A series of dermal opercular bones
+is developed in connection with these arches. The pectoral girdle
+consists almost entirely of dermal clavicular bones. The pelvic girdle
+has disappeared, its place being taken by the enlarged and ossified
+dermal fin-rays of the pelvic fins.
+
+The group includes the vast majority of living fish (see p. 33).
+
+
+_Order_ V. DIPNOI.
+
+The exoskeleton is of two types; dermal bones are largely developed in
+the head region, while the tail and posterior part of the body may be
+naked or may be covered with overlapping scales. The cranium remains
+chiefly cartilaginous, the palato-pterygo-quadrate bar is fused with
+the cranium, and the suspensorium is autostylic. The gill clefts are
+feebly developed and open into a cavity covered by an operculum. The
+notochord is persistent and unconstricted, and the limbs are
+archipterygia. The pelvic fins are without claspers.
+
+
+_Suborder_ (1). SIRENOIDEI[33].
+
+The head has well developed membrane bones. The trunk is covered with
+overlapping scales and bears no bony plates. Three pairs of teeth are
+present, two in the upper and one in the lower jaw, the two principal
+pairs of teeth are borne on the palato-pterygoids and splenials, while
+the third pair are found in the vomerine region. The tail is
+diphycercal in living forms. In the extinct Dipteridae it is
+heterocercal. The pectoral girdle includes both membrane and cartilage
+bones. The pelvic girdle consists of a single bilaterally symmetrical
+piece of cartilage.
+
+This suborder is represented by the living genera _Ceratodus_,
+_Protopterus_ and _Lepidosiren_, and among extinct forms by the
+Dipteridae and others.
+
+
+_Suborder_ (2). ARTHRODIRA.
+
+Bony plates are developed not only on the head but also on the
+anterior part of the trunk, where they consist of a dorsal, a ventral,
+and a pair of lateral plates which articulate with the cranial shield.
+The posterior part of the trunk is naked. The tail is diphycercal. The
+jaws are shear-like, and their margins are usually provided with
+pointed teeth whose bases fuse with the tissue of the jaw and
+constitute dental plates. There seem to have been three pairs of these
+plates, arranged as in the Sirenoidei, the principal ones in the upper
+jaw being borne on the palato-pterygoids. Small pelvic fins are
+present, but pectoral fins are unknown.
+
+The Arthrodira occur chiefly in beds of Devonian and Carboniferous
+age. Two of the best known genera are _Coccosteus_ from the European
+Devonian and _Dinichthys_, a large predatory form from the lower
+Carboniferous of Ohio.
+
+
+FOOTNOTES:
+
+[29] See p. 79.
+
+[30] For this and other groups of extinct fish see A. Smith Woodward,
+_Catalogue of Fossil Fish in the British Museum_, Parts I.-III.
+London, 1889-95.
+
+[31] See p. 127.
+
+[32] See B. Dean, _J. Morphol._ vol. IX. pp. 87-114, 1894, and _Nat.
+Sci._ vol. VIII. p. 245, 1896.
+
+[33] A. Günther, _Phil. Trans._ vol. 161, Part II. 1871, p. 511. T.H.
+Huxley, "On Ceratodus and the classification of fishes," _P.Z.S._
+1876, p. 24.
+
+
+
+
+CHAPTER VI.
+
+THE SKELETON OF THE DOGFISH[34].
+
+_Scyllium canicula._
+
+
+I. EXOSKELETON.
+
+The exoskeleton of the dogfish is mainly composed of placoid scales,
+each of which consists of a little bony base imbedded in the skin,
+bearing a small backwardly-directed spine formed of dentine capped
+with enamel. The scales are larger on the dorsal than on the ventral
+surface, and on the jaws they are specially large and regularly
+arranged in rows, there forming the teeth. The margins of the jaws or
+lips are without scales.
+
+A second exoskeletal structure is found in the fins, all of which,
+both paired and unpaired, have, in addition to their cartilaginous
+endoskeleton, large numbers of long slender horny fibres, the
+fin-rays, which are of exoskeletal origin.
+
+
+II. ENDOSKELETON.
+
+The endoskeleton of the dogfish consists almost entirely of cartilage,
+which however may become calcified in places, e.g. the centrum of each
+vertebra is lined by a layer of calcified tissue.
+
+The endoskeleton is divisible into an =axial= portion consisting of
+the vertebral column, skull, and skeleton of the median fins, and an
+=appendicular= portion consisting of the skeleton of the paired fins
+and their girdles.
+
+
+1. THE AXIAL SKELETON.
+
+A. THE VERTEBRAL COLUMN AND RIBS.
+
+The vertebral column consists of a series of some hundred and thirty
+vertebrae, each of which is united with its predecessor and successor
+in such a way as to allow a large amount of flexibility.
+
+These vertebrae are developed round an unsegmented rod, the
+=notochord=, which forms the axial support of the embryo. The
+notochord remains continuous throughout the whole vertebral column,
+but is greatly constricted opposite the middle of each vertebra, and
+thus rendered moniliform. The vertebrae are divided into two groups,
+an anterior group of trunk vertebrae, and a posterior group of caudal
+or tail vertebrae.
+
+A typical vertebra consists of a middle portion, the =centrum=, a
+dorsal portion, the =dorsal= or =neural arch=, which surrounds the
+spinal cord, and a ventral portion, the =ventral= or =haemal arch=,
+which similarly encloses a space.
+
+The tail vertebrae of the dogfish have this typical arrangement, the
+trunk vertebrae have the haemal arches modified.
+
+Each =centrum= is a short cylinder of cartilage surrounding an
+hourglass-shaped cavity occupied by the notochord. The =neural arches=
+are composed of three separate elements, the =vertebral neural plates=
+(basidorsalia), =intervertebral neural plates= (interdorsalia), and
+=neural spines= (supradorsalia).
+
+The =vertebral neural plates= are in the adult fused with their
+respective centra, and are notched behind for the exit of the ventral
+(motor) roots of the spinal nerves. The =intervertebral neural plates=
+are polygonal pieces alternating with the vertebral neural plates;
+they are notched behind, but at a more dorsal level than are the
+vertebral neural plates, for the exit of the dorsal or sensory roots
+of the spinal nerves.
+
+The =neural spines= are small patches of cartilage filling up the gaps
+between the dorsal ends of the neural plates.
+
+The =haemal arches= (basiventralia) differ much in the trunk and tail
+portions of the vertebral column. In the trunk portion the centra are
+flattened below, and the two halves of the haemal arch diverge from
+one another as blunt =ventri-lateral processes= to
+which short cartilaginous rods, the =ribs=, are attached. Further back
+at about vertebra 37, the two halves of the haemal arch project
+downwards and meet forming a complete arch. Further back still,
+towards the hind end of the tail, the haemal arches bear median
+=haemal spines= (ventrispinalia).
+
+
+B. THE SKULL.
+
+The skull of the dogfish remains cartilaginous throughout the life of
+the animal, and has consequently a far more simple structure than have
+the skulls of higher animals, in which complication has been produced
+by the development of bone.
+
+The skull consists of the following parts:--
+
+(1) a dorsal portion, the =cranium=, which lodges the brain, and to
+the sides of which the capsules of the auditory and olfactory sense
+organs are united. The cranium may be compared to an unsegmented
+continuation of the vertebral column;
+
+(2) a number of ventral structures, disconnected or only loosely
+connected with the cranium. These together constitute the =visceral
+skeleton= forming the jaws and supporting the gills.
+
+(1) THE CRANIUM.
+
+The =Cranium= is an oblong box, with a flattened floor and a more
+irregular roof. Its sides are expanded in front owing to the
+olfactory capsules, and behind owing to the auditory capsules, while
+in the middle they are deeply hollowed to form the orbits.
+
+(_a_) On the dorsal surface of the cranium the following points should
+be noticed. First at the anterior end, the large thin-walled =nasal=
+or =olfactory capsules= (fig. 6, 1), each of which is drawn out into a
+narrow cartilaginous process.
+
+The olfactory capsules have no ventral walls, and are separated from
+one another by the =internasal septum=, which is drawn out into a
+third slender process. These three processes together constitute the
+=rostrum= (fig. 6, 2).
+
+Behind the olfactory capsules comes a large, nearly circular, hole,
+the =anterior fontanelle=, slightly behind which are the two
+=ophthalmic foramina=. The dorsal and ventral boundaries of the orbits
+are respectively formed by the prominent =supra-orbital= and
+=suborbital ridges=. Behind are the =auditory capsules= (fig. 6, 8),
+each of which is marked by a pair of prominent ridges, converging
+towards the middle line to a pair of apertures. These apertures
+communicate with two canals, the =aqueductus vestibuli=, which lead
+into the internal ear. The two ridges lodge respectively the =anterior
+and posterior vertical semicircular canals= of the ear.
+
+(_b_) The principal structures to be noted in a side view of the
+cranium are contained in the =orbit= or eye-cavity. Near the base of
+the orbit at its anterior end is seen the small =orbitonasal foramen=
+(fig. 6, 7), for the passage of blood-vessels, not nerves. Above it is
+the large =ophthalmic foramen= (fig. 6, 5) so prominent in a dorsal
+view of the skull; through it the ophthalmic branches of the fifth and
+seventh nerves pass. Slightly further back near the ventral surface is
+the large =optic foramen= (fig. 6, II.) for the passage of the second
+nerve. Vertically above the optic foramen, near the dorsal surface, is
+the very small =foramen for the fourth nerve= (fig. 6, IV.). Behind
+and a little above the optic foramen is another small aperture, the
+=foramen for the third nerve=. Behind and slightly below this is the
+large =foramen for the sixth and main branches of the fifth and
+seventh nerves= (fig. 6, V.). In front of and slightly below this
+foramen are seen two other small apertures; the more anterior and
+ventral of these (fig. 6, 4) is for the passage of a vessel connecting
+the efferent artery of the hyoid gill with the internal carotid artery
+inside the skull, the more posterior and dorsal is for the
+=interorbital canal= (fig. 6, 3) which unites the two orbital sinuses.
+Above and very slightly in front of the large foramen for the sixth
+and main parts of the fifth and seventh nerves, are two small foramina
+(fig. 6, Va., and VIIa.), through which the =ophthalmic branches of
+the fifth and seventh nerves= enter the orbit. Behind and slightly
+below the large foramen just mentioned is a small hole through which
+the external carotid enters the orbit (fig. 6, 9).
+
+[Illustration Fig. 6. LATERAL VIEW OF THE SKULL OF A DOGFISH
+(_Scyllium canicula_) × 2/3.]
+
+ 1. nasal capsule.
+ 2. rostrum.
+ 3. interorbital canal.
+ 4. foramen for hyoidean artery.
+ 5. foramen for the exit of the
+ ophthalmic branches of
+ Vth and VIIth nerves.
+ 6. foramen through which the
+ external carotid leaves the
+ orbit.
+ 7. orbitonasal foramen.
+ 8. auditory capsule.
+ 9. foramen through which the
+ external carotid enters the
+ orbit.
+ 10. ethmo-palatine ligament.
+ 11. palato-pterygo-quadrate bar.
+ 12. Meckel's cartilage.
+ 13. hyomandibular.
+ 14. cerato-hyal.
+ 15. pharyngo-branchial.
+ 16. epi-branchial.
+ 17. cerato-branchial.
+ 18. gill filaments, nearly all have
+ been cut off short for the
+ sake of clearness.
+ 19. extra-branchial.
+ 20. pre-spiracular ligament.
+ II. III. IV. V. Va. VIIa. foramina
+ for passage of cranial nerves.
+
+Behind the orbit is the =auditory capsule=. This is marked below by a
+prominent =surface for the articulation of the hyomandibular=, above
+which is the deep =postorbital groove= for the passage of a
+blood-vessel, connecting the orbital and anterior cardinal sinuses.
+
+(_c_) Passing to the posterior end of the cranium: in the centre is
+seen the large =foramen magnum= through which the brain and spinal
+cord communicate. The =notochord= enters the skull just below this
+foramen, and on each side of the notochord is a projection, the
+=occipital condyle=, by which the first vertebra articulates with the
+skull.
+
+External to the condyles are the prominent =pneumogastric foramina=
+for the passage of the tenth nerves, and further to the sides, just
+beyond the posterior vertical semicircular canals, are a pair of deep
+pits in which lie the =foramina for the ninth nerves= (fig. 6, IX).
+
+(_d_) The broad and flat ventral surface of the cranium is continued
+in front as the =internasal septum= and terminated laterally by the
+=suborbital ridges=. At a little behind the middle it is traversed by
+two shallow grooves along which the internal carotid arteries run. At
+the divergent ends of these grooves are seen two small apertures
+through which the external carotids enter the orbit (fig. 6, 9), and
+at the point where they meet is a single small aperture through which
+the internal carotid enters the cranium.
+
+(2) THE VISCERAL SKELETON.
+
+The =Visceral skeleton= forms a series of seven cartilaginous arches
+or hoops, surrounding the anterior part of the alimentary canal, and
+enclosing a wide but rather shallow space.
+
+(_a_) The first or =mandibular arch= is the largest of the series, and
+forms the upper and lower jaws. Each half of the upper jaw or
+=palato-pterygo-quadrate= bar is formed by a thick cartilaginous rod
+which meets its fellow in the middle line in front, the two being
+united by ligament. Each half is connected to the cranium just in
+front of the orbit by the =ethmo-palatine ligament= (fig. 6, 10), and
+at its hind end articulates with one of the halves of the lower jaw.
+Each half of the lower jaw or =Meckel's cartilage= (fig. 6, 12) is a
+cartilaginous bar, wide behind but narrow in front, where it is united
+to its fellow by a median ligament. Imbedded in the tissue external to
+the upper jaw are a pair of =labial cartilages=, and a similar but
+smaller pair are imbedded in the tissue external to the lower jaw.
+
+The jaws are developed from a structure whose dorsal and ventral
+portions subsequently become of very different importance. The ventral
+portion forms both upper and lower jaws, the former being developed as
+an outgrowth from the latter. The dorsal portion forms only the
+=pre-spiracular ligament= (fig. 6, 20), a strong fibrous band
+containing a nodule of cartilage, and running from the anterior part
+of the auditory capsule to the point where the jaws are connected with
+the hyomandibular.
+
+(_b_) The =hyoid arch= consists of a pair of cartilaginous rods which
+are attached at their dorsal ends to the cranium, and are united
+ventrally by a broad median plate of cartilage, the =basi-hyal=. Each
+rod is divided into a dorsal portion, the =hyomandibular= and a
+ventral portion, the =cerato-hyal=. The =hyomandibular= (fig. 6, 13)
+is a short stout rod of cartilage projecting outwards, and somewhat
+backwards and downwards from the cranium, with which it articulates
+behind the orbit and below the postorbital groove. Its distal end
+articulates with a rather long slender bar, the =cerato-hyal= (fig. 6,
+14), which is in its turn attached to the side of the =basi-hyal=. The
+=basi-hyal= is a broad plate, rounded in front and drawn out behind
+into two processes to which the two halves of the first branchial arch
+are attached. The posterior surfaces of both hyomandibular and
+cerato-hyal bear slender cartilaginous processes, the =gill rays=. The
+hyoid arch forms the main =suspensorium= or means by which the jaws
+are attached to the cranium. This attachment is chiefly brought about
+by a series of short ligaments which connect the posterior ends of
+both upper and lower jaws with the hyomandibular, but there is also a
+ligament connecting the lower jaw with the cerato-hyal. The attachment
+of the jaws to the cranium is also partially effected by the
+pre-spiracular and ethmo-palatine ligaments.
+
+(_c_) Each of the five =branchial arches= is a hoop, incomplete above
+and formed of four or more pieces of cartilage. The most dorsal
+elements, the =pharyngo-branchials=, are flattened, pointed plates
+whose free inner ends run obliquely backwards, and terminate below the
+vertebral column. They are connected at their outer ends with the
+short broad =epi-branchials= (fig. 6, 16) which lie at the sides of
+the pharynx. From the epi-branchials arise the long =cerato
+branchials= (fig. 6, 17) which run forwards and inwards along the
+ventral wall of the pharynx. The first four cerato-branchials are
+connected with small rods, the =hypo-branchials=, which run backwards
+to meet one another in the middle line. The last two pairs of
+hypo-branchials and the fifth cerato-branchials are connected with a
+broad median plate, the =basibranchial=. Along the outer sides of the
+second, third and fourth cerato-branchials are found elongated curved
+rods, the =extra-branchials= (fig. 6, 19). The epi-branchials and
+cerato-branchials bear gill rays along their posterior borders.
+
+
+C. THE SKELETON OF THE MEDIAN FINS.
+
+The =dorsal fins= have a skeleton consisting of a series of short
+cartilaginous rods, the =basals= or basalia, which slope obliquely
+backwards. Their bases are imbedded in the muscles of the back, while
+their free ends bear a number of small polygonal cartilaginous plates,
+the =radials= or radiale. Associated with this cartilaginous skeleton
+are a number of long slender horny fibres, the fin-rays, which have
+been already referred to in connection with the exoskeleton. The
+skeleton of the other median fins mainly consists of these fibres, the
+cartilaginous portion being reduced or absent.
+
+
+2. THE APPENDICULAR SKELETON.
+
+This includes the skeleton of the two pairs of limbs and of their
+respective girdles.
+
+THE PECTORAL GIRDLE forms a crescent-shaped hoop of cartilage,
+incomplete above and lying just behind the visceral skeleton. The
+mid-ventral part of the hoop is the thinnest portion, and is drawn out
+in front into a short rounded process which is cupped dorsally and
+supports part of the floor of the pericardium (fig. 7, 1). On each
+side of this flattened mid-ventral portion the arch becomes very thick
+and bears on its outer border a surface with which the three basal
+cartilages of the fin articulate. The dorsal ends or scapular portions
+of the girdle form a pair of gradually tapering horns.
+
+THE PECTORAL FIN articulates with the pectoral girdle by means of
+three basalia or basal cartilages, the =propterygium=, =meso
+pterygium= and =meta-pterygium=. The most anterior and the smallest of
+these is the =propterygium= (fig. 7, 5), while the most posterior
+one, the =meta-pterygium= (fig. 7, 3), is much the largest. Along the
+outer borders of the three basalia are arranged a series of close set
+cartilaginous pieces, the =radiale=. The propterygium supports only a
+single radial, which is however much larger than any of the others.
+The meso-pterygium also supports only a single radial which divides
+distally.
+
+[Illustration FIG. 7. SEMIDORSAL VIEW OF THE PECTORAL GIRDLE AND FINS
+OF A DOGFISH (_Scyllium canicula_) × 2/3.
+
+The gaps between the radiale are blackened.
+
+ 1. hollow in the mid-ventral part
+ of the pectoral girdle which
+ supports the pericardium.
+ 2. dorsal (scapular portion) of
+ pectoral girdle.
+ 3. meta-pterygium.
+ 4. meso-pterygium.
+ 5. propterygium.
+ 6. propterygial radial.
+ 7. meso-pterygial radial.
+ 8. meta-pterygial radial.
+ 9. outline of the distal part of
+ the fin which is supported
+ by horny fin-rays.]
+
+The meta-pterygium bears about twelve long narrow radials, the first
+nine of which are traversed by a transverse joint at about two-thirds
+of the way from their origin. Succeeding the radials are a series of
+small polygonal pieces of cartilage arranged in one or more rows and
+attached to the ends of the radials, and finally the fin is completed
+by the dermal fin-rays.
+
+[Illustration FIG. 8. DORSAL VIEW OF THE PELVIC GIRDLE AND FINS OF A
+MALE DOGFISH (_Scyllium canicula_).
+
+ 1. pelvic girdle.
+ 2. basi-pterygium.
+ 3. clasper.
+ 4. radiale.]
+
+THE PELVIC GIRDLE is much smaller than the pectoral. It is formed of a
+stout nearly straight bar of cartilage placed transversely across the
+ventral region of the body. The bar has no dorsal or lateral
+extensions, and is terminated by short blunt processes. It bears on
+its posterior surface a pair of facets with which the pelvic fins
+articulate.
+
+THE PELVIC FIN is smaller and more simply constructed than is the
+pectoral. It consists of a long, somewhat curved rod, the
+=basi-pterygium= (fig. 8, 2), running directly backwards on the inner
+side of the fin, and articulating in front with the pelvic girdle.
+From its outer side arise a series of about fourteen parallel
+cartilaginous radials which bear smaller polygonal pieces. The
+anterior one or two of these radials may articulate independently with
+the pelvic girdle. In the adult male dogfish the distal end of the
+basi-pterygium bears a stout rod nearly as long as itself, and grooved
+on the dorsal surface. This is the skeleton of the =clasper= (fig. 8,
+3).
+
+FOOTNOTE:
+
+[34] See Marshall and Hurst's _Practical Zoology_, 4th ed. London,
+1895, p. 214.
+
+
+
+
+CHAPTER VII.
+
+THE SKELETON OF THE CODFISH. (_Gadus morrhua._)
+
+
+I. EXOSKELETON.
+
+The exoskeleton includes
+
+(1) =Scales.= These are of the type known as =cycloid= and consist of
+flat rounded plates composed of concentrically arranged laminae of
+calcified matter, with the posterior margin entire. The anterior end
+of each scale is imbedded in the skin and is overlapped by the
+preceding scales.
+
+(2) The =teeth=. These are small, pointed, calcified structures
+arranged in large groups on the premaxillae, mandible, vomer, and
+superior and inferior pharyngeal bones.
+
+(3) The =fin-rays=. These are delicate, nearly straight bony rods
+which support the fins.
+
+
+II. ENDOSKELETON.
+
+The endoskeleton of the Codfish, though partially cartilaginous, is
+mainly ossified.
+
+It is divisible into an =axial portion=, including the skull,
+vertebral column, ribs, and skeleton of the median fins, and an
+=appendicular portion=, including the skeleton of the paired fins and
+their girdles.
+
+1. THE AXIAL SKELETON.
+
+A. THE VERTEBRAL COLUMN.
+
+This consists of a series of some fifty-two vertebrae, all completely
+ossified.
+
+
+It is divisible into two regions only, viz. the =trunk= region, the
+vertebrae of which bear movable ribs, and the =caudal= or =tail=
+region, the vertebrae of which do not bear movable ribs.
+
+=Trunk vertebrae.=
+
+These are seventeen in number; the ninth may be described as typical
+of them all. It consists of a short deeply biconcave =centrum= whose
+two cavities communicate by a narrow central canal. From the dorsal
+surface of the anterior half of the centrum arise two strong plates,
+the dorsal or =neural processes=, which are directed obliquely
+backwards and meet forming the dorsal or =neural arch=. This is
+produced into a long backwardly-directed dorsal or =neural spine=.
+
+From the lower part of the anterior edge of each neural arch arise a
+pair of blunt triangular projections which overhang the posterior half
+of the preceding centrum, and bear a pair of flattened surfaces which
+correspond to the anterior or =prezygapophyses= of most vertebrae,
+they differ however from ordinary prezygapophyses in the fact that
+they look downwards and outwards. From the posterior end of the
+centrum arise a pair of short blunt processes each of which bears an
+upwardly- and inwardly-directed articulating surface corresponding to
+a =postzygapophysis=.
+
+The two halves of the ventral arch form a pair of large
+=ventri-lateral processes= which arise from the anterior half of the
+centrum and pass outwards and slightly backwards and downwards.
+
+Behind these there arises on each vertebra a second outgrowth which is
+small and flattened, and like the ventri-lateral process serves to
+protect the air-bladder. The surface of the centrum is marked by more
+or less wedge-shaped depressions, one in the mid-dorsal line, and two
+on the ventral surface immediately mesiad to the bases of the
+ventri-lateral process. There are also a number of smaller
+depressions.
+
+The space between one centrum and the next is in the fresh skeleton
+filled up by the gelatinous remains of the =notochord=.
+
+The first few vertebrae differ from the others in having very short
+centra and no ventri-lateral processes.
+
+The first vertebra comes into very close relation to the posterior
+part of the skull, articulating with the exoccipitals. In the next few
+vertebrae the centra gradually lengthen, and at the fourth or fifth
+vertebra the ventri-lateral processes appear and gradually increase in
+size as followed back. They likewise gradually come to arise at a
+lower level on the centrum, and also become more and more downwardly
+directed, till at the last trunk vertebra they nearly meet.
+
+The =neural spines= of the anterior trunk vertebrae are much longer
+than those of the posterior ones, that of the first vertebra being the
+largest and longest of all, and articulating with the skull. The
+spinal nerves pass out through wide notches or spaces between the
+successive neural arches.
+
+=Caudal vertebrae.=
+
+The caudal vertebrae are about thirty-five in number, each consists of
+a centrum with a slender backwardly-directed dorsal or neural arch,
+similar to those of the posterior trunk vertebrae. The two halves of
+the ventral or haemal arch however do not form outwardly-directed
+ventri-lateral processes, but arise on the ventral surface of the
+centrum, and passing downwards meet and enclose a space; they thus
+form a complete canal, and are prolonged into a backwardly-directed
+ventral or =haemal spine=. The anterior haemal arches are much larger
+than the corresponding neural arches, but when followed back they
+gradually decrease in size, till at about the twenty-fourth caudal
+vertebra they are nearly as small as the neural arches. The last
+caudal vertebra is succeeded by a much flattened =hypural= bone or
+=urostyle=, which together with the posterior neural and haemal spines
+supports the tail-fin.
+
+B. THE RIBS.
+
+The =ribs= are slender, more or less cylindrical bones attached to the
+poster-dorsal faces of the ventri-lateral processes of all the trunk
+vertebrae except the first and second. The earlier ones are thicker
+and more curved; the later ones thinner and more nearly straight. The
+ribs are homologous with the distal parts of the haemal arches of the
+caudal vertebrae.
+
+Associated with the ribs are a second series of rib-like bones, the
+=intermuscular bones=. These are slender, curved bones which arise
+from the ribs or from the ventri-lateral processes at a distance of
+about an inch from the centra, and curve upwards, outwards and
+backwards. In the anterior region where the ventri-lateral processes
+are short they arise from the ribs, further back they arise from the
+ventri-lateral processes.
+
+C. THE UNPAIRED OR MEDIAN FINS.
+
+These are six in number, three being =dorsal=, one =caudal= and two
+=anal=.
+
+The =dorsal= and =anal= fins each consist of two sets of structures,
+the =fin-rays= and the =interspinous bones=. Each fin-ray forms a
+delicate, nearly straight, bony rod which becomes thickened and
+bifurcated at its proximal or vertebral end, while distally it is
+transversely jointed and flexible, frequently also becoming more or
+less flattened.
+
+The first dorsal fin has thirteen rays, the second, sixteen to
+nineteen, the third, seventeen to nineteen. The first anal fin has
+about twenty-two, the second anal fourteen. In each fin the posterior
+rays rapidly decrease in size when followed back.
+
+The =interspinous bones= of the dorsal and anal fins alternate with
+the neural and haemal spines respectively, and form short,
+forwardly-projecting bones, each attached proximally to the base of
+the corresponding fin-ray.
+
+The =caudal fin= consists of a series of about forty-three rays which
+radiate from the posterior end of the vertebral column, being
+connected with the urostyle or hypural bone, and with the posterior
+neural and haemal spines without the intervention of interspinous
+bones. Like the other fin-rays those forming the caudal fin are
+transversely jointed, and are widened and frayed out distally.
+
+The tail-fin in the Cod is =homocercal=, i.e. it appears to be
+symmetrically developed round the posterior end of the vertebral
+column, though in reality a much greater proportion is attached below
+the end of the vertebral column than above it. It is a masked
+heterocercal tail.
+
+THE SKULL.
+
+Owing to the fact that very little cartilage remains in the skull of
+the adult Codfish, its relation to the completely cartilaginous skull
+of the Dogfish is not easily seen. Before describing it therefore, the
+skull of the Salmon will be described, as it forms an intermediate
+type.
+
+THE SKULL OF THE SALMON[36].
+
+The Salmon's skull consists of (1) the =chondrocranium=, which remains
+partly cartilaginous and is partly converted into cartilage bone,
+especially in the occipital region, (2) a large series of plate-like
+membrane bones.
+
+THE CHONDROCRANIUM.
+
+This is an elongated structure, wide behind owing to the fusion of the
+large auditory capsules with the cranium, and elongated and tapering
+considerably in front; in the middle it is much contracted by the
+large orbital cavities.
+
+DORSAL SURFACE OF THE CRANIUM.
+
+In the centre of the posterior end of the dorsal surface is the
+=supra-occipital= (fig. 9, A, 1) with a prominent posterior ridge.
+It is separated by two tracts of unossified cartilage from the large
+series of bones connected with the =auditory organ=. The first of
+these is the =epi-otic= (fig. 9, 2), which is separated by only a
+narrow tract of cartilage from the supra-occipital, and is continuous
+laterally with the large =pterotic= (fig. 9, A, 3) which overlaps in
+front a smaller bone, the =sphenotic= (fig. 9, 4). Both epi-otic and
+pterotic are drawn out into rather prominent backwardly-projecting
+processes.
+
+[Illustration FIG. 9. A. DORSAL AND B. VENTRAL VIEW OF THE CRANIUM OF
+A SALMON (_Salmo salar_) from which most of the membrane bones have
+been removed (after PARKER). Cartilage is dotted.
+
+ 1. supra-occipital.
+ 2. epi-otic.
+ 3. pterotic.
+ 4. sphenotic.
+ 5. frontal.
+ 6. median ethmoid.
+ 7. parietal.
+ 8. lateral ethmoid.
+ 9. parasphenoid.
+ 10. vomer.
+ 11. exoccipital.
+ 12. opisthotic.
+ 13. alisphenoid.
+ 14. orbitosphenoid.
+ 16. foramen for passage of an artery.
+ 17. pro-otic.
+ 18. articular surface for hyomandibular.
+
+II. VII. IX. X. foramina for the passage of cranial nerves.]
+
+The greater part of the remainder of the dorsal surface is formed of
+unossified cartilage which is pierced by three large vacuities or
+=fontanelles=. The anterior fontanelle is unpaired, and lies far
+forward near the anterior end of the long cartilaginous snout, the two
+larger posterior ones lie just in front of the supra-occipital and
+lead into the cranial cavity. In front of the orbit the skull widens
+again, and is marked by two considerable =lateral ethmoid= (fig. 9, 8)
+ossifications. In front of these are a pair of deep pits, the =nasal
+fossae=, at the base of which are a pair of foramina through which the
+olfactory nerves pass out; they communicate with a space, the =middle
+narial cavity=, seen in a longitudinal section of the skull.
+
+The long cartilaginous snout is more or less bifid in front,
+especially in the male (fig. 9).
+
+POSTERIOR END OF THE CRANIUM.
+
+The =foramen magnum= forms a large round hole leading into the cranial
+cavity, and is bounded laterally by the two =exoccipitals= and below
+by them, and to a very slight extent by the =basi-occipital=, the
+three bones together forming a concave =occipital condyle= by which
+the vertebral column articulates with the skull.
+
+The exoccipitals are connected laterally with a fourth pair of
+auditory bones, the =opisthotics=, and just meet the epi-otics
+dorsolaterally, while dorsally they are separated by a wide tract of
+unossified cartilage from the supra-occipital.
+
+The opisthotics are connected laterally with the pterotics.
+
+SIDE OF THE CRANIUM.
+
+At the posterior end is seen the =basi-occipital= in contact above
+with the =exoccipital=, which is pierced by a prominent foramen for
+the exit of the tenth nerve. In front of this lies a small foramen,
+sometimes double, for the ninth nerve.
+
+[Illustration FIG. 10. LATERAL VIEW OF THE CHONDROCRANIUM OF A SALMON
+(_Salmo salar_) (after PARKER). A few membrane bones are also shown.
+Cartilage is dotted.
+
+ 1. supra-occipital.
+ 2. epi-otic.
+ 3. pterotic.
+ 4. opisthotic.
+ 5. exoccipital.
+ 6. basi-occipital.
+ 7. parasphenoid.
+ 8. sphenotic.
+ 9. alisphenoid.
+ 10. orbitosphenoid.
+ 11. lateral- or ectethmoid.
+ 12. olfactory pit; the vomerine teeth are seen just below.
+ 14. pro-otic.
+ 15. basisphenoid.
+ 16. foramen for the passage of an artery.
+ 17. anterior fontanelle.
+ 18. posterior fontanelle.
+
+I. II. V. VII. IX. X. foramina for the passage of cranial nerves.]
+
+In front of the exoccipital is the large =pro-otic= pierced by two
+prominent foramina. Through the more dorsal of these (fig. 10, VII.)
+the facial nerve passes out, while the more ventral (fig. 10, 16) is
+for the passage of an artery. Dorsal to the exoccipital are the
+=opisthotic= and =pterotic=, and dorsal to the pro-otic is the
+=sphenotic=. The =pterotic= is marked by a prominent groove often
+lined by cartilage, which is continued forwards along a tract of
+cartilage between the pro-otic and sphenotic. With this groove the
+hyomandibular articulates.
+
+There are considerable ossifications in the sphenoidal region of the
+side of the cranium. The anterior boundary of the posterior fontanelle
+is formed by the large =alisphenoid=, which is continuous behind with
+the pro-otic and sphenotic, and below with a slender =basisphenoid=.
+Both in front of and behind the basisphenoid there are considerable
+vacuities in the walls of the cranium; through the posterior of these
+openings (fig. 10, V.) the main part of the trigeminal nerve passes
+out, and through the anterior one, the optic (fig. 10, II.). The
+alisphenoid is continuous in front with the =orbitosphenoid= (fig. 10,
+10), which is pierced by the foramen for the exit of the first nerve
+(fig. 10, I.), and in front of the orbitosphenoid there is a large
+vacuity. The =lateral ethmoid= is seen in the side view as well as in
+the dorsal view. Further forwards are seen the olfactory pits, and the
+long cartilaginous snout.
+
+A =ventral view= of the cartilaginous cranium shows much the same
+points as the side view. The basisphenoid appears on the surface
+immediately in front of the basi-occipital.
+
+THE SKULL WITH MEMBRANE BONES.
+
+The =dorsal surface=. The greater part of the dorsal surface in front
+of the supra-occipital is overlaid by a pair of large rough _frontals_
+(figs. 9, A, 5, and 10, 5). They cover the posterior fontanelles and
+stretch over from the sphenotic to the lateral ethmoid, forming a roof
+for the orbit. They meet in the middle line behind, but in front are
+separated by a narrow tract of unossified cartilage, and are
+overlapped by the _median ethmoid_ (figs. 9, A, 6, and 11, 6). At the
+sides of the supra-occipital behind the frontals are a pair of small
+_parietals_ (figs. 9, A, 7, and 11, 7).
+
+[Illustration FIG. 11. LATERAL VIEW OF THE SKULL OF A SALMON (_Salmo
+salar_) (after PARKER). Cartilage is dotted.
+
+ 1. supra-occipital.
+ 2. epi-otic.
+ 3. pterotic.
+ 4. sphenotic.
+ 5. frontal.
+ 6. median ethmoid.
+ 7. parietal.
+ 8. nasal.
+ 9. lachrymal.
+ 10. suborbital.
+ 11. supra-orbital.
+ 12. cartilaginous sclerotic.
+ 13. ossification in sclerotic.
+ 14. meso-pterygoid.
+ 15. meta-pterygoid.
+ 16. palatine.
+ 17. jugal.
+ 18. quadrate.
+ 19. maxillae
+ 20. premaxillae.
+ 21. articular.
+ 22. angular.
+ 23. dentary.
+ 24. hyomandibular.
+ 25. symplectic.
+ 26. epi-hyal.
+ 27. cerato-hyal.
+ 28. hypo-hyal.
+ 29. glosso-hyal.
+ 30. opercular.
+ 31. sub-opercular.
+ 32. infra-opercular.
+ 33. pre-opercular.
+ 34. supratemporal.
+ 35. branchiostegal rays.
+ 36. basi-branchiostegal.]
+
+In a =ventral view= the cranium is seen to be chiefly covered by two
+large membrane bones, the _parasphenoid_ (fig. 9, B, 9) behind, the
+_vomer_ in front. A view of the =posterior end= differs from that of
+the cartilaginous cranium only in the fact that the end of the
+_parasphenoid_ appears lying ventral to the basi-occipital.
+
+The =lateral view= differs very markedly from that of the
+cartilaginous cranium, there being a great development of membrane
+bone in connection with the jaws and branchial apparatus. Lying
+dorsally are seen the _median ethmoid_, _frontal_, _parietal_, and
+=supra-occipital= as before. Lying external to the middle of the
+_median ethmoid_ is seen the small _nasal_ (fig. 11, 8), and below the
+hinder part is the _lachrymal_. The _lachrymal_ (fig. 11, 9) forms the
+first of a series of seven small bones which surround the orbit
+forming the =orbital ring=. Of these the one lying immediately in the
+mid-ventral line of the orbit is the _suborbital_, while the one lying
+in the mid-dorsal line and attached to the frontal is the
+_supra-orbital_ (fig. 11, 11). The orbit has a cartilaginous
+_sclerotic_ in which are two small ossifications (fig. 11, 13)
+laterally placed.
+
+BONES OF THE UPPER JAW.
+
+The =palato-pterygo-quadrate bar= is in a very different condition
+from that of the dogfish, it is partially cartilaginous, partially
+converted into cartilage bone, partially overlapped by membrane bone.
+It is narrow in front but becomes much broader and deeper when
+followed back. Its anterior end forms the =palatine= which bears
+teeth, and in front is completely ossified, while behind the cartilage
+is only sheathed by bone.
+
+Just behind the palatine the outer part of the cartilage is ossified,
+forming two small bones, the =pterygoid= and =meso-pterygoid=, while
+behind them is a larger, somewhat square bone, the =meta-pterygoid=
+(fig. 11, 15).
+
+Below the meta-pterygoid is a tract of unossified cartilage, and then
+comes the =quadrate= (fig. 11, 18).
+
+The lower angle of the quadrate bears a cartilaginous =condyle= with
+which the mandible articulates. In front of the palatine the
+cartilaginous snout is overlapped by three membrane bones, the
+_jugal_, _maxilla_ and _premaxillae_.
+
+The _premaxillae_ (fig. 11, 20), the largest of these, overlaps the
+maxilla behind; both bones bear teeth. The _jugal_ (fig. 11, 17) lies
+above the maxilla and overlaps it in front.
+
+THE LOWER JAW.
+
+The =lower jaw= is a strong bar and is like the upper jaw, partly
+cartilaginous, forming =Meckel's cartilage=, partly ossified, and
+sheathed to a considerable extent in membrane bone.
+
+The outer side and posterior end is ossified, forming the large
+=articular= (fig. 11, 21), but the condyle is cartilaginous and the
+anterior part of the articular forms merely a splint on the outer side
+of Meckel's cartilage, which extends beyond it for a considerable
+distance. The angle of the jaw just below the condyle is formed by a
+small _angular_ (fig. 11, 22), and the anterior two-thirds of the jaw
+is sheathed in the large tooth-bearing _dentary_ (fig. 11, 23).
+
+THE HYOID ARCH.
+
+The =hyoid arch= has a number of ossifications in it and is closely
+connected with the mandibular arch.
+
+The =hyomandibular= (fig. 11, 24) is a large bone which articulates
+with a shallow groove lined by cartilage and formed partly in the
+pterotic, partly in front of it. The hyomandibular is overlapped in
+front by the meta-pterygoid, while below it tapers and is succeeded by
+a small area of unossified cartilage followed by the forwardly
+directed =symplectic= which fits into a groove in the quadrate.
+
+The unossified tract between the hyomandibular and symplectic is
+continuous in front with a strong bar, which remains partly
+cartilaginous and is partly converted into cartilage bone. The
+proximal part is ossified, forming the =epi-hyal=, the middle part
+forms the =cerato-hyal= (fig. 11, 27), in front of which is the small
+=hypo-hyal=. The hyoid arches of the two sides are united by the large
+tooth-bearing =glosso-hyal= (fig. 11, 29). Attached to the lower
+surface of the hyoid arch are a series of twelve flat _branchiostegal
+rays_ (fig. 11, 35). Each overlaps the one in front of it, the
+posterior one being the largest. The branchiostegal rays of the two
+sides are united in front by an unpaired membrane bone, the
+_basi-branchiostegal_ (fig. 11, 36).
+
+=Opercular bones.= Behind the hyomandibular there is a large bony
+plate, the =operculum=, formed of four large membrane bones. The
+anterior of these, the _pre-opercular_ (fig. 11, 33), is crescentic in
+shape, and with its upper end a small _supratemporal_ (fig. 11, 34) is
+connected.
+
+Behind the upper part of the pre-opercular is the largest of the
+opercular bones, the _opercular proper_. Its lower edge overlaps the
+sub-opercular, and both opercular and sub-opercular are overlapped by
+the _infra-opercular_ (fig. 11, 32) in front. The infra-opercular is
+in its turn overlapped by the _pre-opercular_.
+
+BRANCHIAL ARCHES.
+
+There are five branchial arches, the first four of which bear gill
+rays. Each of the first three consists of a shorter upper portion
+directed obliquely backwards and outwards, and a longer lower portion
+forming a right angle with the upper and directed obliquely forwards
+and inwards. The greater part of each arch is ossified.
+
+The upper part of either of the first two consists of a short tapering
+=pharyngo-branchial= directed inwards, and of a long =epi-branchial=
+tipped with cartilage at both ends. The junction of the upper and
+lower parts is formed by a cartilaginous hinge-joint between the
+epi-branchial and cerato-branchial. The =cerato-branchial= is a long
+bony rod separated by a short area of cartilage from the
+=hypo-branchial=, which is succeeded by the =basibranchial= meeting
+its fellow in the middle line. The =fourth arch= has a short
+epi-branchial and no ossified pharyngo-branchial, while the fifth is
+reduced to little more than the cerato-branchial, which bears a few
+teeth on its inner edge. All the branchial arches have projecting from
+their surfaces a number of little processes which act as strainers.
+The first and fourth arches have one series of these, the second and
+third have two.
+
+
+THE SKULL OF THE CODFISH[37].
+
+A full description having been already given of the Salmon's skull,
+that of the Codfish will be described in a briefer manner. The skull
+is very fully ossified, and the great number of plate-like bones
+render it a very complicated structure.
+
+THE CRANIUM.
+
+At the posterior end of the dorsal surface is the large
+=supra-occipital=, which is drawn out behind into the large blade-like
+=occipital spine=. On each side of the supra-occipital are the small
+irregular _parietals_, while in front of it the roof of the skull is
+mainly formed by the very large unpaired _frontal_.
+
+A complicated series of bones are developed in connection with the
+=auditory capsule=, which forms a large projecting mass united with
+the side of the cranium and drawn out behind into a pair of strong
+processes, the =epi-otic= and =parotic= processes. Both these
+processes are connected behind with a large V-shaped bone, the
+_post-temporal_ (fig. 13, 1), which will be described when dealing
+with the pectoral girdle. The =epi-otic process= is formed by the
+=epi-otic=, which is continuous in front with the parietal. The
+=parotic process= is formed by two larger bones, a more dorsal one,
+the =pterotic=, and a more ventral and internal one, the =opisthotic=,
+which is continuous in front with the large =pro-otic=. Intervening
+between the pterotic and frontal is another rather large bone, the
+=sphenotic=, this articulates below with the pro-otic. The pterotic
+and sphenotic together give rise to a large concave surface by which
+the hyomandibular articulates with the cranium. Several of the cranial
+nerves pass out through the bones of the auditory capsule. The ninth
+leaves by a foramen near the posterior border of the opisthotic, the
+fifth and seventh by a notch in the anterior border of the pro-otic.
+
+A number of bones are likewise developed in connection with the orbit
+forming the =orbital ring=. Of these the most anterior, the
+_lachrymal_, is much the largest, the others are five to seven in
+number, the most ventral being the _suborbital_. The sclerotic coat of
+the eye is cartilaginous.
+
+Two pairs of bones and one unpaired bone are developed in connection
+with the =olfactory capsules=, of these, the _nasals_ are narrow bones
+lying next the lachrymals, but nearer the middle line; they overlap
+the second pair of bones, the irregular =lateral ethmoids=. These meet
+one another in the middle line, and are overlapped behind by the
+frontal. They articulate laterally with the lachrymal and palatine,
+and ventrally with the parasphenoid.
+
+In a =posterior view= the foramen magnum and the four bones which
+surround it and together form the occipital segment are well seen. On
+the ventral side is the =basi-occipital=, terminated posteriorly by a
+slightly concave surface which articulates with the centrum of the
+first vertebra. The sides of the foramen magnum are formed by the
+=exoccipitals=, a pair of very irregular bones, pierced by a pair of
+prominent foramina for the exit of the tenth nerves. The exoccipitals
+also bear a pair of surfaces for articulation with corresponding ones
+on the neural arch of the first vertebra. The most dorsal of the four
+bones is the supra-occipital.
+
+On the ventral surface of the cranium in front of the basi-occipital
+is seen the _parasphenoid_, a very long narrow bone which underlies
+the greater part of the cranium. Behind, it articulates dorsally with
+the basi-occipital and dorsolaterally with the pro-otics and
+opisthotics, in front it articulates dorsally with the lateral ethmoid
+and ventrally with the vomer. At the sides of the parasphenoid are the
+small =alisphenoids= articulating above with the postfrontals, in
+front with the frontals, and behind with the pro-otics.
+
+The _vomer_ is an unpaired bone lying immediately in front of the
+parasphenoid. In front it terminates with a thickened curved margin
+bearing several rows of small teeth; behind it tapers out into a long
+process which underlies the anterior part of the parasphenoid.
+Immediately dorsal to the vomer is another median bone, the _median
+ethmoid_; this is truncated in front and tapers out behind into a
+process which fits into a groove on the ventral side of the frontal.
+
+BONES IN CONNECTION WITH THE UPPER JAW.
+
+These bear a close resemblance to those of the Salmon. The most
+anterior bone is the _premaxillae_, a thick curved bone meeting its
+fellow in the middle line. The point of junction of the two is drawn
+out into a short process, and the oral surface is thickly covered with
+small teeth. The dorsal ends of the premaxillae are seen in the fresh
+skull to meet a large patch of cartilage. Behind the premaxillae is the
+_maxilla_, a long rod-like toothless bone, somewhat expanded at the
+upper end where it articulates with the premaxillae and vomer.
+
+Articulating in front with the anterior end of the maxilla and with
+the =lateral ethmoid= is a very irregular bone, the =palatine= (fig.
+12, 1); it articulates behind with two flat bones, the =pterygoid= and
+=meso-pterygoid=. The pterygoid is united behind with two more bones,
+the =quadrate= (fig. 12, 4) and =meta-pterygoid=. The =quadrate= is a
+rather stout irregular bone, bearing on its lower surface a prominent
+saddle-shaped articulating surface for the mandible. The palatine,
+pterygoid and quadrate bones are the ossified representatives of the
+palato-pterygo-quadrate bar of the Dogfish.
+
+[Illustration FIG. 12. MANDIBULAR AND HYOID ARCHES OF A COD (_Gadus
+morrhua_) × 1/2 (Brit. Mus.).
+
+ 1. palatine.
+ 2. meso-pterygoid.
+ 3. pterygoid.
+ 4. quadrate.
+ 5. symplectic.
+ 6. meta-pterygoid.
+ 7. hyomandibular.
+ 8. angular.
+ 9. articular.
+ 10. dentary.
+ 11. inter-hyal.
+ 12. epi-hyal.
+ 13. cerato-hyal.
+ 14. hypo-hyal.
+ 15. uro-hyal.
+ 16. branchiostegal rays.]
+
+The quadrate is united behind with the =symplectic= (fig. 12, 5), and
+the meta-pterygoid with the symplectic and =hyomandibular=, both of
+which bones will be described immediately in connection with the hyoid
+arch.
+
+
+THE LOWER JAW.
+
+The =lower jaw or mandible= like that of the Salmon is partly
+cartilaginous, forming =Meckel's cartilage=, partly formed of
+cartilage bone, partly of membrane bone. Meckel's cartilage is of
+course not seen in the dried skull.
+
+The lower jaw includes one cartilage bone, the =articular= (fig. 12,
+9), this is a large bone connected by a saddle-shaped surface with the
+quadrate. Meckel's cartilage lies in a groove on its under surface,
+and projects beyond it in front. The _angular_ is a small thick bone
+united to the lower surface of the articular at its posterior end. The
+_dentary_ (fig. 12, 10) is a large tooth-bearing bone meeting its
+fellow in the middle line in front, while the articular fits into a
+deep notch at its posterior end.
+
+
+THE HYOID ARCH.
+
+The =hyomandibular= (fig. 12, 7) is a large irregular bone,
+articulating by a prominent rounded head with the sphenotic and
+pterotic. It is united in front with the meta-pterygoid and
+symplectic, and sends off behind a strong process which articulates
+with the opercular. The =symplectic= is a long somewhat triangular
+bone drawn out in front into a process which fits into a groove on the
+inner surface of the quadrate. The distal portion of the hyoid arch is
+strongly developed and consists of first the =inter-hyal= (fig. 12,
+11), a short bony rod, which articulates dorsally with a patch of
+cartilage intervening between the posterior part of the hyomandibular
+and the symplectic. Below it is united with the apex of the triangular
+=epi-hyal=, a bone suturally connected with the large =cerato-hyal=
+(fig. 12, 13) which unites distally with two small =hypo-hyals=. To
+the cerato-hyal are attached a series of seven strong curved
+cylindrical rods, the _branchiostegal rays_. The first of these is the
+smallest and they increase in size up to the last. The four dorsal
+ones are attached to the outer surface of the cerato-hyal, the three
+ventral ones to its inner surface. Interposed between the hypo-hyals
+of the two sides is an unpaired somewhat triangular plate, the
+uro-hyal or _basi-branchiostegal_ (fig. 12, 15).
+
+
+THE BRANCHIAL ARCHES.
+
+The =branchial arches= are five in number and consist of the following
+parts on each side. The dorsal end is formed of the =supra-pharyngeal=
+bone, a large irregular bone covered ventrally with teeth of a fair
+size, and representing the fused =pharyngo-branchials= of the four
+anterior arches. Its external surface is continuous with four small
+=epi-branchials= which pass horizontally backwards and outwards. Their
+distal ends meet four long =cerato-branchials= which are directed
+forwards and inwards and form the principal part of the arches.
+
+Each of the first three cerato-branchials articulates ventrally with a
+=hypo-branchial=, and the hypo-branchials of the two sides are united
+in the middle line by an unpaired =basibranchial=. The third
+hypo-branchial is much flattened. The fourth cerato-branchial is
+united by cartilage with the posterior surface of the third
+hypo-branchial, which it meets near the middle line.
+
+The fifth arch consists only of the cerato-branchial, a wide structure
+covered with teeth and generally called the =inferior pharyngeal
+bone=.
+
+The skeleton of the =operculum= consists of the same four bones as in
+the Salmon, namely the _opercular_, the _infra-opercular_, the
+_pre-opercular_ and the _sub-opercular_. Of these the anterior bone,
+the _pre-opercular_, is the largest, while the _infra-opercular_ is
+the smallest. The _opercular_ has a facet for articulation with the
+hyomandibular.
+
+
+2. THE APPENDICULAR SKELETON.
+
+THE PECTORAL GIRDLE.
+
+This is of a highly specialised type. Membrane bones are greatly
+developed, and the cartilage bones, the =scapula= and =coracoid=, are
+much reduced in size and importance.
+
+[Illustration FIG. 13. THE RIGHT HALF OF THE PECTORAL GIRDLE AND RIGHT
+PECTORAL FIN OF A COD (_Gadus morrhua_) × 1/2 (Brit. Mus.).
+
+ 1. post-temporal.
+ 2. supra-clavicle.
+ 3. clavicle.
+ 4. coracoid.
+ 5. scapula.
+ 6. post-clavicle.
+ 7. brachial ossicles.
+ 8. dermal fin-rays.]
+
+The largest bone in the shoulder girdle is the _clavicle_ (fig. 13,
+3), which is irregularly crescent shaped, thick in front and tapering
+off behind. To the outer side of its upper part is attached a thick
+cylindrical bone, the _supra-clavicle_, which passes upwards and is
+connected with a strong =V= shaped bone, the _post-temporal_. The apex
+of the =V= meets the supra-clavicle, the inner limb articulates with
+the epi-otic process, the outer with the parotic process. Projecting
+downwards from the upper part of the clavicle is a long bony rod,
+flattened proximally and cylindrical and pointed distally, this is
+the _post-clavicle_ (fig. 13, 6).
+
+The =scapula= (fig. 13, 5) is a small irregular plate of bone attached
+to the inner side of the middle of the _clavicle_. The =coracoid=[38]
+is a larger plate of similar character, irregularly triangular in
+shape, attached to the inner side of the clavicle immediately below
+the scapula. The scapula and coracoid bear the pectoral fin.
+
+THE PECTORAL FINS.
+
+Each of these consists of four small irregular bones, the =brachial
+ossicles= (fig. 13, 7), bearing a series of about nineteen dermal
+_fin-rays_. The brachial ossicles represent the reduced and modified
+radiale and basalia of cartilaginous fish such as the dogfish. The
+fin-rays (fig. 13, 8) which form the whole external portion of the fin
+are long slender rods having essentially the same character as those
+of the unpaired fins.
+
+THE PELVIC GIRDLE.
+
+The =pelvic girdle= in the Cod as in other Teleosteans is entirely
+absent, its place being taken by the enlarged basi-pterygia of the
+fins.
+
+THE PELVIC FINS.
+
+These have a very anomalous position in the Cod, being attached to the
+throat in front of the pectoral girdle. Each consists of a basal
+portion, the =basi-pterygium=, and of a number of dermal rays. The
+basi-pterygium consists of an expanded ventral portion which meets its
+fellow below in the middle line, and to which the rays are attached,
+and of an inwardly-directed dorsal portion which also meets its fellow
+and is imbedded in the flesh. The rays are six in number and are long
+slender structures similar to those of the other fins.
+
+
+FOOTNOTES:
+
+[35] See T.J. Parker's _Zootomy_, London, 1884, p. 86.
+
+[36] See W.K. Parker and G.T. Bettany, _The Morphology of the Skull_,
+London, 1877, chap. 3.
+
+[37] T.J. Parker, _Zootomy_, London, 1884, p. 91.
+
+[38] According to G. Swirski, _Schultergurtel des Hechtes_, Dorpat,
+1880, the true coracoid is aborted, and the so-called coracoid of
+Teleosteans is really the precoracoid.
+
+
+
+
+CHAPTER VIII.
+
+GENERAL ACCOUNT OF THE SKELETON IN FISHES[39].
+
+
+EXOSKELETON.
+
+The most primitive type of exoskeleton is that found in Elasmobranchs
+and formed of =placoid= scales; these are tooth-like structures
+consisting of dentine and bone capped with enamel, and have been
+already described (p. 4). In most Elasmobranchs they are small and
+their distribution is fairly uniform, but in the Thornback skate,
+_Raia clavata_, they have the form of larger, more scattered spines.
+In adult Holocephali and in _Polyodon_ and _Torpedo_ there is no
+exoskeleton, in young Holocephali, however, there are a few small
+dorsal ossifications.
+
+The plates or scales of many Ganoids may have been formed by the
+gradual fusion of elements similar to these placoid scales, and often
+bear a number of little tooth-like processes. In _Lepidosteus_,
+_Polypterus_, and many extinct species, these _ganoid_ scales, which
+are rhomboidal in form and united to one another by a peg and socket
+articulation, enclose the body in a complete armour. In _Trissolepis_
+part of the tail is covered by rhomboidal scales, while rounded scales
+cover the trunk and remainder of the tail. _Acipenser_ and
+_Scaphirhynchus_ have large dermal bony plates which are not
+rhomboidal in shape and do not cover the whole body. In _Acipenser_ a
+single row extends along the middle of the back and two along each
+side.
+
+The majority of Teleosteans have thin flattened scales which differ
+from those of Ganoids in being entirely mesodermal in origin,
+containing no enamel. There are two principal types of Teleostean
+scales, the cycloid and ctenoid. A =cycloid= scale is a flat thin
+scale with concentric markings and an entire posterior margin. A
+=ctenoid= scale differs in having its posterior margin pectinate. The
+Dipnoi have overlapping cycloid scales. The rounded scales of _Amia_
+and of many fossil ganoids such as _Holoptychius_ are shaped like
+cycloid scales, but differ from them in being more or less coated with
+enamel. In Eels and some other Teleosteans the scales are completely
+degenerate and have almost disappeared. Some Teleosteans, like _Diodon
+hystrix_, have scales with triradiate roots from which arise long
+sharp spines directed backwards. These scales, which resemble teeth,
+contain no enamel; they become erect when the fish inflates its body
+into a globular form. Many Siluroids have dermal armour in the form of
+large bony plates which are confined to the anterior part of the body.
+In _Ostracion_ the whole body is covered by hexagonal plates, closely
+united together.
+
+The =fin-rays= are structures of dermal origin which entirely or
+partially support the unpaired fins, and assist the bony or
+cartilaginous endoskeleton in the support of the paired fins.
+
+In Elasmobranchs, Dipnoi, and Chondrosteous ganoids the skeletons of
+the fins are, as a rule, about half of exoskeletal, half of
+endoskeletal origin, the proximal and inner portion being
+cartilaginous and endoskeletal, the distal and outer portion being
+exoskeletal, and consisting of horny or of more or less calcified
+fin-rays. In bony Ganoids and Teleosteans the endoskeletal parts are
+greatly reduced and the fins come to consist mainly of the fin-rays,
+which are ossified and frequently become flattened at their distal
+ends.
+
+The fin-rays of the ventral part of the caudal fin are carried by the
+haemal arches; those of the dorsal and anal fins and of the dorsal
+part of the caudal fin generally by interspinous bones, which in adult
+Teleosteans alternate with the neural and haemal spines. In Dipnoi
+these interspinous bones articulate with the neural and haemal spines.
+In many Siluroids the anterior rays of the dorsal and pectoral fins
+are developed into large spines which often articulate with the
+endoskeleton, or are sometimes fused with the dermal armour plates.
+Similar spines may occur in Ganoids in front of both the dorsal and
+anal fins. _Polypterus_ has a small spine or _fulcrum_ in front of
+each segment of the dorsal fin. Such spines are often found
+fossilised, and are known as _ichthyodorulites_.
+
+Similar spines are found in many Elasmobranchs, but they are simply
+inserted in the flesh, not articulated to the endoskeleton. They also
+differ from the spines of Teleosteans and Ganoids in the fact that
+they are covered with enamel, and often have their edges serrated like
+teeth. In the extinct Acanthodii they generally occur in front of all
+the fins, paired and unpaired.
+
+In _Trygon_, the Sting-ray, the tail bears a serrated spine which is
+used for purposes of offence and defence. Many ichthyodorulites may
+have been spines of this nature fixed to the tail, rather than spines
+situated in front of the fins. The spines, which are always found in
+front of the dorsal fin in Holocephali, agree with those of
+Elasmobranchs in containing enamel, and with those of Teleosteans in
+being articulated to the endoskeleton.
+
+TEETH.
+
+The teeth of fish[40] are subject to a very large amount of variation,
+perhaps to more variation than are those of any other class of
+animals. Sometimes, as in adult Sturgeons, they are entirely absent,
+sometimes they are found on all the bones of the mouth, and also on
+the hyoid and branchial arches. The teeth are all originally developed
+in the mucous membrane of the mouth, but they afterwards generally
+become attached to firmer structures, especially to the jaws. In
+Elasmobranchs, however, they are generally simply imbedded in the
+tough fibrous integument of the mouth. Their attachment to the jaws
+may take place in three different ways.
+
+[Illustration FIG. 14. DIAGRAM OF A SECTION THROUGH THE JAW OF A SHARK
+(_Odontaspis americanus_) showing the succession of teeth (Brit. Mus.
+from specimen and diagram).
+
+ 1. teeth in use.
+ 2. teeth in reserve.
+ 3. skin.
+ 4. cartilage of the jaw.
+ 5. encrusting calcification of cartilage.
+ 6. connective tissue.
+ 7. mucous membrane of the mouth.]
+
+(1) By an elastic hinge-joint, as in the Angler (_Lophius_), and the
+Pike (_Esox lucius_). In the Angler the tooth is held by a fibrous
+band attaching its posterior end to the subjacent bone, in the Pike by
+uncalcified elastic rods in the pulp cavity.
+
+(2) By ankylosis, i.e. by the complete union of the calcified tooth
+substance with the subjacent bone. This is the commonest method among
+fish.
+
+(3) By implantation in sockets. This method is not very common among
+fish. The teeth are sometimes, as in _Lepidosteus_, ankylosed to the
+base of the socket. In this genus there is along each ramus of the
+mandible a median row of large teeth placed in perfect sockets, and
+two irregular lateral rows of small teeth ankylosed to the jaw.
+
+Dentine, enamel and cement are all represented in the teeth of fishes,
+but the enamel is generally very thin, and cement is but rarely
+developed. Dentine forms the main bulk of the teeth; it is sometimes
+of the normal type, but generally differs from that in higher
+vertebrates in being vascular, and is known as _vasodentine_. A third
+type occurs, known as _osteodentine_; it is traversed by canals
+occupied by marrow, and is closely allied to bone.
+
+[Illustration FIG. 15. PART OF THE LOWER JAW OF A SHARK (_Galeus_)
+(from OWEN after ANDRÉ).
+
+1. teeth in use.
+
+2. reserve teeth folded back.
+
+3. part of the caudal spine of a Sting-ray (_Trygon_) which has
+pierced the jaw and affected the growth of the teeth.]
+
+The teeth are generally continually renewed throughout life, but
+sometimes one set persists.
+
+The teeth of Selachii are fundamentally identical with placoid
+scales. They are developed from a layer of dental germs which occurs
+all over the surface of the skin, except in the region of the lips. At
+this point the layer of tooth-producing germs extends back into the
+mouth, being projected by a fold of the mucous membrane (fig. 14, 7).
+Here new teeth are successively formed, and as they grow each is
+gradually brought into a position to take the place of its predecessor
+by the shifting outwards of the gum over the jaw. Owing to this
+arrangement sharks have practically an unlimited supply of teeth
+(figs. 14 and 15).
+
+Two principal types of teeth are found in ELASMOBRANCHS. In Sharks and
+Dogfish, on the one hand, the teeth are very numerous, simple, and
+sharp-pointed, and are with or without serrations and lateral cusps.
+Many Rays and fossil Elasmobranchs, on the other hand, have broad
+flattened teeth adapted for crushing shells. Intermediate conditions
+occur between these two extremes. Thus in _Cestracion_ and many
+extinct sharks, such as _Acrodus_, while the median teeth are sharp,
+the lateral teeth are more or less flattened and adapted for crushing.
+In various species belonging to the genus _Raia_ the teeth of the male
+are sharp, while those of the female are blunt. A very specialised
+dentition is met with in the Eagle-rays (Myliobatidae), in which the
+jaws are armed with flattened angular tooth-plates, arranged in seven
+rows, forming a compact pavement; the plates of the middle row are
+very wide and rectangular, those of the other rows are much smaller
+and hexagonal. Lastly, in _Cochliodus_ the individual crushing teeth
+are fused, forming two pairs of spirally-coiled dental plates on each
+side of each jaw. _Pristis_, the Saw-fish, has a long flat
+cartilaginous snout, bearing a double row of persistently-growing
+teeth planted in sockets along its sides. Each tooth consists of a
+number of parallel dentinal columns, united at the base, but elsewhere
+distinct.
+
+In the HOLOCEPHALI--_Chimaera_, _Hariotta_ and _Callorhynchus_--only
+three pairs of teeth or dental plates occur, two pairs in the upper
+jaw, one in the lower. These structures persist throughout life and
+grow continuously. The upper tooth structures are attached
+respectively to the ethmoid or vomerine region of the skull, and to
+the palato-pterygoids. The vomerine teeth are small, while those
+attached to the mandible and the palato-pterygoid region are large and
+bear several roughened ridges adapted for grinding food. The teeth of
+the two opposite sides of the jaw meet in a median symphysis. The
+teeth of _Chimaera_ are more adapted for cutting, those of
+_Callorhynchus_ for crushing. Many extinct forms are known, some of
+whose teeth are intermediate in structure between those of _Chimaera_
+and _Callorhynchus_.
+
+The teeth of GANOIDS are also extremely variable. Among living forms,
+the Holostei are more richly provided with teeth than are any other
+fishes, as they may occur on the premaxillae, maxillae, palatines,
+pterygoids, parasphenoid, vomers, dentaries, and splenials. Among the
+Chondrostei, on the other hand, the adult Acipenseridae are toothless;
+small teeth however occur in the larval sturgeon, and in _Polyodon_
+many small teeth are found attached merely to the mucous membrane of
+the jaws. Many fossil Ganoids have numerous flattened or knob-like
+teeth, borne on the maxillae, palatines, vomers and dentaries. Others
+have a distinctly heterodont dentition. Thus in _Lepidotus_ the
+premaxillae bear chisel-like teeth, while knob-like teeth occur on the
+maxillae, palatines and vomers. In _Rhizodus_ all the teeth are
+pointed, but while the majority are small a few very large ones are
+interspersed.
+
+In TELEOSTEANS, too, the teeth are eminently variable both in form and
+mode of arrangement. They may be simple and isolated, or compound, and
+may be borne on almost any of the bones bounding the mouth cavity, and
+also as in the Pike, on the hyoid and branchial arches. The splenial
+however never bears teeth and the pterygoid and parasphenoid only
+rarely, thus differing from the arrangement in the Holostei.
+
+The isolated teeth are generally conical in form and are ankylosed to
+the bone that bears them. Such teeth are, with a few exceptions such
+as _Balistes_, not imbedded in sockets nor replaced vertically.
+
+In some fish beak-like structures occur, formed partly of teeth,
+partly of the underlying jaw bones. These beaks are of two kinds: (1)
+In _Scarus_, the parrot fish, the premaxillae and dentaries bear
+numerous small, separately developed teeth, which are closely packed
+together and attached by their proximal ends to the bone, while their
+distal ends form a mosaic. Not only the teeth but the jaws which bear
+them are gradually worn away at the margins, while both grow
+continuously along their attached edge. (2) In Gymnodonts, e.g.
+_Diodon_, the beaks are formed by the coalescence of broad calcified
+horizontal plates, which when young are free and separated from one
+another by a considerable interval.
+
+In some Teleosteans the differentiation of the teeth into biting teeth
+and crushing teeth is as complete as in _Lepidosteus_. Thus in the
+Wrasse (_Labrus_), the jaws bear conical slightly recurved teeth
+arranged in one or two rows, with some of the anterior ones much
+larger than the rest. The bones of the palate are toothless, while
+both upper and lower pharyngeal bones are paved with knob-like
+crushing teeth; such pharyngeal teeth occur also in the Carp but are
+attached only to the lower pharyngeal bone, the jaw bones proper being
+toothless.
+
+In DIPNOI the arrangement of the teeth is very similar to that in
+Holocephali. The mandible bears a single pair of grinding teeth
+attached to the splenials, and a corresponding pair occur on the
+palato-pterygoids. In front of these there are a pair of small conical
+vomerine teeth loosely attached to the ethmoid cartilage. The
+palato-pterygoid teeth of _Ceratodus_ are roughly semicircular in
+shape with a smooth convex inner border, and an outer border bearing a
+number of strongly marked ridges. The teeth of the extinct Dipteridae
+resemble those of _Ceratodus_ but are more complicated.
+
+
+ENDOSKELETON.
+
+SPINAL COLUMN[41].
+
+The spinal column of fishes is divisible into only two regions, a
+caudal region in which the haemal arches or ribs meet one another
+ventrally, and a precaudal region in which they do not meet.
+
+The various modifications of the spinal column in fishes can be best
+understood by comparing them with the arrangement in the simplest type
+known, namely _Amphioxus_. In _Amphioxus_ the notochord is immediately
+surrounded by a structureless cuticular layer, the _chordal sheath_.
+Outside this is the _skeletogenous layer_, which in addition to
+surrounding the notochord and chordal sheath embraces the nerve cord
+dorsally, and laterally sends out septa forming the _myomeres_.
+
+The CARTILAGINOUS GANOIDS[42] _Acipenser_, _Polyodon_ and
+_Scaphirhynchus_ are the simplest fishes as regards their spinal
+column. The notochord remains permanently unconstricted and is
+enclosed in a chordal sheath, external to which is the skeletogenous
+layer. In this layer the development of cartilaginous elements has
+taken place. In connection with each _neuromere_, or segment as
+determined by the points of exit of the spinal nerves, there are
+developed two pairs of ventral cartilages, the ventral arches
+(basiventralia) and intercalary pieces (interventralia); and at least
+two pairs of dorsal pieces, the neural arches (basidorsalia) and
+intercalary pieces (interdorsalia). The lateral parts of the
+skeletogenous layer do not become converted into cartilage, so there
+are no traces of vertebral centra. The ventral or haemal arches meet
+one another ventrally and send out processes to protect the ventral
+vessels. The neural arches do not meet, but are united by a
+longitudinal elastic band.
+
+In Cartilaginous ganoids the only indications of metameric
+segmentation are found in the neural and haemal arches. The case is
+somewhat similar with the Holocephali and Dipnoi.
+
+In the HOLOCEPHALI the notochord grows persistently throughout life,
+and is of uniform diameter throughout the whole body except in the
+cervical region and in the gradually tapering tail. The chordal sheath
+is very thick and includes a well-marked zone of calcification which
+separates an outer zone of hyaline cartilage from an inner zone. There
+are also a number of cartilaginous pieces derived from the
+skeletogenous layer which are arranged in two series, a dorsal series
+forming the neural arches and a ventral series forming the haemal
+arches. These do not, except in the cervical region, meet one another
+laterally round the notochord and form centra. To each neuromere there
+occur a pair of basidorsals, a pair of interdorsals, and one or two
+supradorsals. In the tail the arrangement is irregular.
+
+In the DIPNOI as in the Holocephali the notochord grows persistently
+and uniformly, and the chordal sheath is thick and cartilaginous
+though there are no metamerically arranged centra. The neural and
+haemal arches and spines are cartilaginous and interbasalia
+(intercalary pieces) are present. The basidorsalia and basiventralia
+do not in _Ceratodus_ meet round the notochord and enclose it except
+in the anterior part of the cervical and posterior part of the caudal
+region.
+
+In ELASMOBRANCHII the chordal sheath is weak and the skeletogenous
+layer strong. Biconcave cartilaginous vertebrae are developed, and as
+is the case in most fishes, constrict the notochord _vertebrally_.
+
+Two distinct types of vertebral column can be distinguished in
+Elasmobranchs[43]:
+
+1. In many extinct forms and in the living Notidanidae, _Cestracion_,
+and _Squatina_, the dorsal and ventral arches do not meet one another
+laterally round the centrum, and consequently readily come away from
+it.
+
+2. In most living Elasmobranchs the arches meet laterally round the
+centrum.
+
+The vertebrae are never ossified but endochondral calcification nearly
+always takes place, though it very rarely reaches the outer surface of
+the vertebrae. Elasmobranchs are sometimes subdivided into three
+groups according to the method in which this calcification takes
+place:
+
+1. =Cyclospondyli= (_Scymnus_, _Acanthias_), in which the calcified
+matter is deposited as one ring in each vertebra.
+
+2. =Tectospondyli= (_Squatina_, _Raia_, _Trygon_), in which there are
+several concentric rings of calcification.
+
+3. =Asterospondyli= (Notidanidae, _Scyllium_, _Cestracion_), in which
+the calcified material instead of forming one simple ring, extends out
+in a more or less star-shaped manner.
+
+In _Heptanchus_ the length of the vertebral centra in the middle of
+the trunk is double that in the anterior and posterior portions, and
+as the length of the arches does not vary, the long centra carry more
+of them than do the short centra.
+
+In many Rays the skull articulates with the vertebral column by
+distinct occipital condyles.
+
+In BONY GANOIDS the skeletogenous layer becomes calcified
+ectochondrally in such a way that the notochord is pinched in at
+intervals, and distinct vertebrae are produced. Ossification of the
+calcified cartilage rapidly follows. In _Amia_ the vertebrae are
+biconcave, in _Lepidosteus_ they are opisthocoelous, cup and ball
+joints being developed between the vertebrae in a manner unique among
+fishes. The notochord entirely disappears in the adult _Lepidosteus_,
+but at one stage in larval life it is expanded vertebrally and
+constricted intervertebrally in the manner usual in the higher
+vertebrata, but unknown elsewhere among fishes.
+
+The tail of _Amia_ is remarkable from the fact that as a rule to each
+neuromere, as determined by the exit of the spinal nerves, there are
+two centra, a posterior one which bears nothing, and an anterior one
+which bears the neural and haemal arches, these being throughout the
+vertebral column connected with the centra by cartilaginous discs.
+
+In most TELEOSTEANS but not in the Plectognathi the neural arches are
+continuous with the centra, which are nearly always deeply biconcave.
+
+In some cases many of the anterior vertebrae are ankylosed together
+and to the skull. The vertebrae often articulate with one another by
+means of obliquely placed flattened surfaces, the zygapophyses. The
+centrum in early stages of development is partially cartilaginous, but
+the neural arches and spines in the trunk at any rate, pass directly
+from the membranous to the osseous condition.
+
+
+FINS.
+
+The most primitive fins are undoubtedly the unpaired ones, which
+probably originally arose as ridges or folds of skin along the
+mid-dorsal line of the body, and passed thence round the posterior end
+on to the ventral surface, partially corresponding in position and
+function to the keel of a ship.
+
+In long 'fish' which pass through the water with an undulating motion
+such simple continuous fins may be the only ones found, as in
+_Myxine_. To support these median fins skeletal structures came to be
+developed; these show two very distinct forms, viz. cartilaginous
+endoskeletal pieces, the _radiale_, and horny exoskeletal fibres, the
+_fin-rays_. Mechanical reasons caused the fin to become concentrated
+at certain points and reduced at intervening regions. Thus a terminal
+caudal fin arose and became the chief organ of propulsion, and the
+dorsal and ventral fins became specialised to act as balancing organs.
+
+In some of the earlier Elasmobranchs, the Pleuracanthidae, the
+endoskeletal cartilaginous radiale are directly continuous with
+outgrowths from the dorsal and ventral arches of the vertebrae, and
+form the main part of the fin. In later types of Elasmobranchs the
+horny exoskeletal fin-rays have comparatively greater prominence. In
+bony fish, as has been already stated, the horny fibres are replaced
+by bony rays of dermal origin, and at the same time complete reduction
+and disappearance of the cartilaginous radiale takes place.
+
+THE CAUDAL FIN.
+
+The caudal region of the spinal column in fishes is of special
+importance. It is distinctly marked off from the rest of the spinal
+column by the fact that the ventral or haemal arches meet one another
+and are commonly prolonged into spines, while in the trunk region they
+do not meet but commonly diverge from one another.
+
+In some fish the terminal part of the caudal region of the spinal
+column retains the same direction as the rest of the spinal column.
+The blade of the caudal fin is then divided into two nearly equal
+portions, and is said to be =diphycercal=. This condition is generally
+regarded as the most primitive one; it occurs in the Ichthyotomi,
+Holocephali, all living Dipnoi, _Polypterus_ and some extinct
+Crossopterygii, and a few Selachii and Teleostei. It occurs also in
+deep-sea fish belonging to almost every group, and under these
+conditions obviously cannot be regarded as primitive, but must be
+looked on as a feature induced by the peculiar conditions of life.
+
+In the great majority of fish the terminal part of the caudal region
+of the spinal column is bent dorsalwards, and the part of the blade of
+the caudal fin which arises on the dorsal surface is much smaller than
+is that arising on the ventral surface. Such a fin is said to be
+=heterocercal=.
+
+Strictly speaking all fish whose tails are not diphycercal have
+heterocercal tails, but the term is commonly applied to two-bladed
+tails in which the spinal column forms a definite axis running through
+the dorsal blade, while the ventral blade is enlarged and generally
+forms the functional part of the tail. Such heterocercal tails are
+found in nearly all Elasmobranchii, together with the living
+cartilaginous Ganoidei, and many extinct forms belonging to the same
+order; _Lepidosteus_, _Amia_, and the Dipteridae among Dipnoi, have
+tails which, though obviously heterocercal, are not two-bladed.
+
+The vast majority of the Teleostei and some extinct Ganoidei have
+heterocercal tails of the modified type to which the term =homocercal=
+is applied. The hypural bones which support the lower half of the tail
+fin become much enlarged, and frequently unite to form a wedge-shaped
+bone which becomes ankylosed to the last ossified vertebral centrum.
+The fin-rays then become arranged in such a way as to produce a
+secondary appearance of symmetry. Some homocercal fish such as the
+Perch have the end of the notochord protected by a calcified or
+completely ossified sheath, the =urostyle=, to which several neural
+and haemal arches may be attached, and which becomes united with the
+centrum of the last vertebra; in others such as the Salmon the end of
+the notochord is protected only by laterally placed bony plates.
+
+THE SKULL.
+
+It is often impossible to draw a hard and fast line between the
+cranium and the vertebral column. This is the case for instance in
+_Acipenser_ (fig. 18, 16) among Chondrostei, in _Amia_ among Holostei,
+and in _Ceratodus_ and _Protopterus_ among Dipnoi. The occipital
+region of the skull in _Amia_ is clearly formed of three cervical
+vertebrae whose centra have become absorbed into the cranium, while
+the neural arches and spines are still distinguishable.
+
+The simplest type of cranium is that found in ELASMOBRANCHS: it
+consists of a simple cartilaginous box, which is generally immovably
+fixed to the vertebral column, though in some forms, like _Scymnus_
+and _Galeus_, a joint is indicated, and in others, such as the
+Rays, one is fairly well developed. The cranium in Elasmobranchs
+is never bony, though the cartilage is sometimes calcified. It is
+drawn out laterally into an antorbital process in front of the eye,
+and a postorbital process behind it. The nasal capsules are always
+cartilaginous, and the eye, as a general rule, has a cartilaginous
+sclerotic investment. The cranium is often prolonged in front into
+a rostrum which is enormously developed in _Pristis_ and some Rays.
+The cartilaginous roof of the cranium is rendered incomplete by the
+presence of a large hole, the anterior fontanelle.
+
+[Illustration FIG. 16. A. SKULL OF _Notidanus_ × 1/2 (Brit. Mus.). B.
+SKULL OF _Cestracion_ × 1/3 (after GEGENBAUR). In neither case are the
+branchial arches shown.
+
+ 1. rostrum.
+ 2. olfactory capsule.
+ 3. ethmo-palatine process.
+ 4. palatine portion of palato-pterygo-quadrate bar.
+ 5. quadrate portion of bar.
+ 6. Meckel's cartilage.
+ 7. teeth.
+ 8. labial cartilage.
+ 9. hyomandibular.
+ 10. postorbital process.
+ II. optic foramen.]
+
+Two pairs of labial cartilages (fig. 16, B, 8) are often present. They
+lie imbedded in the cheeks outside the anterior region of the jaws,
+and are specially large in _Squatina_.
+
+As regards the visceral arches[44] the simplest and most primitive
+condition of the jaws is that of the Notidanidae, in which the
+mandibular and hyoid arches are entirely separate. In these primitive
+fishes the palato-pterygo-quadrate bar articulates with the
+postorbital process (fig. 16, 10), while further forwards it is united
+to the cranium by the ethmo-palatine ligament. The hyoid arch is small
+and is broadly overlapped by the mandibular arch. The term
+=autostylic= is used to describe this condition of the suspensorium.
+From this condition we pass in the one direction to that of
+_Cestracion_ (fig. 16, B), in which the whole of the palato-pterygo
+quadrate bar has become bound to the cranium, and in the other to that
+of _Scyllium_. In _Scyllium_ (fig. 6), while the ethmo-palatine
+ligament is retained, the postorbital articulation of the palato
+pterygo-quadrate has been given up, so that the palato-pterygo
+quadrate comes to abut on the hyomandibular and is attached to it by
+ligaments. The pre-spiracular ligament (fig. 16, 20) running from the
+auditory capsule also assists in supporting the jaws.
+
+Lastly we come to the purely =hyostylic= condition met with in Rays,
+in which the mandibular arch is entirely supported by the
+hyomandibular. In some Rays the hyoid is attached to the posterior
+face of the hyomandibular near its proximal end, and may even come to
+articulate with the cranium.
+
+The =visceral arches of Elasmobranchs= may be summarised as follows:--
+
+1. The =mandibular arch=, consisting of a much reduced dorsal portion,
+the pre-spiracular ligament, and a greatly developed ventral portion
+from which both upper and lower jaws are derived. The mandible
+(Meckel's cartilage) is the original lower member of the mandibular
+arch, and from it arises an outgrowth which forms the upper jaw or
+palato-pterygo-quadrate bar. In _Scymnus_ this bears a few
+branchiostegal rays.
+
+2. The =hyoid arch=, which consists of the hyomandibular and the
+hyoid, and bears branchiostegal rays on its posterior face.
+
+[Illustration FIG. 17. DORSAL VIEW OF THE BRANCHIAL ARCHES OF
+_Heptanchus_. (From GEGENBAUR).
+
+ 1. basi-hyal.
+ 2. cerato-hyal.
+ 3. second hypo-branchial.
+ 4. first cerato-branchial.
+ 5. first epi-branchial.
+ 6. first pharyngo-branchial.
+ 7. pharyngo-branchial, common to the sixth and seventh arches.
+ 8. basibranchial of second arch.
+ 9. basibranchial, common to the sixth and seventh arches.]
+
+3. The =branchial arches=, generally five in number, all of which
+except the last bear branchiostegal rays. In the Notidanidae the
+number of branchial arches is increased beyond the normal series,
+thus in _Hexanchus_ there are six, and in _Heptanchus_ seven. There
+are six also in _Chlamydoselache_ and _Protopterus_.
+
+4. The so-called external branchial arches which are cartilaginous
+rods attached to all the visceral arches. They are especially large in
+_Cestracion_.
+
+The skull in HOLOCEPHALI is entirely cartilaginous. The
+palato-pterygo-quadrate bar is fixed to the cranium, and to it the
+mandible articulates. There is a well-marked joint between the skull
+and the spinal column.
+
+In living Cartilaginous GANOIDS the primitive cartilaginous cranium is
+very massive, and is greatly prolonged anteriorly, while posteriorly
+it merges into the spinal column. Although it is mainly cartilaginous
+a number of ossifications take place in the skull, and membrane bones
+are now found definitely developed, especially in connection with the
+roof of the cranium. In _Acipenser_ (fig. 18) the ossifications in the
+cartilage include the pro-otic, which is pierced by the foramen for
+the fifth nerve, the alisphenoid, orbitosphenoid, ectethmoid,
+palatine, pterygoid, meso-pterygoid, hyomandibular (fig. 18, 11),
+cerato-hyal, all the cerato-branchials, and the first two
+epi-branchials. Most of these structures are, however, partly
+cartilaginous, though they include an ossified area. The membrane
+bones too of _Acipenser_ are very well developed, they include a bone
+occupying the position of the supra-occipital, and form a complete
+dorsal cephalic shield. Resting on the ventral surface are a vomer and
+a very large parasphenoid (fig. 18, 3). There is a bony operculum
+attached to the hyomandibular, and membrane bones representing
+respectively the maxilla and dentary are attached to the jaws. The
+suspensorium is most markedly hyostylic. The palato-pterygo-quadrate
+bar has a very curious shape and is quite separate from the cranium.
+It is connected to the hyomandibular by a thick symplectic ligament
+containing a small bone homologous with the symplectic of
+Teleosteans.
+
+_Polyodon_ differs much from _Acipenser_, the membrane bones not being
+so well developed though they cover the great cartilaginous snout.
+
+[Illustration FIG. 18. LATERAL VIEW OF THE SKULL OF A STURGEON
+(_Acipenser sturio_). Nearly all the membrane bones have been removed
+(Brit. Mus.).
+
+ 1. nasal cavity.
+ 2. orbit.
+ 3. parasphenoid.
+ 4. vomer.
+ 5. pterygoid.
+ 6. maxilla. (The dotted line running from 6 passes into the mouth
+ cavity.)
+ 7. dentary.
+ 8. symplectic.
+ 10. palatine.
+ 11. hyomandibular.
+ 12. pharyngo-branchial.
+ 13. epi-branchial.
+ 14. cerato-branchial.
+ 15. hypo-branchial.
+ 16. coalesced anterior vertebrae.
+ 17. inter-hyal.
+ 18. cerato-hyal.
+ 19. rib.]
+
+The skull in _Polypterus_ (Crossopterygii) shows a great advance
+towards the condition met with in Teleostei. The cranium remains to a
+great extent unossified, and large dorsal and ventral fontanelles
+pierce its walls. It is covered by a great development of membrane
+bones, paired nasals, frontals, parietals, supra- and post-temporals,
+and dermo-supra-occipitals among others being present. The
+palato-pterygo-quadrate bar is fused to the cranium, and in connection
+with it the following paired membrane bones appear, palatine, ecto-,
+meso- and meta-pterygoid, and further forwards jugal, vomer, maxilla
+and premaxillae. The membrane bones developed in connection with each
+ramus of the mandible are the dentary, angular, and splenial, in
+addition to the cartilage bone the articular. Several large opercular
+bones occur. There are also a pair of large jugular or gular plates,
+and several large opercular bones.
+
+In Bony Ganoids both cartilage bone and membrane bone is well
+developed. The pro-otics and exoccipitals are well ossified, but the
+supra-occipital and pterotics are not. Lateral ethmoids are developed,
+and there are ossifications in the sphenoidal region which vary in
+different forms. The place of the cartilaginous palato-pterygo
+quadrate is taken by a series of bones, the quadrate behind and the
+palatine, ecto-, meso- and meta-pterygoids in front. In _Lepidosteus_,
+however, the palatine and pterygoid are membrane bones, as they are in
+_Polypterus_ and the Frog. Paired maxillae, premaxillae, vomers and a
+parasphenoid occur forming the upper jaw and roof of the mouth, and a
+series of membrane bones are found investing the mandible and forming
+the operculum.
+
+In _Amia_[45] membrane bones are as freely developed as they are in
+Teleosteans; they include on each side a squamosal, four opercular
+bones, a lachrymal, a pre-orbital, one or two suborbitals, two large
+postorbitals and a supratemporal; while investing the mandible,
+besides the dentary, splenial, angular, and supra-angular, there is an
+unpaired jugular. The articular too is double and a mento-meckelian
+occurs. In _Amia_ teeth are borne on the premaxillae, maxillae, vomers,
+palatines and pterygoids.
+
+Bony Ganoids are the lowest animals in which squamosal bones are
+found, and they do not occur in Teleosteans.
+
+The suspensorium in bony Ganoids, as in the Chondrostei, is hyostylic,
+and there are two ossifications in the hyomandibular cartilage, viz.
+the hyomandibular, and the symplectic.
+
+The skull of TELEOSTEI is very similar to those of _Lepidosteus_ and
+_Amia_. Although the bony skull is greatly developed and very
+complicated, much of the original cartilaginous cranium often
+persists. Membrane bones are specially developed on the roof of the
+skull where they include the parietal, frontal, and nasal bones. The
+same bones are developed in connection with the upper jaw and roof of
+the mouth as in bony Ganoids, but only two membrane bones occur in the
+lower jaw, viz. the angular and dentary. A number of large
+ossifications take place in the cartilage of the auditory capsules. In
+some forms parts of the last pair of branchial arches are broadened
+out and form the pharyngeal bones which bear teeth. The opercular
+bones and those of the upper and lower jaws are quite comparable to
+those of bony Ganoids.
+
+A full account of the Teleostean skull has been given in the case of
+the Salmon (pp. 87-96) and the Cod (pp. 96-101).
+
+In DIPNOI the skull is mainly cartilaginous, but both cartilage- and
+membrane-bone occur also. Cartilage-bone is found in the ossified
+exoccipitals, while of membrane-bones _Protopterus_ has among unpaired
+bones a fronto-parietal, a median ethmoid, and a parasphenoid, and
+among paired bones nasals and large supra-orbitals. The skull of
+_Ceratodus_ (fig. 19) has an almost complete roof of membrane bones,
+including some whose homology is doubtful. The ethmo-vomerine region
+is always cartilaginous, but bears small teeth. The palato-pterygo
+quadrate bar is ossified and firmly united to the cranium, and the
+mandible articulates directly with it (autostylic). Membrane bones are
+freely developed in connection with the mandible, dentary, splenial,
+and angular bones being all present. There are two opercular bones.
+
+In the extinct Dipteridae the cranium is very completely covered with
+plates of dermal bone, and the skeleton in general is more ossified
+than is the case in recent Dipnoi.
+
+Six pairs of branchial arches occur in _Protopterus_; _Ceratodus_ and
+_Lepidosiren_ have five, like most other fish. The branchial arches
+bear gill rakers.
+
+[Illustration FIG. 19. DORSAL (TO THE LEFT) AND VENTRAL (TO THE RIGHT)
+VIEWS OF THE CRANIUM OF _Ceratodus miolepis_ (after GÜNTHER).
+
+ 1. cartilaginous part of the quadrate with which the mandible
+ articulates.
+ 2. scleroparietal.
+ 3. frontal.
+ 4. ethmoid.
+ 5. nares.
+ 6. orbit.
+ 7. pre-opercular (squamosal).
+ 8. second rib.
+ 9. first rib.
+ 10. vomerine tooth.
+ 11. palato-pterygoid tooth.
+ 12. palato-pterygoid.
+ 13. parasphenoid.
+ 14. interopercular.]
+
+RIBS.
+
+As has been already mentioned (p. 24), although ribs commonly appear
+to be the cut-off ends of the transverse processes, they are really
+elements derived from the ventral or haemal arch.
+
+In Elasmobranchii and other cartilaginous fish they have the form of
+small cartilaginous structures imperfectly separated from the
+diverging halves of the ventral arch, and are often absent.
+
+In Teleostei and bony Ganoids they often have different attachments in
+different parts of the body. In the tail region they are not
+differentiated from the two halves of the ventral arch, which meet in
+the middle line, and are prolonged into a haemal spine. In the
+posterior trunk region they sometimes form distinct processes
+diverging from the two halves of the ventral arch; while further
+forward they may shift their attachment so as to arise from the dorsal
+side of the two halves of the ventral arch and at some distance from
+their ends, which now diverge as ventri-lateral processes.
+
+APPENDICULAR SKELETON.
+
+PECTORAL GIRDLE.
+
+The simplest type of pectoral girdle is found in Elasmobranchs. It is
+entirely cartilaginous and consists of a curved ventrally-placed rod,
+ending dorsally in two horn-like scapular processes which are
+sometimes attached to the cranium or vertebral column. In Rays the
+shoulder girdle is very large, and has a distinct suprascapular
+portion forming a broad plate attached to the neural spines of the
+vertebrae. There is often a cup-like glenoid cavity for the
+articulation of the limb; this cavity is specially large in Rays and
+is much pierced by holes. In Dipnoi the cartilaginous girdle still
+occurs, but on it there is a deposit of membrane bone forming the
+clavicle, infraclavicle, and supra-clavicle. These bones, which with
+the exception of the clavicle, are unknown in higher vertebrates, are
+better developed in Ganoids, and best of all in Teleosteans. They are
+connected by the supratemporal with the epi-otic and opisthotic
+regions of the cranium. Owing to this development of dermal bone, the
+original cartilaginous arch becomes much reduced, but ossifications
+representing the scapula and coracoid occur in bony Ganoids and
+Teleosteans.
+
+PELVIC GIRDLE.
+
+In Elasmobranchs the pelvic girdle consists of a short ventral rod of
+cartilage representing the ischium and pubis, which does not send up
+dorsal iliac processes. In _Chimaera_ the pelvic girdle has a
+flattened pointed iliac portion, and ventrally an unpaired movable
+cartilaginous plate which bears hooks and is supposed to be copulatory
+in function. Claspers of the usual type are present as well. The
+Dipnoi have a primitive kind of pelvis in the form of a cartilaginous
+plate lying in the mid-ventral line and drawn out into three horns
+anteriorly. In Ganoids the pelvis has almost entirely disappeared,
+though small cartilaginous vestiges of it remain in _Polypterus_. In
+Teleosteans even these vestiges are gone, and in these fish and
+Ganoids the place of the pelvis is taken by the enlarged basi-pterygia
+(meta-pterygia) of the fins.
+
+PAIRED FINS[46].
+
+As regards the origin of the limbs or paired fins of fishes there are
+two principal views. One view, that of Gegenbaur, considers that limbs
+and their girdles are derived from visceral arches which have migrated
+backwards. The other view, which probably now has the greater number
+of supporters, considers that the paired fins of fishes are of
+essentially the same nature as the median fins.
+
+According to Gegenbaur's view[47] the =archipterygium= of _Ceratodus_
+(fig. 20) represents the lowest type of fin; it consists of a central
+cartilaginous axis bearing a large number of radiale. The dorsal or
+pre-axial radiale are more numerous than the ventral or postaxial, and
+at the margin of the fin[48] the cartilaginous endoskeletal radiale
+are replaced by horny exoskeletal fin-rays.
+
+[Illustration FIG. 20. LATERAL VIEW OF THE SKELETON OF _Ceratodus
+miolepis_ (after GÜNTHER).
+
+ 1. ethmoid.
+ 2. scleroparietal.
+ 3. frontal.
+ 4. cartilaginous posterior part of cranium.
+ 5. pre-opercular (squamosal).
+ 6. opercular.
+ 7. suborbital.
+ 8. orbit.
+ 9. pectoral girdle.
+ 10. proximal cartilage of pectoral fin.
+ 11. pectoral fin.
+ 12. pelvic girdle.
+ 13. pelvic fin.
+ 14. spinal column.
+ 15. caudal fin (diphycercal).]
+
+It is impossible here to give a full discussion of the rival views,
+but some of the points which support Gegenbaur's view may be
+mentioned. The fact that migration of visceral arches has to be
+assumed is no difficulty, as it is obvious that migration in the
+opposite direction has taken place in many Teleosteans such as the
+Cod, whose pelvic fins are attached to the throat in front of the
+pectorals. If migration did take place, the pelvic fins being older
+than the pectoral should be the more modified, and this is the case.
+Again, if the pectoral girdle is a modified branchial arch, it must at
+some period have carried a gill, and in _Protopterus_ it does bear a
+vestigial gill.
+
+According to the view more prevalent at the present time, the paired
+fins have been derived from two continuous folds of skin and their
+skeletal supports running forward from the anal region along the sides
+of the body, their character being similar to the fold that gave rise
+to the median fins. In support of this view it may be argued that the
+paired and unpaired fins are often identical in structure, and that
+some Elasmobranch embryos do show a ridge running between the pectoral
+and pelvic fins. Then from this continuous fold two pairs of smaller
+folds may have been specialised off, and in each a number of
+cartilaginous radiale may have been developed. The fin of
+_Cladoselache_ from the Carboniferous of Ohio apparently illustrates
+this condition. It consists of certain basal pieces which do not
+project beyond the body wall and bear a number of unsegmented
+cartilaginous radiale, which show crowding together and are sometimes
+bifurcated distally; they extend throughout the whole fin from the
+body wall to the margin. From this fin the archipterygium might be
+easily derived by the enlargement of one of the middle radiale and the
+segmentation and partial fusion of them all.
+
+Whether the archipterygium be a primitive or secondary type of fin,
+when it is once reached it is easy to derive all the other types from
+it. The fins of the other living Dipnoi,--_Protopterus_ and
+_Lepidosiren_--are simply archipterygia from which the radiale have
+almost or completely disappeared, leaving only the segmented axes.
+Archipterygia too are found in the pectoral fins of the Ichthyotomi,
+but the postaxial radiale are much reduced.
+
+The =ichthyopterygium=, or type of fin, characteristic of many modern
+Elasmobranchs such as _Scyllium_, may have been derived from the
+archipterygium by the gradual reduction of the rays on the postaxial
+side of the axis and their condensation on the pre-axial side. The
+Ichthyotomi such as _Xenacanthus_ show one stage in the reduction of
+the postaxial rays, and a further stage is seen in the Notidanidae and
+some other sharks like _Scymnus_ and _Acanthias_, in which a few
+postaxial rays still remain. The condensation of the pre-axial rays
+when further continued leads to one of the rays getting an attachment
+to the girdle. Thus the fin comes to articulate with the girdle by two
+basalia or basal pieces; a third attachment is formed in the same way
+and the three basalia are called respectively pro-, meso-, and
+meta-pterygia. By some authors the meta-pterygium and by others the
+meso-pterygium is regarded as homologous with the axis of the
+archipterygium.
+
+The pectoral fins of Elasmobranchs vary very much in their mode of
+attachment. In some of the sharks, including the Notidanidae and
+_Scyllium_, all three basalia articulate with the pectoral girdle,
+while in others such as _Cestracion_ the meta-pterygium is excluded.
+In Rays the propterygium and the meta-pterygium are long and narrow
+and diverge much from one another; other basalia work their way in
+between the meso-pterygium and meta-pterygium, and come to articulate
+with the pectoral girdle. Sometimes they fuse and form a second
+meso-pterygium. The radiale are greatly elongated and are segmented.
+
+In _Chimaera_ all three basalia are present, but the meso-pterygium
+is shifted and does not articulate with the pectoral girdle[49].
+
+In _Acipenser_ and _Polyodon_ the pectoral fin is built on the same
+type as in Elasmobranchs, but becomes modified from the fact that the
+propterygium is replaced by dermal bone which forms a large =marginal
+ray=. Extra meso-pterygia are formed in the same way as in Rays.
+
+In _Polypterus_ the pro-and meta-pterygia have ossified while the
+meso-pterygium remains chiefly cartilaginous; the fin-rays are also
+chiefly ossified.
+
+In _Amia_, _Lepidosteus_, and certain Teleosteans like _Salmo_, not
+only the propterygium but the meso-pterygium is almost suppressed by
+the marginal ray.
+
+In the great majority of Teleosteans a still further stage is reached,
+the endoskeletal elements, the basalia and radiale are almost entirely
+suppressed and the fin comes to consist entirely of ossified fin-rays
+of dermal origin.
+
+In some Teleosteans--_Exocaetus_, a herring, and _Dactylopterus_, a
+gurnard--the pectoral fins are so enormously developed that by means
+of them the fish is able to fly through the air for considerable
+distances. The skeleton of these great fins is almost entirely
+composed of dermal bone.
+
+PELVIC FIN.
+
+The pelvic fin is almost always further removed from the
+archipterygial condition, and is in general more modified than is the
+pectoral. Thus in the Ichthyotomi, while the pectoral fins are
+archipterygia similar to those of _Ceratodus_, the pelvic fins consist
+of an axis bearing rays on the postaxial side only, and prolonged
+distally into a clasper. In Dipnoi however the pelvic fins are very
+similar to the pectoral. In Elasmobranchs the meso-pterygium is
+missing, the propterygium is small or absent, and the fin is mainly
+composed of the meta-pterygium (generally called basi-pterygium) and
+its radiale. The males in Elasmobranchii and Holocephali have the
+distal end of the meta-pterygium prolonged into a clasper.
+
+In Ganoids and in Teleosteans the loss of the pelvic girdle causes the
+pelvic fin to be still further removed from the primitive state. There
+is always a large basi-pterygium which lies imbedded in the muscles
+and meets its fellow at its proximal end. In Cartilaginous Ganoids it
+has a secondary segmentation. Its relation to its fellow is subject to
+much variation in Teleosteans, sometimes as in the Perch the two are
+in contact throughout, sometimes as in the Salmon they meet distally
+as well as proximally, but are elsewhere separated by a space,
+sometimes as in the Pike and Bony Ganoids they diverge widely. The
+radiale are articulated to the basi-pterygium. In Cartilaginous
+Ganoids and _Polypterus_ they are well developed, in other Ganoids and
+in Teleosteans they are in the main replaced by dermal fin-rays.
+
+In some Teleosteans such as the Cod the pelvic fins have migrated from
+their usual position and come to be attached to the throat in front of
+the pectoral fins. Fish with this arrangement are grouped together as
+=jugulares=.
+
+
+FOOTNOTES:
+
+[39] The following general works on fishes may be referred to:
+Bashford Dean, _Fishes, Living and Fossil_, New York, 1895. A.
+Günther, _An Introduction to the Study of Fishes_, Edinburgh, 1880.
+A.A.W. Hubrecht and M. Sagemehl, _Fische_ in Bronn's _Classen und
+Ordnungen des Thierreichs_, Band VI. Leipzig, 1876.
+
+[40] See W.G. Ridewood, _Nat. Sci._ vol. VIII. 1896, p. 380. Full
+references are there given to the literature of the subject.
+
+[41] See H. Gadow and E.C. Abbott, _Phil. Trans._ vol. 186 (1895) B.
+pp. 163-221.
+
+[42] C. Hasse, _Zeitschr. wiss. Zool._ LVII. 1893, p. 76.
+
+[43] C. Hasse, _Das natürliche System der Elasmobranchier auf
+Grundlage des Baues und der Entwickelung ihrer Wirbelsäule_, Jena,
+1879 and 1885, and "Die fossilen Wirbel, Morph. Studien I.-IV.,"
+_Morphol. Jahrb. Bd._ II., III. and IV. 1876-78.
+
+[44] See H.B. Pollard, _Anat. Anz._ X. 1894.
+
+[45] T.W. Bridge, "The Cranial Osteology of _Amia calva_," _J. Anat.
+Physiol. norm. path._ 1876, vol. XI. p. 605. R. Shufeldt, "The
+Osteology of _Amia calva_," _Ann. Rep. of the Commissioner for Fish
+and Fisheries_, Washington, 1885.
+
+[46] A. Smith Woodward, _Nat. Sci._ vol. I. 1892, p. 28. Further
+references are here given on the literature of the subject.
+
+[47] C. Gegenbaur, Ueber das Archipterygium, _Jena Zeitschr. der
+Wirbelthiere_, 2^e Heft, 1873, vol. 7, and _Morphol. Jahrb._ XXII.
+1894, p. 119.
+
+[48] The fins of _Ceratodus_ are very variable, no two being exactly
+alike. Sometimes even the main axis bifurcates. See W.A. Haswell,
+_Linn. Soc. N. S. Wales_, vol. VII. 1882.
+
+[49] Some of these views with regard to the homologies of the parts of
+the fins are not accepted by all anatomists.
+
+
+
+
+CHAPTER IX.
+
+CLASS II. AMPHIBIA[50].
+
+
+AMPHIBIA differ markedly from Pisces in the fact that in the more
+abundant and familiar forms the skin is naked, and that when the
+integument is prolonged into median fins they are devoid of fin-rays.
+The notochord may persist, but bony vertebral centra are always
+developed. These are sometimes biconcave, sometimes procoelous,
+sometimes opisthocoelous. There is only one sacral vertebra, except in
+rare cases. The cartilaginous cranium persists to a considerable
+extent but is more or less replaced by cartilage bone, and overlain by
+membrane bone. The basi-occipital is not completely ossified, and the
+skull articulates with the vertebral column by means of two occipital
+condyles formed by the exoccipitals.
+
+There is a large parasphenoid, but there are no ossifications in the
+basisphenoidal, presphenoidal, and alisphenoidal regions. In most
+cases the epi-otics and opisthotics are ossified continuously with the
+exoccipitals.
+
+The palato-pterygo-quadrate bar is firmly united to the cranium, so
+the skull is autostylic. The palatines and pterygoids are membrane
+bones. Teeth are nearly always borne on the vomers and commonly on the
+maxillae and premaxillae. There are no sternal ribs, and the sternum is
+very intimately related to the pectoral girdle. There are no obturator
+foramina. The limbs are as in the higher vertebrata, divisible into
+upper arm, fore-arm, and manus (wrist and hand), and into thigh, shin,
+and pes (ankle and foot) respectively. The posterior limb is, as a
+rule, pentedactylate, but in nearly every case the pollex is vestigial
+or absent.
+
+
+_Order 1._ URODELA[51].
+
+The Urodela are elongated animals with a naked skin, a persistent
+tail, and generally four short limbs.
+
+The vertebral centra are opisthocoelous or biconcave, and there are
+numerous precaudal vertebrae. Portions of the notochord commonly
+persist in the intervertebral spaces. In the skull there is no
+sphenethmoid forming a ring encircling the anterior end of the brain,
+its place being in many cases partly taken by a pair of
+orbitosphenoids. There is no quadratojugal, and the quadrate is more
+or less ossified. The mandible has a distinct splenial, and the
+articular is ossified.
+
+There is no definite tympanic cavity. The hyoid apparatus is
+throughout life connected to the quadrate by ligament, and a large
+basilingual plate does not occur. The ribs are short structures with
+bifurcated proximal ends. In the pelvis the pubis remains
+cartilaginous, and there is a bifid cartilaginous epipubis. The bones
+of the fore-arm and shin remain distinct, and the manus never has more
+than four digits.
+
+_Suborder_ (1). ICHTHYOIDEA.
+
+The vertebrae are amphicoelous, but the notochord remains but little
+constricted throughout the whole length of the vertebral column. Three
+or four branchial arches nearly always persist in the adult. The
+cartilages of the carpus and tarsus remain unossified.
+
+The Ichthyoidea may be subdivided again into two groups:--
+
+A. _Perennibranchiata_, whose chief distinguishing skeletal characters
+are that the skull is elongated, the premaxillae are not ankylosed, the
+maxillae are vestigial or absent; there are sometimes no nasals, and
+the palatines bear teeth;
+
+ e.g. _Siren_, _Proteus_, _Menobranchus_.
+
+B. _Derotremata_, whose chief distinguishing skeletal characters are
+that there are large maxillae and nasals; teeth are borne by both
+maxillae and premaxillae; there are no palatines; and both pectoral and
+pelvic limbs are always present;
+
+ e.g. _Amphiuma_, _Megalobatrachus_, _Cryptobranchus_.
+
+_Suborder_ (2). SALAMANDRINA.
+
+The vertebrae are opisthocoelous. The skull is broad, and teeth are
+borne by both premaxillae and dentaries. Nasal bones are present. The
+remains of only two branchial arches are found in the adult. The
+carpus and tarsus are more or less ossified.
+
+This suborder includes the Newts (_Molge_), Salamanders
+(_Salamandra_), and _Amblystoma_.
+
+
+_Order_ 2. LABYRINTHODONTIA[52].
+
+These are extinct Amphibia with a greatly developed dermal
+exoskeleton, which is generally limited to the ventral surface. The
+body and tail are long and in some cases limbs are absent. The teeth
+are pointed and often have the dentine remarkably folded. The
+vertebrae are amphicoelous, and are generally well ossified. The skull
+is very solid, and has a greatly-developed secondary roof which hides
+the true cranium and is very little broken up by fossae. Paired
+dermal supra-occipitals are found, and there is an interparietal
+foramen. The epi-otics and opisthotics form a pair of bones distinct
+from the exoccipitals. Four simple limbs of moderate length are
+generally present, and in some cases all four limbs are
+pentedactylate. Among the better known genera of Labyrinthodonts are
+_Mastodonsaurus_, _Nyrania_, and _Archegosaurus_.
+
+
+_Order 3._ GYMNOPHIONA[53].
+
+These animals form a group of abnormal worm-like Amphibia having an
+exoskeleton in the form of subcutaneous scales arranged in rings. The
+vertebrae are biconcave and are very numerous; very few however belong
+to the tail. The skull has a complete secondary bony roof, the
+mandible bears teeth and has an enormous backward projection of the
+angular. The hyoid arch has very slender cornua and no distinct body,
+it is attached neither to the cranium nor to the suspensorium. The
+ribs are very long and there are no limbs or limb girdles.
+
+
+_Order 4._ ANURA.
+
+These are tailless Amphibia, which except in a few instances, are
+devoid of an exoskeleton. The vertebrae are as a rule procoelous, and
+are very few in number. The post-sacral part of the spinal column
+ossifies continuously, forming an unsegmented cylindrical rod, the
+urostyle. Remains of the notochord persist, lying _vertebrally_, i.e.
+enclosed within the centra of the several vertebrae, and not as in
+Urodela lying between one vertebra and the next. The skull is very
+short and wide. The mandible is almost always, if not invariably,
+toothless.
+
+The frontals and parietals on each side are united so as to form a
+pair of fronto-parietals, and a girdle-like sphenethmoid is present.
+
+The quadrate is not generally ossified. A predentary or
+mento-meckelian bone is commonly present in the mandible, and a single
+bone represents the angular and splenial. The branchial arches are
+much reduced in the adult, and the distal ends of the cornua unite to
+form a flat basilingual plate of a comparatively large size.
+
+Ribs are very little developed. Clavicles are present. The ilia are
+very greatly elongated. The anterior limb has four well-developed
+digits and a vestigial pollex, and is of moderate length; the radius
+and ulna have fused. The posterior limb is greatly elongated and is
+pentedactylate; the tibia and fibula are fused, while the calcaneum
+and astragalus are greatly elongated, and it is largely owing to them
+that the length of the limb is so great. The group includes the Frogs
+and Toads, the predominant Amphibia of the present time.
+
+
+FOOTNOTES:
+
+[50] T.H. Huxley, _Amphibia_ (_Encyclopaedia Britannica_).
+
+[51] See R. Wiedersheim, _Morphol. Jahrb._ Bd. III. 1877, p. 459.
+
+[52] See A. Fritsch, _Fauna der Gaskohle_, Prague, 1883-85-86, also
+writings of Cope, Credner, Huxley, H. v. Meyer, Miall.
+
+[53] See R. Wiedersheim, _Anatomie der Gymnophionen_, Jena, 1879.
+
+
+
+
+CHAPTER X.
+
+THE SKELETON OF THE NEWT (_Molge cristata_).
+
+
+I. EXOSKELETON.
+
+The skin of the Newt is quite devoid of any exoskeletal structures.
+The only exoskeletal structures that the animal possesses are the
+teeth, and these are most conveniently described with the
+endoskeleton.
+
+
+II. ENDOSKELETON.
+
+The endoskeleton of the Newt, though ossified to a considerable
+extent, is more cartilaginous than is that of the frog. It is
+divisible into an =axial portion= including the vertebral column,
+skull, ribs, and sternum, and an =appendicular portion= including the
+skeleton of the limbs and their girdles.
+
+1. THE AXIAL SKELETON.
+
+A. THE VERTEBRAL COLUMN.
+
+This consists of about fifty vertebrae arranged in a regular
+continuous series. The first vertebra differs a good deal from any of
+the others; the seventeenth or sacral vertebra and the eighteenth or
+first caudal also present peculiarities of their own. The remaining
+vertebrae are divided by the sacrum into an anterior series of =trunk=
+vertebrae which bear fairly large ribs, and a posterior series of
+=caudal= vertebrae, all of which except the first few are ribless.
+
+THE TRUNK VERTEBRAE.
+
+Any vertebra from the second to the sixteenth may be taken as a type
+of the trunk vertebrae.
+
+The general form is elongated and somewhat hour-glass shaped, and the
+=centra= are convex in front and concave behind; an opisthocoelous
+condition such as this is quite exceptional in Anura. The =notochord=
+may persist intervertebrally[54], but in the centre of each vertebra
+it becomes greatly constricted or altogether obliterated, and replaced
+by marrow. The superficial portion of the centrum is ossified, while
+the articular surfaces are cartilaginous. The =neural arches= are low
+and articulate together by means of =zygapophyses= borne on short
+diverging processes. The anterior zygapophyses look upwards, the
+posterior downwards. Each neural arch is drawn out dorsally into a
+very slight cartilaginous =neural spine=.
+
+On each centrum, at a little behind the middle line, there arise a
+pair of short backwardly-directed =transverse processes=; each of
+which becomes divided into two slightly divergent portions, a dorsal
+portion which meets the tubercular process of the rib and is derived
+from the neural arch, and a ventral portion which meets the capitular
+process of the rib and is derived from the ventral or haemal arch. The
+division between these two parts of the transverse processes can be
+traced back as far as the sacrum.
+
+The =first vertebra= as already mentioned differs much from all the
+others. It has no ribs, and presents anteriorly two slightly divergent
+concave surfaces which articulate with the occipital condyles of the
+skull. Between these surfaces the dorsal portion of the anterior face
+of the centrum is drawn out into a prominent =odontoid process=, the
+occurrence of which renders it probable that the first vertebra of
+the newt is really the axis, and that the atlas with the exception of
+the odontoid process has become fused with the skull. The sacral
+vertebra or =sacrum= differs from the vertebrae immediately in front
+of it only in the fact that its transverse processes are stouter and
+more obviously divided into dorsal and ventral portions.
+
+THE CAUDAL VERTEBRAE.
+
+The =caudal vertebrae= are about twenty-four in number. The anterior
+ones have hour-glass shaped centra, and short backwardly-directed
+transverse processes. The middle and posterior ones have rather
+shorter centra, and are without transverse processes. The neural
+arches resemble those of the trunk vertebrae, but each is drawn out
+into a rather high cartilaginous neural spine abruptly truncated
+anteriorly. All the caudal vertebrae except the first have also a
+haemal arch, which is very similar to the neural arch, and is drawn
+out into a haemal spine quite similar to the neural spine. Both neural
+and haemal arches are ossified continuously with the centra.
+
+B. THE SKULL.
+
+The skull of the newt is divisible into three principal parts:--
+
+(1) an axial part, the =cranium proper=, which encloses the brain and
+to which
+
+(2) the =capsules= of the =auditory and olfactory sense organs= are
+fused;
+
+(3) the skeleton of the =jaws and hyoid apparatus=. The skull is much
+flattened and expanded, though not so much as in the frog.
+
+(1) THE CRANIUM PROPER.
+
+The =cranium proper= or =brain case= is an unsegmented tube which
+remains partly cartilaginous, and is partly converted into cartilage
+bone, partly sheathed by membrane bone. The roof and floor of the
+cartilaginous cranium are, as is the case also in the frog, pierced
+by holes or fontanelles, and these are so large that the main part of
+the roof and floor comes to be formed by membrane bone.
+
+Two pairs of large ossifications take place in the cranial walls. Of
+these the more posterior on each side represents the =exoccipital= and
+all three =periotic= bones. It bears a small convex patch of cartilage
+for articulation with the atlas, and with its fellow forms the
+boundary of the foramen magnum.
+
+Two foramina pierce the exoccipital just in front of the occipital
+condyle and transmit respectively the glossopharyngeal and
+pneumogastric (fig. 21, X) nerves. Lying laterally to these nerve
+openings is seen a patch of cartilage, the =stapes=, which is
+homologous with the stapes or proximal element of the columellar chain
+in the frog. Further forward in front of the stapes is the small
+opening for the exit of the facial nerve, and seen in a lateral view
+close to the orbitosphenoid, that for the trigeminal (fig. 21, C, 5).
+
+In front of these large bones the lateral parts of the cranial walls
+remain cartilaginous for a short distance, and then there follow two
+elongated bones, the =orbitosphenoids= (fig. 21, B and C, 11), pierced
+by the foramina for the exit of the optic nerves. These bones partly
+correspond to the sphenethmoid of the frog.
+
+The _membrane bones_ connected with the cranium are the _parietals_,
+_frontals_ and _prefronto-lachrymals_ on the dorsal surface, and the
+_parasphenoid_ on the ventral surface.
+
+The _parietals_ (fig. 21, A and C, 6) roof over the posterior part of
+the great dorsal fontanelle and overlap the exoccipito-periotics. They
+meet one another along a sinuous suture in the middle line, as do also
+the _frontals_ which overlap them in front. The _frontals_ and
+_parietals_ both extend for a short distance down the sides of the
+cranium and meet the orbitosphenoids. The _prefronto-lachrymals_ (fig.
+21, A and C, 7) connect the frontals with the maxillae.
+
+[Illustration FIG. 21. A DORSAL, B VENTRAL, AND C LATERAL VIEWS OF THE
+SKULL OF A NEWT (_Molge cristata_) × 2-1/2 (after PARKER).
+
+The cartilage is dotted, the cartilage bones are marked with dots and
+dashes, the membrane bones are left white.
+
+ 1. premaxillae.
+ 2. anterior nares.
+ 3. posterior nares.
+ 4. nasal.
+ 5. frontal.
+ 6. parietal.
+ 7. prefronto-lachrymal.
+ 8. maxillae.
+ 9. vomero-palatine.
+ 10. parasphenoid.
+ 11. orbitosphenoid.
+ 12. pterygoid.
+ 13. squamosal.
+ 14. pro-otic region of exoccipito-periotic.
+ 15. quadrate.
+ 16. quadrate cartilage.
+ 17. exoccipital region of exoccipito-periotic.
+ 18. articular.
+ 19. articular cartilage.
+ 20. dentary.
+ 21. splenial.
+ 22. middle narial passage.
+ II. V. VII. IX. X. foramina for the exit of cranial nerves.]
+
+On the ventral surface is the large _parasphenoid_ (fig. 21, B, 10),
+which is widest behind and overlapped anteriorly by the
+vomero-palatines.
+
+(2) THE SENSE CAPSULES.
+
+The =auditory capsules= become almost completely ossified continuously
+with the exoccipitals; they have been already described.
+
+The =nasal capsules= are large and quite unossified though they are
+overlain by membrane bone. They appear on the dorsal surface between
+the anterior nares and the nasal process of the premaxillae. They
+enclose the nasal organs, bound the inner side of the anterior narial
+opening, and are connected with one another posteriorly by a
+cartilaginous area.
+
+Developed in connection with the nasal capsules are a pair of rather
+large _nasals_ (fig. 21, A and C, 4), which lie on the dorsal surface
+immediately in front of the frontals. Each forms part of the posterior
+boundary of one of the anterior nares, and the two are separated from
+one another in the middle line by the nasal process of the premaxillae
+(fig. 21, A, 1), and the opening of the =middle narial passage= (fig.
+21, A and B, 22), which passes right through the skull.
+
+On the ventral surface of the skull and forming the greater part of
+the boundary of the posterior nares are two large bones, the
+_vomero-palatines_ (fig. 21, B and C, 9). Each consists of a wide
+anterior portion, partly separated from its fellow in the middle line
+by the ventral opening of the middle narial passage, and of a long
+pointed posterior portion which is separated from its fellow by the
+_parasphenoid_, and bears a row of small pointed teeth formed of
+dentine capped with enamel.
+
+(3) THE JAWS.
+
+The =upper jaw= of the newt is a discontinuous structure divided into
+two parts, an anterior part which consists of membrane bones, the
+_maxillae_ and _premaxillae_, and a posterior part which remains mainly
+cartilaginous.
+
+The _premaxillae_ are united, forming a single bone, which in a
+ventral view is seen to meet the maxillae and vomero-palatines, and in
+a dorsal view to send back a nasal process (fig. 21, A, 1) between the
+nasals.
+
+The _maxillae_ are large bones, each terminating in a point
+posteriorly. A single row of teeth similar to those on the
+vomero-palatines runs along the outer margin of the maxillae and
+premaxillae.
+
+The posterior part of the upper jaw forms a mass of cartilage which
+extends forwards towards the maxillae as a long pointed process whose
+ventral surface and sides are overlapped by a membrane bone, the
+_pterygoid_ (fig. 21, 12).
+
+The suspensorial bones include the =quadrate= and _squamosal_. The
+=quadrate= (fig. 21, 15) which forms the true =suspensorium= is
+directed forwards and outwards, and is terminated by a patch of
+cartilage with which the mandible articulates.
+
+The lower jaw or mandible remains partly cartilaginous, while its
+ossifications include two membrane bones and one cartilage bone. The
+cartilage bone is the =articular= (fig. 21, C, 18), it forms the
+posterior part of the ramus, extends forwards for some distance along
+its inner side, and is terminated posteriorly by a patch of cartilage
+which articulates with the quadrate. The _dentary_ (fig. 21, C, 20) is
+a large bone which forms the anterior part and nearly all the outer
+half of each ramus, and bears teeth similar to those of the upper jaw.
+Attached to its inner face is a long slender _splenial_ (fig. 21, C,
+21).
+
+THE HYOID APPARATUS.
+
+This consists of the hyoid arch and part of the first two branchial
+arches.
+
+The =hyoid arch= (fig. 29, A, 2) consists of a pair of =cornua=, each
+of which is divided into two halves. The dorsal half forming the
+=cerato-hyal= is mainly ossified though tipped with cartilage, and is
+connected by ligament with the suspensorium. The ventral half
+(=hypo-hyal=) is cartilaginous, and is connected with the
+basibranchial.
+
+The =branchial arches= consist of a median piece, the =basibranchial=,
+which is ossified in the centre and cartilaginous at either end, and
+of two pairs of =cerato-branchials= which are attached to the
+cartilaginous part (fig. 29, A, 8) of the basibranchial. The first
+cerato-branchial is chiefly ossified, the second (fig. 29, A, 4) is a
+good deal smaller and is cartilaginous. Both are united dorsally to a
+single =epi-branchial=, which is terminated by a small cartilaginous
+area at the free end but is elsewhere well ossified.
+
+C. THE RIBS.
+
+The ribs are short imperfectly ossified structures, bifid at their
+proximal end where they articulate with the transverse processes, and
+tipped both proximally and distally with cartilage. The dorsal portion
+of the proximal end corresponds to the =tuberculum= of the ribs of
+higher animals, and the ventral portion to the =capitulum=. Some of
+the anterior ribs have a step-like notch on their dorsal surfaces.
+
+The second to twelfth ribs are fairly equal in size, but further back
+they decrease slightly. The ribs which connect the sacral vertebrae
+with the ilia are however large. The short ribs borne on the anterior
+caudal vertebrae are cartilaginous.
+
+D. THE STERNUM.
+
+The sternum (fig. 22, A, 6) is a rather broad plate of cartilage,
+drawn out posteriorly into a median process marked by a prominent
+ridge. On its antero-lateral margins it bears surfaces for
+articulation with the pectoral girdle.
+
+2. THE APPENDICULAR SKELETON.
+
+A. THE PECTORAL GIRDLE.
+
+This is of a very simple character, and remains throughout life in an
+imperfectly ossified condition. It consists of a dorsal =scapular
+portion=, and a ventral =coracoid portion= partially divided into an
+anterior part, the precoracoid, and a posterior part, the =coracoid=.
+
+[Illustration FIG. 22. A VENTRAL, AND B LATERAL VIEW OF THE SHOULDER
+GIRDLE AND STERNUM OF AN OLD MALE CRESTED NEWT (_Molge cristata_) × 3
+(after PARKER).
+
+ 1. scapula.
+ 2. suprascapula.
+ 3. coracoid.
+ 4. glenoid cavity.
+ 5. precoracoid.
+ 6. sternum.]
+
+The =scapular portion= is a slightly curved oblong plate; its proximal
+third the =scapula= (fig. 22, 1) is ossified and bounds part of the
+well-marked =glenoid cavity= (fig. 22, 4); its distal portion forms a
+large oblong cartilaginous plate, the =suprascapula= (fig. 22, 2).
+
+The =precoracoid= (fig. 22, 5) forms a small forwardly-directed
+cartilaginous plate. The =coracoid= (fig. 22, 3) forms a much larger
+plate, the greater part of which is unossified and overlaps its fellow
+in the middle line, the two being overlapped by the sternum. Around
+the glenoid cavity is an area which is mainly ossified and is
+continuous with the scapula.
+
+B. THE ANTERIOR LIMB.
+
+This is divisible into three parts, the =upper arm= or =brachium=, the
+=fore-arm= or =antibrachium=, and the =manus=.
+
+The =upper arm= includes a single bone, the =humerus=.
+
+The =humerus= is a slender bone cylindrical in the middle and expanded
+at either end, the proximal part forms a rounded =head= which
+articulates with the glenoid cavity. Along the proximal part of the
+anterior or pre-axial surface runs a strong =deltoid ridge=. The
+proximal part of the postaxial surface also bears a small outgrowth.
+
+The =fore-arm= contains two bones, the =radius= and =ulna=, both of
+which are small and imperfectly ossified at their terminations.
+
+The =radius= (fig. 23, B, 11) or pre-axial bone is rather the larger
+of the two, and is considerably expanded at its proximal end. The
+=ulna= or postaxial bone is somewhat expanded distally, but is not
+drawn out proximally into an olecranon process.
+
+The =manus= consists of two parts, a group of small bones forming the
+=carpus= or =wrist=, and the =hand=.
+
+The =carpus= is in a very simple unmodified condition as compared with
+that of the Frog. It consists of a proximal row of two bones and a
+distal row of four, with one, the =centrale=, interposed between. All
+these bones are small and polygonal and are imbedded in a plate of
+cartilage.
+
+The bones of the proximal row are a smaller pre-axial bone, the
+=radiale= (fig. 23, B, 13), and a larger postaxial bone, which
+represents the fused =ulnare= and =intermedium= of the very simple
+carpus described on pp. 26 and 27.
+
+The four bones of the distal row are respectively =carpalia= 2, 3, 4
+and 5.
+
+The =hand= consists of four digits, that corresponding to the thumb of
+the human hand, judging from the analogy of the frog probably being
+the one that is absent.
+
+[Illustration FIG. 23. A RIGHT POSTERIOR, AND B RIGHT ANTERIOR LIMB OF
+A NEWT × 1-1/2 (_Molge cristata_).
+
+ 1. femur.
+ 2. tibia.
+ 3. fibula.
+ 4. tibiale.
+ 5. intermedium.
+ 6. fibulare.
+ 7. centrale of tarsus.
+ 8. tarsale 1.
+ 9. tarsalia 4 and 5 fused.
+ 10. humerus.
+ 11. radius.
+ 12. ulna.
+ 13. radiale.
+ 14. intermedium and ulnare fused.
+ 15. centrale of carpus, the pointing line passes across carpale 2.
+ 16. carpale 3.
+ 17. carpale 5.
+ I. II. III. IV. V. digits.]
+
+Each digit consists of a somewhat elongated =metacarpal= and of two or
+three phalanges. The metacarpals are contracted in the middle and
+expanded at either end. They are connected with the carpus by
+cartilage, and the articulations between the several phalanges, and
+between the metacarpals and phalanges are also cartilaginous. The
+second, third, and fifth digits have two phalanges apiece, the fourth,
+which is the longest, has three. The second metacarpal in the
+specimen examined and figured articulates partly with carpale 2,
+partly with carpale 3.
+
+C. THE PELVIC GIRDLE.
+
+The pelvic girdle of the Newt is in a much less modified condition
+than is that of the Frog (see p. 165). It consists of a dorsal
+element, the =ilium=, a posterior ventral element, the =ischium=, and
+an anterior ventral element, the =pubis=, to which is attached an
+=epipubis=.
+
+The =ilium= is a somewhat cylindrical bone which at its ventral end
+meets the ischium, and forms part of the =acetabulum=. It is then
+directed upwards and slightly backwards, and is attached to the ribs
+of the sacral vertebra.
+
+The =ischia= are a pair of somewhat square bones which meet one
+another in the middle line; they form part of the acetabulum, and are
+united to the ilia above.
+
+In front of the ischia is a narrow cartilaginous area which represents
+the =pubes=. Projecting forwards from it is a bifid cartilaginous
+=epipubis=.
+
+D. THE POSTERIOR LIMB.
+
+This is divisible into a proximal portion, the =thigh=, a middle
+portion, the =crus= or =shin=, and a distal portion, the =pes=.
+
+The =thigh= consists of a single bone, the =femur= (fig. 23, A, 1),
+which has a thin shaft and expanded ends. The anterior part of the
+pre-axial border and posterior part of the postaxial border bear
+slight outgrowths.
+
+The =crus= or =shin= includes two short bones, the =tibia= and
+=fibula=, which are nearly equal in length. The pre-axial bone or
+tibia is a straight bone thickest at its proximal end, the postaxial
+bone or =fibula= (fig. 23, A, 3) is a rather stouter curved bone of
+nearly equal diameter throughout.
+
+The =pes= includes the =tarsus= or =ankle=, and the =foot=.
+
+The =tarsus= consists of eight small bones arranged in a proximal row
+of three, the =tibiale=, =intermedium= and =fibulare=, and a distal
+row of four =tarsalia=, with one bone, the =centrale= (fig. 23, A, 7),
+interposed between the two rows. In the specimen examined, the
+=tibiale=, is a small bone articulating with the tibia, the
+=intermedium= (fig. 23, A, 5) is larger and articulates with both
+tibia and fibula, the =fibulare= is the largest of the three and
+articulates with the fibula.
+
+The bones of the distal row are =tarsalia 1=, =2=, =3=, and a bone
+representing =4= and =5= fused. In the specimen examined tarsale 1 is
+pushed away dorsally (fig. 23, A, 8), so as to lie between the tibiale
+and tarsale 2. All the tarsal bones are small and somewhat polygonal,
+and are connected with one another, and with the tibia and fibula on
+the one hand, and with the metatarsals on the other by a thin layer of
+cartilage.
+
+The five =digits= of the foot each consist of a =metatarsal= and of a
+certain number of =phalanges=. In the specimen examined, owing to the
+shifting of tarsale 1, the first metatarsal as well as the second
+articulates with tarsale 2, while the fifth metatarsal articulates
+partially with the bone representing the fused tarsalia 4 and 5,
+partially with the fibulare. All the bones of the digits except the
+distal phalanges are terminated at each end by cartilaginous
+epiphyses, the distal phalanx of each digit has a cartilaginous
+epiphysis only on its proximal end.
+
+The first, second, and fifth digits have two phalanges apiece, the
+third and fourth have three.
+
+Figure 31 B, showing a Newt's tarsus copied from Gegenbaur, has
+precisely the arrangement generally regarded as primitive for the
+higher vertebrates, except that tarsalia 4 and 5 are fused.
+
+
+FOOTNOTES:
+
+[54] i.e. between one vertebra and the next.
+
+
+
+
+CHAPTER XI.
+
+THE SKELETON OF THE FROG[55] (_Rana temporaria_).
+
+
+I. EXOSKELETON.
+
+The skin of the frog is smooth and quite devoid of scales or other
+exoskeletal structures. The only exoskeletal structures met with in
+the frog are:--
+
+1. The =teeth=, which are most conveniently described with the
+endoskeleton.
+
+2. The horny covering of the calcar or prehallux (see p. 167).
+
+
+II. ENDOSKELETON.
+
+The endoskeleton of the adult frog consists partly of cartilage,
+partly of bone and each of these types of tissue occurs in two forms.
+The cartilage may be hyaline, as in the omosternum and xiphisternum,
+or may be more or less calcified as in part of the suprascapula and
+the epiphyses of the limb bones. The bone may be cartilage bone, or
+membrane bone.
+
+The skeleton is divisible into an =axial portion= consisting of the
+skull, vertebral column, and sternum, and an =appendicular portion=
+consisting of the skeleton of the limbs and their girdles.
+
+1. THE AXIAL SKELETON.
+
+A. THE VERTEBRAL COLUMN.
+
+The vertebral column is a tube, formed of a series of ten bones which
+surround and protect the spinal cord. Of these ten bones nine are
+vertebrae, while the tenth is a straight rod, the =urostyle=, and is
+almost as long as all the vertebrae put together. The second to eighth
+vertebrae inclusive have a very similar structure, but the first and
+ninth differ from the others.
+
+Any one of the second to eighth vertebrae forms a bony ring with a
+somewhat thickened floor, the =centrum= or body, which articulates
+with the centra of the immediately preceding and succeeding vertebrae.
+The articulating surfaces are covered with cartilage and are
+procoelous, or convex in front and concave behind. The eighth vertebra
+is however amphicoelous or biconcave. The centrum of each vertebra
+encloses an isolated vestige of the notochord. The =neural arch= forms
+the roof and sides of the neural canal, which is very spacious in the
+anterior vertebrae, but becomes more depressed in the posterior ones.
+The arch bears the =neural spine=, a low median ridge of variable
+character, and is drawn out in front and behind, forming the two pairs
+of articulating surfaces or =zygapophyses= by means of which the
+vertebrae are attached together. Of these the anterior articulating
+surfaces or =prezygapophyses= look upwards and slightly inwards, while
+the posterior articulating surfaces or =postzygapophyses= look
+downwards and slightly outwards. The sides of the neural arches are
+drawn out into a pair of prominent =transverse processes=. Those of
+the second vertebra look somewhat forwards, those of the third look
+directly outwards or somewhat forwards, while those of the fourth,
+fifth, and sixth are directed slightly backwards, and those of the
+seventh and eighth nearly straight outwards. All the transverse
+processes are terminated by very small cartilaginous =ribs=.
+
+SPECIAL VERTEBRAE.
+
+The =first vertebra= is a ring-like structure with a much depressed
+centrum. It bears in front two oval concave surfaces for articulation
+with the condyles of the skull, while the centrum is terminated behind
+by a prominent convex surface. There are as a rule no transverse
+processes, and the postzygapophyses look downwards and outwards.
+Occasionally however transverse processes do occur. Projecting
+forwards from the centrum is a minute process better developed in the
+Newt. This resembles an odontoid process, and it has hence been
+supposed that the first vertebra is homologous with the axis of
+mammalia, and that the atlas of the frog is fused with the skull.
+
+The =ninth vertebra= has very stout transverse processes directed
+backwards and somewhat upwards. They articulate with the pelvic girdle
+and hence this vertebra is regarded as the =sacrum=. The neural arch
+is much depressed, the centrum is convex in front and bears on its
+posterior surface two short rounded processes for articulation with
+the urostyle.
+
+The =urostyle= is a long rod-like bone forming the posterior
+unsegmented continuation of the vertebral column. It is probably
+equivalent to three vertebrae, the tenth, eleventh, and twelfth fused
+together, and to an unsegmented rod of cartilage which lies ventral to
+the notochord. The anterior end is expanded and bears two concave
+articular surfaces by means of which it articulates with the sacrum. A
+prominent ridge runs along the dorsal surface, but gradually
+diminishes when traced back. The anterior portion contains a canal
+which is a continuation of the neural canal. At a point not far from
+the anterior end, this canal communicates with the exterior by a pair
+of minute holes which correspond with the intervertebral foramina.
+
+B. THE SKULL[56].
+
+The skull of the Frog consists of three principal parts:--
+
+(1) an axial part, the =cranium proper=, which encloses the brain. To
+it are firmly fused
+
+(2) the =capsules of the olfactory and auditory sense organs=,
+
+(3) lastly there is the =hyoid apparatus= and the =skeleton of the
+jaws=.
+
+The skull is by no means so completely ossified as is the vertebral
+column, but in addition to the cartilage bone, there is a great
+development of membrane bone in connection with it.
+
+The skull has a peculiarly flattened and expanded form depending on
+the wide lateral separation of the jaws from the cranium.
+
+
+(1) THE CRANIUM PROPER or Brain case.
+
+This is an unsegmented tube, which is widest behind. It remains to a
+considerable extent cartilaginous, but is partly converted into
+cartilage bone, partly sheathed in membrane bone. Its roof is
+imperfect, being pierced by three holes or =fontanelles=, one large
+anterior fontanelle (fig. 25, A, 9), and two smaller posterior
+fontanelles (fig. 25, A, 10).
+
+The cartilage bones of the cranium proper are the two =exoccipitals=
+and the =sphenethmoid=.
+
+The =exoccipitals= (figs. 24, 25, and 26, 6) are a pair of irregular
+bones bounding the foramen magnum at the posterior end of the skull.
+They almost completely surround the foramen magnum, and bear a pair of
+oval convex surfaces, the =occipital condyles=, with which the first
+vertebra articulates. The bones generally called the exoccipitals of
+the frog include the =epi-otic= and =opisthotic= elements of many
+skulls, in addition to the exoccipitals.
+
+[Illustration FIG. 24. A DORSAL, AND B VENTRAL VIEWS OF THE CRANIUM OF
+A COMMON FROG (_Rana temporaria_) × 2 (after PARKER).
+
+In this and the next two figs. cartilage is dotted, cartilage bones
+are marked with dots and dashes, membrane bones are left white.
+
+ 1. sphenethmoid.
+ 2. fronto-parietal.
+ 3. pterygoid.
+ 4. squamosal.
+ 6. exoccipital.
+ 7. parasphenoid.
+ 8. pro-otic.
+ 9. quadratojugal.
+ 10. maxillae.
+ 11. nasal.
+ 12. premaxillae.
+ 13. anterior nares.
+ 14. vomer.
+ 15. posterior nares.
+ 16. palatine.
+ 18. columella.
+ 19. quadrate.
+ 20. occipital condyle.
+ II. optic foramen.
+ V. VII. foramen for exit of trigeminal and facial nerves.
+ IX. X. foramina for exit of glossopharyngeal and pneumogastric
+ nerves.]
+
+The patch of unossified cartilage immediately external to the
+occipital condyle is pierced by two small foramina, through which the
+ninth and tenth nerves leave the cranial cavity. The ninth nerve
+passes through the more external of these foramina, the tenth through
+the one nearer the condyle. The foramina lie however very close
+together and are sometimes confluent. The cranial walls for a
+considerable distance in front of the occipitals are unossified, but
+the anterior end of the cranial cavity is encircled by another
+cartilage bone, the =sphenethmoid= (figs. 24 and 25, 1) or girdle
+bone. This partly corresponds to the orbitosphenoids of the Newt's
+skull. Anteriorly it is pierced by a pair of small foramina through
+which the ophthalmic branches of the trigeminal nerve pass out.
+
+The anterior part of the cranial cavity is divided into two halves by
+a vertical plate, the =mesethmoid=. Some little distance behind the
+sphenethmoid the ventro-lateral walls of the cartilaginous cranium are
+pierced by a pair of rather prominent holes, the =optic foramina=
+(figs. 24 and 25, B, II), and at a similar distance further back,
+occupying a kind of notch in the pro-otic are the large =trigeminal
+foramina=, through which the fifth and seventh nerves leave the
+cranium. Between the trigeminal and optic foramina are the very small
+foramina for the sixth nerves (fig. 25, B, VI).
+
+The _membrane bones_ of the cranium proper include the
+_fronto-parietals_ and the _parasphenoid_.
+
+The _fronto-parietals_ (figs. 24 and 26, A, 2) form a pair of long
+flat bones closely applied to one another in the middle line, the line
+of junction being the =sagittal suture=. They cover over the
+fontanelles and overlap the sphenethmoid in front.
+
+The _parasphenoid_ (figs. 24 and 26, B, 7) is a bone shaped like a
+dagger with a very short handle. It lies on the ventral surface of the
+cranium, the blade being directed forwards and underlying the
+sphenethmoid; its lateral processes underlie the auditory capsules.
+
+
+(2) THE SENSE CAPSULES.
+
+The sense capsules are cartilaginous or bony structures which surround
+the olfactory and auditory organs and are closely united to the
+cranium.
+
+[Illustration FIG. 25. A DORSAL AND B VENTRAL VIEW OF THE CRANIUM OF A
+COMMON FROG (_Rana temporaria_) from which the membrane bones have
+mostly been removed. × 2 (after PARKER).
+
+ 1. sphenethmoid.
+ 2. palatine.
+ 3. pterygoid.
+ 4. quadrate.
+ 5. columella.
+ 6. exoccipital.
+ 7. ventral cartilaginous wall of cranium.
+ 8. pro-otic.
+ 9. anterior fontanelle.
+ 10. right posterior fontanelle.
+ 11. quadratojugal.
+ 12. nasal capsule.
+ II. V. VI. IX. X. foramina for exit of cranial nerves.]
+
+The =auditory capsules= are fused with the sides of the posterior
+end of the cranium just in front of the exoccipitals. They are
+largely cartilaginous, but include in their anterior walls a pair
+of irregular cartilage bones, the =pro-otics= (figs. 24 and 25, 8).
+The cartilaginous area lying ventral to the pro-otic and external
+to the exoccipital is pierced by a rather prominent hole, the
+=fenestra ovalis=, which forms a communication between the internal
+ear cavity, and a space the tympanic cavity, which lies at the side
+of the head, and is bounded externally by the tympanic membrane. The
+fenestra ovalis is occupied by a minute cartilaginous structure, the
+=stapes=, and articulated partly to this and partly to a slight recess
+in the pro-otic is the =columella= (fig. 25, B, 5), a rod in part
+bony and in part cartilaginous, whose outer end is attached to the
+tympanic membrane. The columella and stapes are together homologous
+with the mammalian auditory ossicles and with the hyomandibular of
+Elasmobranchs. Sometimes the term columella is used to include the
+whole ossicular chain,--the columella together with the stapes.
+
+The =olfactory= or =nasal capsules= (fig. 25, B, 12) are fused with
+the anterior end of the cranium and differ from the auditory capsules
+in being to a great extent unossified. There are however two pairs of
+membrane bones developed in connection with them, the _vomers_ and the
+_nasals_. They are drawn out into three pairs of cartilaginous
+processes, on the dorsal surface into the =prenasal= and =alinasal=
+processes which bound the external nares, and on the ventral surface
+towards the middle line into the forwardly-projecting =rhinal=
+processes.
+
+The _nasals_ (figs. 24 and 26, 11) form a pair of triangular bones
+lying dorsolaterally in front of the fronto-parietals. Their bases are
+turned towards one another and their apices are directed outwards and
+backwards. They correspond in position with the prefrontals of the
+reptilian skull as well as with the nasals.
+
+The _vomers_ are a pair of irregular bones lying on the ventral
+surface of the olfactory capsules. Each bears on its inner and
+posterior angle a group of minute pointed teeth, while its outer
+border is drawn out into three or four small slightly diverging
+processes, the two posterior of which form the inner boundary of the
+=posterior nares= (fig. 24, B, 15).
+
+(3) THE JAWS.
+
+The =upper jaw= consists of a rod of cartilage connected with the
+cranium near its two ends, but widely separated from it in the middle.
+It is almost completely overlain by membrane bone. With its posterior
+end the lower jaw articulates.
+
+The membrane bones of the upper jaw include first the _premaxillae_, a
+small bone meeting its fellow in the middle line, and forming the
+extreme anterior end of the upper jaw. It gives off on its dorsal
+surface a backwardly-projecting process. It is connected behind with
+the _maxillae_ (figs. 24 and 26, 10), a long flattened bone which forms
+the greater part of the margin of the upper jaw, and gives off near
+its anterior end a short process which projects upwards and meets the
+nasal.
+
+[Illustration FIG. 26. A, LATERAL VIEW OF THE SKULL, B, POSTERIOR VIEW
+OF THE CRANIUM OF A COMMON FROG (_Rana temporaria_) × 2 (after
+PARKER).
+
+ 1. sphenethmoid.
+ 2. fronto-parietal.
+ 3. pterygoid.
+ 4. squamosal.
+ 5. tympanic membrane.
+ 6. exoccipital.
+ 7. parasphenoid.
+ 8. pro-otic.
+ 9. quadratojugal.
+ 10. maxillae.
+ 11. nasal.
+ 12. premaxillae.
+ 13. anterior nares.
+ 14. mento-meckelian.
+ 15. dentary.
+ 16. angulo-splenial.
+ 17. basilingual plate.
+ 19. quadrate.
+ 20. columella.
+ 21. occipital condyle.
+ 22. anterior cornu of the hyoid (cerato-hyal).
+ 23. foramen magnum.
+
+ II. IX. X. foramina for the exit of cranial nerves.]
+
+Both maxillae and premaxillae are grooved ventrally, and bear, attached
+to the outer more prominent margin of the groove, a row of minute
+conical teeth. These teeth are =pleurodont=, that is, are ankylosed by
+their bases and outer sides to the margin of the jaw. Each tooth is a
+hollow cone, the basal part of which is formed of bone, the apical
+part of dentine, capped by a very weak development of enamel.
+
+The posterior end of the maxillae is overlapped by a small bone, the
+_quadratojugal_ (figs. 24 and 26, 9), whose posterior end forms part
+of the articular surface for the lower jaw. Just behind the
+quadratojugal there is a small unossified area which lies at the angle
+of the mouth, and is connected by a narrow bar of cartilage with the
+cranium; this forms the =quadrate= (figs. 24 and 26, 19). A
+backwardly-directed outgrowth from the cartilaginous bar more or less
+completely surrounds the tympanic membrane, forming the tympanic ring.
+When followed back the maxillae and quadratojugal diverge further and
+further from the cranium, till the angle of the mouth comes to be
+separated from the foramen magnum by a space nearly double the width
+of the cranium. This space is bridged over to a considerable extent by
+two triradiate bones, the _pterygoid_ and _squamosal_.
+
+The _pterygoid_ (figs. 24 and 26, 3) is a large bone, whose anterior
+limb runs forwards meeting the maxillae and palatine; while its inner
+limb meets the auditory capsule and parasphenoid, and its outer limb
+runs backwards and outwards to the angle of the mouth. The _palatine_
+is a small transversely-placed bone, which connects the pterygoid with
+the anterior part of the sphenethmoid. The _squamosal_ (figs. 24 and
+26, 4) is a T-shaped bone whose anterior arm is pointed and passes
+forwards to meet the pterygoid. The posterior upper arm is closely
+applied to the pro-otic, while the posterior lower arm meets the
+pterygoid and quadratojugal at the angle of the jaw, and surrounds the
+narrow cartilaginous bar of the quadrate which goes to join the
+cranium. The squamosal is probably homologous with the squamosal
+together with the pre-opercular of Bony Ganoids.
+
+The quadrate and squamosal form the =suspensorium= by which the lower
+jaw is connected with the cranium.
+
+The =lower jaw= or =mandible= consists of a pair of cartilaginous rods
+(=Meckel's cartilages=) in connection with each of which there are
+developed two membrane bones and one cartilage bone. The cartilage
+bone is the =mento-meckelian= (fig. 26, A, 14), a very small
+ossification at the extreme anterior end. The membrane bones are the
+_angulo-splenial_ and the _dentary_. The _angulo-splenial_ is a strong
+flat bone which forms the inner and lower part of the mandible for the
+greater part of its length. Its dorsal surface is produced into a
+slight =coronoid process=. The _dentary_ (fig. 26, A, 15) is a flat
+plate which covers the outer surface of the anterior half of the
+mandible, as far forwards as the mento-meckelian. The lower jaw is
+devoid of teeth. The part of Meckel's cartilage which in most
+vertebrates ossifies, forming the articular bone, remains unossified
+in the Frog.
+
+THE HYOID APPARATUS.
+
+The =hyoid= of the adult Frog is formed of the modified hyoid and
+branchial arches of the tadpole. It consists of a broad thin plate of
+cartilage, the =basilingual plate= (fig. 29, B, 1), drawn out into two
+pairs of long processes, the =cornua=. The basilingual plate is
+broader in front than behind, and is formed from the fused ventral
+ends of the hyoid and branchial arches of the tadpole.
+
+The =anterior cornua= (fig. 29, B, 2) form a pair of long slender
+cartilaginous rods which project from the body of the hyoid at first
+forwards, then backwards, and finally upwards and somewhat forwards
+again, to be united to the auditory capsules just below the fenestrae
+ovales. They are formed from the dorsal portion of the hyoid arch of
+the tadpole and are homologous with the cerato-hyals of the Dogfish.
+
+The =posterior cornua= form a pair of straight bony rods diverging
+outwards from the posterior end of the basilingual plate. They are
+formed from the fourth branchial arches of the tadpole, and differ
+from the rest of the hyoid apparatus in being well ossified.
+
+The =columellar chain=, which has been already described (p. 157),
+should be mentioned with the hyoid as it is homologous to the
+hyomandibular of fishes.
+
+The =sternum= of the =Frog=, though regarded as part of the axial
+skeleton, is so intimately connected with the pectoral girdle, that it
+will be described with the appendicular skeleton.
+
+
+2. THE APPENDICULAR SKELETON.
+
+This consists of the skeleton of the two pairs of limbs and their
+respective girdles. It is at first entirely cartilaginous but the
+cartilage becomes later on mainly replaced by bone. The only bone
+developed in connection with the appendicular skeleton, which has no
+cartilaginous predecessor, is the _clavicle_.
+
+
+A. THE PECTORAL GIRDLE.
+
+This consists originally of two half rings of cartilage encircling the
+sides of the body a short way behind the head. These two halves meet
+one another in the ventral middle line, and separate the anterior
+elements of the sternum from the posterior ones.
+
+Each half-ring bears on the middle of its outer and posterior surface
+a prominent cup, the =glenoid cavity=, with which the proximal
+arm-bone articulates. This cup divides the half-arch into a dorsal
+=scapular= and a ventral =coracoid= portion.
+
+The =scapular portion= consists of two parts, the =suprascapula= and
+the =scapula=.
+
+The =suprascapula= (fig. 30, A, 2) is a wide, thin plate attached by
+its ventral and narrowest border to the scapula. Its proximal and
+anterior half is imperfectly ossified, its whole border or sometimes
+only its dorsal and posterior borders consist of unaltered hyaline
+cartilage, while the rest of it is composed of calcified cartilage.
+The =scapula= (fig. 30, A, 3) is a fairly stout rod of bone
+constricted in the middle, and forming the dorsal half of the glenoid
+cavity.
+
+The =coracoid portion= consists of three parts, the =coracoid=,
+=precoracoid= and _clavicle_.
+
+The largest and most posterior of these is the _coracoid_ (fig. 30, A,
+4) which like the scapula, is contracted in the middle and expanded at
+the ends, especially at the ventral end. It forms a large part of the
+glenoid cavity. The ventral ends of the coracoids which meet one
+another in the middle line are unossified, and form narrow strips of
+calcified cartilage, the =epicoracoids= (fig. 30, A, 5); these are
+often regarded as sternal elements.
+
+The =precoracoid= forms a narrow strip of cartilage lying in front of
+the coracoid, from which it is separated by the wide =coracoid
+foramen= (fig. 30, A, 9). The dorsal end is continuous with an area of
+unossified cartilage which separates the coracoid and scapula and
+forms part of the glenoid cavity.
+
+The _clavicle_ is a narrow membrane bone closely attached to the
+anterior surface of the precoracoid, its dorsal end is expanded.
+
+
+THE STERNUM.
+
+The sternum consists of four parts arranged in two groups; two parts
+to each group. The anterior members are the episternum and omosternum.
+
+The =episternum= (fig. 30, A, 10) is a thin almost circular plate of
+cartilage much of which remains hyaline.
+
+The =omosternum= (fig. 30, A, 11) is a slender bony rod widest at its
+posterior end; it connects the episternum with the ventral ends of the
+precoracoids.
+
+The =sternum proper= is a short rod of cartilage sheathed in bone; it
+is contracted in the middle and expanded at each end. It bears
+attached to its posterior end a broad somewhat bilobed plate of
+partially calcified cartilage, the =xiphisternum= (fig. 30, A, 13).
+
+B. THE ANTERIOR LIMB.
+
+This is divisible into three parts, the =upper arm= or =brachium=, the
+=fore-arm= or =antibrachium=, and the =manus=.
+
+All the larger bones have their ends formed by prominent epiphyses
+which do not unite with the shaft till late in life. Their
+articulating surfaces are covered by hyaline cartilage.
+
+In the =upper arm= there is a single bone, the =humerus=.
+
+This has a more or less cylindrical shaft and articulates by a
+prominent rounded =head= with the glenoid cavity. The distal end shows
+a large rounded swelling on either side of which is a =condylar
+ridge=, the inner or postaxial one being the larger. A prominent
+=deltoid ridge= runs along the proximal half of the anterior surface,
+and in the male frog a second equally prominent ridge runs along the
+distal half of the posterior surface.
+
+The =fore-arm= consists of two bones, the =radius= and =ulna=, united
+together and forming the =radio-ulna=. The two bones are quite fused
+at their proximal ends where they form a deep cup which articulates
+with the distal end of the humerus, and is drawn out into a rather
+prominent backwardly-projecting =olecranon process=, which ossifies
+from a centre distinct from that of the shaft. The distal end is
+distinctly divided by a groove into an anterior radial and a posterior
+ulnar portion.
+
+The =manus= consists of two parts, the =wrist= or =carpus= and the
+=hand=.
+
+The =carpus=[57] consists of six small bones arranged in two rows. The
+three bones of the proximal row are the =ulnare=, =radiale= and
+=centrale=. The =ulnare= and =radiale= are about equal in size and
+articulate regularly with the radio-ulna. The =centrale= is pushed out
+of its normal position and lies partly on the pre-axial side, partly
+in front of the radiale. Of the three bones of the distal row the two
+pre-axial ones, =carpalia 1= and =2=, are small; carpale 2 articulates
+with the second metacarpal, carpale 1 with both the first and second.
+The third bone is large and articulates with the third, fourth and
+fifth metacarpals, it represents =carpalia 3-5=, with probably in
+addition the representative of a second centrale.
+
+The =hand= consists of four complete digits, and a vestigial =pollex=
+reduced to a short metacarpal.
+
+Each of the four complete digits consists of a =metacarpal= and a
+variable number of =phalanges=. The first digit, as just mentioned,
+has no phalanges, the second and third have two, and the fourth and
+fifth have three.
+
+C. THE PELVIC GIRDLE.
+
+The pelvic girdle of the Frog is much modified from the simple or
+general type found in the Newt (p. 149).
+
+It is a V-shaped structure consisting of two halves which are fused
+together in the middle line posteriorly, while in front they are
+attached to the ends of the transverse processes of the sacral
+vertebra. Each half bears at its posterior end a deep cup, the
+=acetabulum=, with which the head of the femur articulates.
+
+Each half of the pelvis ossifies from two centres. The anterior and
+upper half of the acetabulum, and the long laterally compressed bar
+extending forwards to meet the sacral vertebra ossify from a single
+centre and are generally called the =ilium=; it is probable however
+that they represent both the =ilium= and =pubis= of mammals[58]. The
+posterior part of this bone meets its fellow in a median symphysis.
+
+The posterior third of the acetabulum is formed by a small bone, the
+=ischium=, which likewise meets its fellow in a median symphysis.
+
+The ventral portion of the pelvic girdle never ossifies, even in old
+animals being formed only of calcified cartilage. This is generally
+regarded as the pubis, but it perhaps corresponds to the =acetabular
+bone= of mammals.
+
+
+D. THE POSTERIOR LIMB.
+
+This corresponds closely to the anterior limb and, like it, is
+divisible into three parts, the =thigh=, the =shin= or =crus= and the
+=pes=.
+
+As was the case with the anterior limb, all the long bones have their
+ends formed by prominent epiphyses which do not unite with the shaft
+till late in life.
+
+In the =thigh= there is only a single bone, the =femur=.
+
+The =femur= is a moderately long, slender bone with a well-ossified
+hollow shaft slightly curved in a sigmoid manner. Both ends are
+expanded, the proximal end is hemispherical and articulates with the
+acetabulum, the distal end is larger and more laterally expanded.
+
+The =shin= likewise includes a single bone, the =tibio-fibula=, but
+this, as can be readily seen by the grooves at the proximal and distal
+ends of the shaft, is formed by the fusion of two distinct bones, the
+=tibia= and =fibula=. The tibio-fibula is longer and straighter than
+the femur.
+
+The =pes= consists of two parts, the =ankle= or =tarsus= and the
+=foot=.
+
+The =tarsus= consists of two rows of structures, very different in
+size. The proximal row consists of two long bones, the =tibiale= and
+=fibulare=, which are united by common epiphyses at the two ends,
+while in the middle they are widely separated. The tibiale lies on the
+tibial or pre-axial side, and the fibulare which is the larger of the
+two bones on the fibular or postaxial side. The distal row of tarsals
+consists of three very small pieces of calcified cartilage. The
+postaxial of these is the largest, it articulates with the second and
+third metatarsals and is probably homologous with tarsalia 2 and 3
+fused. The middle one is very small, it articulates with the first
+metatarsal and is probably tarsale 1. The pre-axial one articulates
+with the metatarsal of the calcar, a structure to be described
+immediately, and has been regarded as a =centrale=.
+
+The =foot= includes five complete digits and a supplemental toe as
+well. Each of the five digits consists of a long =metatarsal= with
+epiphyses at both ends, and of a variable number of phalanges. The
+first digit or =hallux= and the second have two phalanges, the third
+three, the fourth, which is the largest, four, and the fifth, three.
+The distal phalanges have epiphyses only at their proximal ends, the
+others at both ends.
+
+On the pre-axial side of the hallux is the supplemental digit, the
+=prehallux= or =calcar=. It consists of a short metatarsal and one or
+two phalanges, and is terminated distally by a horny covering of
+epidermal origin.
+
+
+FOOTNOTES:
+
+[55] See A. Ecker, _Die anatomie des Frosches_, Braunschweig 1864,
+translated by G. Haslam, Oxford, 1889, also A.M. Marshall, _The Frog_,
+5th edition, Manchester and London, 1894.
+
+[56] W.K. Parker, _Phil. Trans._ 161, 1871, p. 137, and W.K. Parker
+and G.T. Bettany, _The Morphology of the Skull_, London, 1877, p. 136.
+
+[57] See G.B. Howes and W. Ridewood, _P.Z.S._, 1888, p. 141.
+
+[58] See bottom of p. 187.
+
+
+
+
+CHAPTER XII.
+
+GENERAL ACCOUNT OF THE SKELETON IN AMPHIBIA.
+
+
+EXOSKELETON.
+
+The exoskeleton, at any rate in most living forms, is very slightly
+developed in Amphibia. The only representatives of the epidermal
+exoskeleton are (1) the minute horny beaks found coating the
+premaxillae and dentaries in _Siren_ and the tadpoles of most Anura,
+(2) the nails borne by the first three digits of the pes in _Xenopus_
+and by the Japanese Salamander _Onychodactylus_, (3) the horny
+covering of the calcar or prehallux of frogs. The Urodela and nearly
+all the Anura, which form the vast majority of living Amphibia, have
+naked skins. A few Anura belonging to the genera _Ceratophrys_ and
+_Brachycephalus_ have bony dermal plates developed in the skin of the
+back, and these plates become united with some of the underlying
+vertebrae.
+
+In the Gymnophiona the integument bears small cycloid scales arranged
+in rings which are equal in number to the vertebrae. These scales
+contain calcareous concretions. Scales also occur between the
+successive rings.
+
+In the Labyrinthodontia the dermal exoskeleton is in many genera
+greatly developed. It is generally limited to the ventral surface and
+consists principally of a buckler formed of three bony plates, one
+median and two lateral. These plates protect the anterior part of the
+thorax, and are closely connected with the adjacent endoskeleton. They
+probably represent the interclavicle and clavicles. Behind this
+buckler numerous scutes are generally developed, which often cover the
+whole ventral surface, and may cover the whole body.
+
+
+TEETH[59].
+
+In Amphibia teeth are generally present on the maxillae, premaxillae and
+vomers, and except in Anura on the dentaries; sometimes they occur on
+the palatines as in many Urodela, most Labyrinthodontia, and the
+Gymnophiona; less commonly on the pterygoids as in _Menobranchus_,
+_Siredon_, some Labyrinthodontia, and _Pelobates cultripes_[60], or on
+the splenials as in _Siren_ and _Menobranchus_, or parasphenoid as in
+_Pelobates cultripes_, _Spelerpes belli_ and _Batrachoseps_. In some
+Anura such as _Bufo_ and _Pipa_ the jaws are toothless.
+
+In Gymnophiona, _Menobranchus_, and _Siredon_, the teeth are arranged
+in two concentric curved rows. The teeth of the outer row are borne on
+the premaxillae and maxillae if present, (the maxillae are absent in
+_Menobranchus_), the teeth of the second row on the vomers and
+pterygoids in _Menobranchus_ and _Siredon_, and on the vomers and
+palatines in Gymnophiona. In some Gymnophiona there is a double row of
+mandibular teeth. The vomerine, palatine and parasphenoid teeth of all
+forms are numerous and are not arranged in rows.
+
+The teeth of all living Amphibia are simple conical structures
+ankylosed to the bone, and consisting of dentine, coated or capped
+with a thin layer of enamel. In the Labyrinthodontia teeth of more
+than one size are sometimes present. The dentine of the basal part of
+the larger teeth is in some genera very greatly folded, causing the
+structure to be highly complicated. These folds, the intervals between
+which are filled with cement, radiate inwards from the exterior and
+outwards from the large pulp cavity. The basal part of the teeth of
+_Ceratophrys_ (Anura) has a similar structure.
+
+
+ENDOSKELETON.
+
+VERTEBRAL COLUMN.
+
+Four regions of the vertebral column can generally be recognised in
+Amphibia, viz. the cervical, the trunk or thoraco-lumbar, the sacral
+and the caudal regions. In the limbless Gymnophiona, however, only
+three regions, the cervical, thoracic, and post-thoracic can be made
+out. The cervical region is limited to a single vertebra which
+generally differs from the others in having no transverse processes or
+indication of ribs. It is generally called the atlas, but it commonly
+bears a small process arising from the anterior face of the centrum
+which resembles the odontoid process of higher animals, and renders it
+probable that the first vertebra of Amphibia corresponds to the axis,
+not to the atlas. Amphibia generally have a single sacral vertebra.
+
+Three elements go to make up the vertebral column in Amphibia, viz.
+
+1. the notochord,
+
+2. the long vertebral centra,
+
+3. intervertebral cartilage which forms the joints between successive
+centra.
+
+The relations which these three elements bear to one another are
+subject to much variation. The successive stages can be well traced in
+the Urodela.
+
+1. The first stage is found in larval Urodeles in general and in adult
+Ichthyoidea, and some Salamandrina. In these forms the notochord
+persists and retains approximately the same diameter throughout the
+whole length of the vertebral column. Bony biconcave centra are
+present and constrict it to a certain extent vertebrally, while
+intervertebrally there is a development of cartilage. The connection
+between the bony vertebrae is effected mainly by the expanded
+notochord.
+
+2. In the next stage, as seen in _Gyrinophilus porphyriticus_, the
+growth of intervertebral cartilage has caused the almost complete
+obliteration of the notochord intervertebrally, and its entire
+disappearance vertebrally, i.e. in the centre of each vertebra. The
+intervertebral cartilage now forms the main connection between
+successive vertebrae, and sometimes cases are found whose condition
+approaches that of definite articulations. Readily recognisable
+remains of the notochord are still found at each end of the
+intervertebral constriction.
+
+3. In the third stage differentiation and absorption of the
+intervertebral cartilage has given rise to definitely articulating
+opisthocoelous vertebrae. These are found in most adult Salamandrina.
+
+ * * * * *
+
+The transverse processes of the earlier trunk vertebrae are divided
+into two parts, a dorsal part which meets the tubercular process of
+the rib and is derived from the neural arch, and a ventral part which
+meets the capitular process of the rib, and is derived from the
+ventral or haemal arch. In the caudal vertebrae and often also in the
+posterior trunk vertebrae the two processes are fused.
+
+_Siren_ and _Proteus_, although they possess minute posterior limbs,
+have no sacral vertebrae, while _Cryptobranchus lateralis_ has two.
+The caudal vertebrae, except the first, have haemal arches very
+similar to the neural arches.
+
+In Labyrinthodontia the centra of the vertebrae are generally well
+ossified biconcave discs. In some forms however, like _Euchirosaurus_,
+the centra are imperfectly ossified, and consist of bony rings
+traversed by a wide notochordal canal. Each ring is formed of four
+pieces, a large well-ossified neural arch, a basal piece, and a pair
+of lateral pieces. Vertebrae of this type are called _rachitomous_.
+
+In the tail region of other forms each vertebra consists of an
+anterior centrum bearing the neural arch, and a posterior
+intercentrum[61] bearing chevron bones. Vertebrae of this type are
+called _embolomerous_. Haemal arches similar to the neural arches are
+often found as in Urodela. The transverse processes are sometimes well
+developed and are divided into tubercular and capitular portions.
+
+In Gymnophiona the vertebrae are biconcave and are very numerous, they
+sometimes number about two hundred and thirty. Only quite the last few
+are ribless and so can be regarded as post-thoracic vertebrae. The
+first vertebra has nothing of the nature of an odontoid process.
+
+In Anura the number of vertebrae is very greatly reduced, only nine
+and the urostyle being present. Of these, eight are presacral and one
+sacral. The urostyle is post-sacral and corresponds to three or more
+modified vertebrae. The first vertebra is without transverse
+processes, the remaining presacral vertebrae have the transverse
+processes fairly large, while the sacral vertebra has them very large,
+forming in some genera widely expanded plates. The urostyle is a long
+cylindrical rod which articulates with the sacrum generally by two
+facets. Ankylosed to its anterior end are the remains of two neural
+arches.
+
+In Anura remains of the notochord are found in the centre of each
+vertebra, i.e. vertebrally, while in the Urodela they only occur
+intervertebrally.
+
+The vertebrae in Anura are, as a rule, procoelous. The eighth vertebra
+is however generally amphicoelous, while the ninth commonly has one
+convexity in front, and two behind.
+
+In some forms such as _Bombinator_, _Pipa_, _Discoglossus_ and
+_Alytes_ they are opisthocoelous; in others like _Pelobates_ they are
+variable.
+
+
+THE SKULL[62].
+
+CRANIUM AND MANDIBLE.
+
+In the Amphibian skull there are as a rule far fewer bones than in the
+skull of bony fish. The primordial cartilaginous cranium often
+persists to a great extent. Only quite a few ossifications take place
+in it; namely in the occipital region--the exoccipitals, further
+forwards--the pro-otics, and at the boundary of the orbital and
+ethmoidal regions--the sphenethmoid. The basi-occipital and
+basisphenoid are never ossified. As in Mammalia there are two
+occipital condyles formed by the exoccipitals.
+
+Large vacuities commonly occur in the cartilage of both floor and roof
+of the primordial cranium. These are roofed over to a greater or less
+extent by the development of membrane bone. Thus on the roof of the
+cranium there are paired parietals, frontals, and nasals, and on its
+floor are paired vomers, and a median unpaired parasphenoid.
+
+In all living forms the parietals meet and there is no interparietal
+foramen, though this exists in Labyrinthodonts.
+
+The palato-pterygo-quadrate bar is united at each end with the
+cranium, but elsewhere in most cases forms a wide arch standing away
+from it. The suspensorium is, as in Dipnoi and Holocephali,
+autostylic. The palato-pterygo-quadrate bar sometimes remains entirely
+cartilaginous, sometimes its posterior half is ossified forming the
+quadrate. In connection with it a number of membrane bones are
+generally developed, viz. the maxillae, premaxillae, palatines,
+pterygoids, quadratojugals, and squamosals. The pterygoids are,
+however, sometimes partially formed by the ossification of cartilage.
+The cartilage of the lower jaw and its investing membrane bones
+generally have much the same relations as in bony fishes.
+
+URODELA. The skulls of the various Urodeles show an interesting series
+of modifications and differ much from one another, but all agree in
+the absence of the quadratojugals, in the fact that the palatines lie
+parallel to the axis of the cranium, and in the large size of the
+parasphenoid.
+
+The lower types _Menobranchus_, _Siren_, _Proteus_, and _Amphiuma_
+have longer and narrower skulls than do the higher types.
+
+_Menobranchus_ has a very low type of skull which remains throughout
+life in much the same condition as that of a young tadpole or larval
+salamander. The roof and floor of the cranium internal to the membrane
+bones are formed of fibrous tissue, not of well-developed cartilage.
+The epi-otic regions of the skull are ossified, forming a pair of
+large bones which lie external to, and distinct from, the
+exoccipitals. _Proteus_ and the Labyrinthodonts are the only other
+Amphibia which have these elements separately ossified. The parietals
+send a pair of long processes forwards along the sides of the
+frontals. Nasals and maxillae are absent, as is likewise the case in
+_Proteus_. Teeth are borne on the vomers, premaxillae, pterygoids,
+dentaries and angulo-splenials. The suspensorium is forwardly
+directed.
+
+The skull of _Siren_ resembles that of _Menobranchus_ in several
+respects, as in the forward direction of the suspensorium and in the
+absence of maxillae, but differs in the possession of nasals, in the
+toothless condition of the premaxillae and dentaries, and in the fusion
+and dentigerous condition of the vomers and palatines.
+
+_Amphiuma_ has a skull which, though narrow and elongated, differs
+from those of _Menobranchus_, _Proteus_, and _Siren_, and resembles
+those of higher types in the following respects:--
+
+(1) the suspensorium projects nearly at right angles to the cranium
+instead of being directed forwards, (2) the maxillae are well
+developed, and the premaxillae are completely ankylosed together, (3)
+there are no palatines.
+
+The skulls of _Megalobatrachus_, _Cryptobranchus_ and _Siredon_
+resemble those of the highest Urodeles the Salamanders in their wide
+form, in having the pro-otics distinct from the exoccipitals which are
+ossified continuously with the epi-otics and opisthotics, and in
+having no palatines, but differ in having the two premaxillae
+separate, and in the arrangement of the vomerine teeth which in
+_Megalobatrachus_ and _Cryptobranchus_ are placed along the anterior
+boundaries of the bones, these meeting in the middle line. In
+_Siredon_ the vomers are separated by the very large parasphenoid.
+
+The suspensorium in _Megalobatrachus_ and _Cryptobranchus_ projects at
+right angles to the cranium; in _Siredon_ it projects somewhat
+downwards and forwards as in the Salamandrina.
+
+Modifications of the vomers, pterygoids and palatines accompany the
+changes of the larval ichthyoid _Siredon_ into the adult salamandroid
+_Amblystoma_, the vomers especially come to resemble to a much greater
+extent those of the Salamandrina.
+
+The ossification of the skull in the Salamandrina is carried further
+than in the Ichthyoidea, though the supra-occipital and basi-occipital
+are not ossified. The skull differs from that in the Ichthyoidea in
+the size of the part of the vomero-palatines which lies in front of
+the teeth, in the frequent union of the two premaxillae and in the
+ossification of all the periotic bones continuously with the
+exoccipital.
+
+The skull differs from that of Anura in the following respects:--
+
+(1) the bones of the upper jaw do not form a complete arch standing
+away from the cranium, and the maxillae are not united to the quadrates
+by quadratojugals, (2) the long axis of the suspensorium passes
+obliquely downwards and forwards instead of downwards and backwards,
+(3) there is no sphenethmoid encircling the anterior end of the
+brain, its place being partly taken by a pair of orbitosphenoids, (4)
+there is no definite tympanic cavity.
+
+[Illustration FIG. 27. DORSAL VIEW OF THE SKULL OF A LABYRINTHODONT
+(_Capitosaurus nasutus_) × 1/9 (from VON ZITTEL).
+
+ 1. premaxillae.
+ 2. nasal.
+ 3. maxillae.
+ 4. anterior nares.
+ 5. frontal.
+ 6. prefrontal.
+ 7. lachrymal.
+ 8. jugal.
+ 9. orbit.
+ 10. parietal.
+ 11. postfrontal.
+ 12. postorbital.
+ 13. interparietal foramen.
+ 14. squamosal.
+ 15. supratemporal.
+ 16. quadratojugal.
+ 17. quadrate.
+ 18. epi-otic.
+ 19. dermo-supra-occipital.
+ 20. exoccipital.
+ 21. foramen magnum.]
+
+LABYRINTHODONTIA. The skull in Labyrinthodontia is remarkable for its
+extreme solidity, the large number of bones which are present, and the
+extent to which the roofing over of the temporal and other fossae has
+taken place. In many forms the surface of the bones is as in
+Crocodiles, strongly sculptured (fig. 27, right half) with ridges and
+grooves which probably lodged sensory organs. The bones forming the
+roof of the skull are generally very uniform in size, perhaps the most
+noticeable of them being the paired dermo-supra-occipitals (fig. 27,
+19). Paired dermo-supra-occipitals occur also in certain Ganoids. The
+Labyrinthodont skull also bears resemblance to that of many fish in
+the development of a pair of long pointed epi-otics (fig. 27, 18),
+which remain permanently distinct from the surrounding bones. The
+parietals are small and enclose between them the interparietal foramen
+(fig. 27, 13). In some forms in which the head is protected with an
+armour of scutes, these do not roof over the interparietal foramen,
+and from this fact it has been inferred that the Labyrinthodonts had a
+functional pineal eye. Both supra- and infra-temporal fossae are
+partially or completely roofed over by the postorbitals and large
+supra-temporals (fig. 27, 15).
+
+There is generally a ring of bones in the sclerotic coat of the eye.
+The pterygoids do not meet in the middle line, being separated by the
+parasphenoid. The palatines bear teeth, and in some genera
+(_Archegosaurus_) form long splints lying along the inner side of the
+maxillae and more or less surrounding the posterior nares. In others
+(_Nyrania_) they lie in the normal position near the middle line, one
+on each side of the parasphenoid. The vomers bear teeth and sometimes
+meet in the middle line; they are sometimes confluent with the
+parasphenoid. On the ventral surface of the cranium there are
+generally large palatal vacuities.
+
+In the mandible there is often a well-marked postglenoid process, and
+the articular is generally completely ossified.
+
+[Illustration FIG. 28. A, VENTRAL VIEW OF THE CRANIUM; B, LATERAL VIEW
+OF THE CRANIUM AND MANDIBLE OF _Siphonops annulatus_ (after
+WIEDERSHEIM).
+
+ 1. anterior nares.
+ 2. naso-premaxillae.
+ 3. frontal.
+ 4. parietal.
+ 5. maxillae.
+ 6. vomer.
+ 7. orbit.
+ 8. quadrate united with the pterygoid in front.
+ 9. squamosal.
+ 10. exoccipital.
+ 11. dentary.
+ 12. angular.
+ 13. basi-occipital and basisphenoid fused.
+ 14. posterior narial opening surrounded by the palatine.
+ X. pneumogastric foramen.]
+
+GYMNOPHIONA. The skull bears a considerable resemblance to that of
+Labyrinthodonts, especially in the arrangement of the bones which
+bound the mouth cavity. The cranium is very hard, and is covered by
+a complete bony roof formed mainly of the exoccipitals, parietals,
+frontals, prefrontals, nasals and premaxillae. The nasals and
+premaxillae are sometimes ossified continuously. There is a median
+unpaired ethmoid whose dorsal end appears at the surface wedged in
+between the frontals and parietals. The bone generally regarded as the
+squamosal[63] is very large, and it and the maxillae generally together
+surround the orbit, which, in _Epicrium_, has in it a ring of bones.
+The palatines form long tooth-bearing bones fused with the inner sides
+of the maxillae; they nearly surround the posterior nares.
+
+The quadrate bears the knob, and the angular the cup for the
+articulation of the mandible,--a very primitive feature. The mandible
+is also noticeable for the enormous backward projection of the
+angular.
+
+ * * * * *
+
+ANURA. In Anura the skull is very short and wide owing to the
+transverse position of the suspensorium. There is often a small
+ossification representing the quadrate. Sometimes as in _Hyla_ and
+_Alytes_ there is a fronto-parietal fontanelle.
+
+As compared with the skull in Urodela the chief characteristics of the
+skull of Anura are:--
+
+1. the presence of a sphenethmoid,
+
+2. the union of the frontals and parietals on each side,
+
+3. the occasional occurrence of small supra- and basi-occipitals,
+
+4. the backward growth of the maxillae and its connection with the
+suspensorium by means of the quadratojugal,
+
+5. the dagger-like shape of the parasphenoid,
+
+6. the occurrence of a definite tympanic cavity,
+
+7. the frequent occurrence of a predentary or mento-meckelian
+ossification in the mandible.
+
+The skull of _Pipa_ is abnormal, being greatly flattened and
+containing little cartilage. The fronto-parietals are fused, and there
+is no sphenethmoid. The quadrates are well developed and the
+squamosals and parasphenoid differ much from those of other Anura.
+
+
+HYOID AND BRANCHIAL ARCHES.
+
+In larval Amphibia the hyoid and four branchial arches are generally
+present, and in adult Ichthyoidea they are frequently almost as well
+represented as in the larva, and are of use in strengthening the
+swallowing apparatus. They are very well seen in _Siredon_, and
+consist of a hyoid attached by ligaments to the suspensorium, followed
+by four branchial arches of which the first and second are united by a
+copula (fig. 29, D, 8), while the third and fourth are not. The hyoid
+is not always the largest and best preserved of the arches, for
+sometimes as in _Spelerpes_ one of the branchials is far larger than
+the hyoid. Four branchial arches occur in _Siren_ as in _Siredon_, but
+in _Proteus_ there are only three.
+
+In some larval Labyrinthodontia (_Branchiosaurus_) four branchial
+arches are known to occur, and their arrangement is almost precisely
+similar to that in _Siredon_.
+
+In Gymnophiona the remains of only three branchial arches occur in
+addition to the hyoid. The four arches are all very similar to one
+another, each consists of a curved rod of uniform diameter throughout.
+The hyoid is united with the first branchial arch, but has no
+attachment to the cranium.
+
+In larval Anura (fig. 29, C) the arrangement of the hyoid and
+branchial arches is much as in Urodela. In the adult, however, the
+ventral parts of all the arches unite, forming a compact structure,
+the _basilingual plate_ (fig. 29, B, 1).
+
+[Illustration FIG. 29. VISCERAL ARCHES OF AMPHIBIA.
+
+ A. _Molge cristata_ (after PARKER).
+ B. _Rana temporaria_ adult (after PARKER).
+ C. Tadpole of _Rana_ (after MARTIN ST ANGE).
+ D. _Siredon pisciformis_ (after CREDNER).
+
+In each case the ossified portions are slightly shaded, while the
+cartilaginous portions are left white.
+
+ 1. basilingual plate.
+ 2. hyoid arch.
+ 3. first branchial arch.
+ 4. second do.
+ 5. third branchial arch.
+ 6. fourth do.
+ 7. thyro-hyal.
+ 8. copula.]
+
+The dorsal parts of the first three branchial arches disappear, but
+those of the fourth become ossified and form the short, stout
+thyro-hyals or posterior cornua. The dorsal parts of the hyoid arch in
+the adult form a pair of long bars, the anterior cornua, which are
+united to the periotic region of the skull in front of the fenestra
+ovalis either by short ligaments or by fusion as in _Bufo_. In _Pipa_
+and _Xenopus_ the first and second branchial arches persist as well as
+the fourth (thyro-hyal), but in _Pipa_ the hyoid is wanting.
+
+
+RIBS.
+
+Ribs are generally very poorly developed in Amphibia. In Anura they
+are in most cases absent; when present they generally form minute
+unossified appendages attached to the transverse processes, but in
+_Discoglossus_ and _Xenopus_ the anterior vertebrae are provided with
+distinct ribs. In Urodela and Labyrinthodontia they are generally
+short structures, each as a rule attached to the vertebra by a
+bifurcated proximal end. The number of rib-bearing vertebrae varies,
+but the first and the posterior caudal vertebrae are always ribless.
+The anterior caudal vertebrae too are generally ribless, but sometimes
+a few of them bear small ribs. In _Spelerpes_ the last two trunk
+vertebrae are ribless, and hence may be regarded as lumbar vertebrae.
+
+In Gymnophiona ribs are better developed than in any other Amphibia;
+they occur on all the vertebrae except the first and last few, and are
+attached to the transverse processes, sometimes by single, sometimes
+by double heads.
+
+Sternal ribs are almost unknown in Amphibia, but traces of them occur
+in _Menobranchus._
+
+
+STERNUM.
+
+In Amphibia the sternum is not very well developed; sometimes as in
+Gymnophiona and _Proteus_ no traces of it occur, and in the Urodela it
+is never ossified. It is always very intimately related to the
+pectoral girdle. In the Salamandrina it has the form of a broad thin
+plate of cartilage, grooved and overlapped by the coracoid.
+
+[Illustration FIG. 30. SHOULDER-GIRDLE AND STERNUM OF
+
+ A. An old male common Frog (_Rana temporaria_).
+ B. An adult female _Docidophryne gigantea_ (after PARKER).
+
+In both A and B the left suprascapula is removed. The parts left
+unshaded are ossified; those marked with small dots consist of hyaline
+cartilage, those marked with large dots of calcified cartilage.
+
+ 1. calcified cartilage of suprascapula.
+ 2. ossified portion of suprascapula.
+ 3. scapula.
+ 4. coracoid.
+ 5. epicoracoid.
+ 6. precoracoid.
+ 7. clavicle.
+ 8. glenoid cavity.
+ 9. coracoid foramen.
+ 10. episternum.
+ 11. omosternum.
+ 12. sternum.
+ 13. xiphisternum.]
+
+In most Anura the sternum consists of a number of parts arranged in
+series. At the anterior end is a flat cartilaginous plate with a
+bony basal stalk. This plate is called the episternum, and its stalk
+the omosternum. The continuity of the sternum is now interrupted
+by a pair of cartilaginous structures, the epicoracoids, which are
+shoulder-girdle elements, and represent the unossified ventral ends of
+the coracoids. In some cases cartilaginous epiprecoracoids can also
+be distinguished. Further back is the long sternum proper, while last
+comes the xiphisternum, a broad expanded plate of cartilage.
+
+In some Anura such as _Pipa_ and _Hyla_ the number of sternal elements
+is considerably reduced.
+
+
+APPENDICULAR SKELETON.
+
+
+PECTORAL GIRDLE.
+
+The most primitive Amphibian shoulder-girdle is found in the Urodela.
+It consists of a dorsal element, the scapula, a posterior ventral
+element, the coracoid, and an anterior ventral element, the
+precoracoid. The clavicle is not developed, and the two coracoids
+overlap in the middle line. The shoulder-girdle remains largely
+cartilaginous but the proximal end of the scapula is ossified, and the
+ossification may extend through part of the coracoid and precoracoid.
+
+In Labyrinthodontia there is an exoskeletal ventral buckler formed of
+three plates, a median one, which probably represents an
+interclavicle, and two lateral ones, which are probably clavicles.
+Traces of endoskeletal structures, probably corresponding to the
+precoracoid and scapula, are also known in some cases. The Gymnophiona
+and some of the Labyrinthodontia have lost the pectoral girdle and
+limbs.
+
+The ossification of the shoulder-girdle has gone on much further in
+Anura than it has in Urodela. Clavicles are present and the scapula
+and coracoid of each side are ossified from separate centres. The
+distal part of the scapula forms a large imperfectly ossified plate,
+the suprascapula.
+
+The shoulder-girdle of Anura is however subject to considerable
+variations. In the Toads (Bufonidae) the epicoracoids or unossified
+ventral ends of the coracoids and precoracoids overlap in the middle
+line (fig. 30, B, 5). This arrangement is called _Arciferous_. In the
+Frogs,--Ranidae, and other forms belonging to the group
+_Firmisternia_,--the epicoracoids do not overlap but form a narrow
+cartilaginous bar separating the ventral ends of the coracoids (fig.
+30, A, 5).
+
+
+ANTERIOR LIMB.
+
+In many Amphibia and especially in the Urodela the anterior limb has a
+very simple unmodified arrangement. The humerus is straight and of
+moderate length, its ends are rounded for articulation on the one hand
+with the shoulder-girdle, and on the other hand with the radius and
+ulna. In the Urodela the radius and ulna are distinct. In the Anura
+they have fused, though the line of junction of the two is not
+obliterated. Their proximal ends are hollowed for articulation with
+the convex end of the humerus.
+
+The manus in all recent Amphibia agrees in never having more than four
+complete digits, but is subject to considerable variation, this
+statement applying especially to the carpus.
+
+In the larva of Salamandra (fig. 31, A), except that the pollex is
+absent[64], the manus retains completely the condition which is
+generally regarded as primitive for the higher Vertebrata. It consists
+of an anterior row of three elements, the ulnare, intermedium, and
+radiale, and a posterior row of four, the carpalia 2, 3, 4, and 5.
+Interposed between the two rows is a centrale. _Menobranchus_ has a
+similar very simple carpus. In most other Amphibia this simplicity is
+lost. This loss may be due to:--
+
+(_a_) fusion of certain structures, e.g. in the adult _Salamandra_ the
+intermedium and ulnare have fused,
+
+(_b_) displacement of structures, e.g. in _Bufo viridis_, the centrale
+has been pushed up till it comes to articulate with the radius,
+
+(_c_) the development of supernumerary elements, especially of extra
+centralia. In _Megalobatrachus_ two or even three centralia sometimes
+occur.
+
+[Illustration FIG. 31. A, RIGHT ANTIBRACHIUM AND MANUS OF A LARVAL
+SALAMANDER (_Salamandra maculosa_) (after GEGENBAUR).
+
+B, RIGHT TARSUS AND ADJOINING BONES OF _Molge sp._ (after GEGENBAUR).
+
+ 1. radius.
+ 2. ulna.
+ 3. radiale.
+ 4. intermedium.
+ 5. ulnare.
+ 6. centrale.
+ 7. carpale 2.
+ 8. " 3.
+ 9. " 4.
+ 10. " 5.
+ 11. tibia.
+ 12. fibula.
+ 13. tibiale.
+ 14. intermedium.
+ 15. fibulare.
+ 16. centrale.
+ 17. tarsale 1.
+ 18. tarsalia 4 and 5 fused.
+ I. II. III. IV. V. digits.]
+
+In the great majority of Amphibia while one digit, probably the first,
+is absent, the other four digits are well developed. In the forms
+however with degenerate limbs like _Amphiuma_, _Siren_ and _Proteus_
+the number of digits is still further reduced. In _Siren_ there are
+three or four, in _Proteus_ three, and in _Amphiuma_ two or three
+digits in the manus.
+
+In Anura the pollex is represented only by a short metacarpal. There
+are sometimes traces of a prepollex. The carpus often has two
+centralia and the intermedium is absent.
+
+In Labyrinthodontia the limbs are generally very simple and resemble
+those of Urodela. In some forms, however, the manus differs from that
+of all living Amphibia in possessing five well-developed digits.
+
+
+PELVIC GIRDLE.
+
+The simplest Amphibian pelvis is that of some of the Labyrinthodontia;
+thus in _Mastodonsaurus_ it consists dorsally of a short broad ilium
+placed vertically and attached to the sacrum, and ventrally of a small
+pubis and of a large ischium meeting its fellow in the middle line. In
+some Labyrinthodonts the pubes as well as the ischia meet in a ventral
+symphysis, and in many there are no obturator foramina. In _Siren_,
+Gymnophiona and some Labyrinthodontia the pelvic girdle and limbs are
+absent.
+
+In Urodela the ventral element of the pelvis on each side forms a flat
+plate which meets its fellow of the opposite side. The anterior part
+of the plate, representing the pubis, generally remains cartilaginous
+throughout life; the posterior part representing the ischium is in
+almost every case well ossified. Attached to the anterior end of the
+pubes there is an unpaired bifid cartilaginous structure, the
+epipubis. The ilia are vertically placed.
+
+In most Anura the pelvis is peculiarly modified in correlation with
+the habits of jumping. The long bone generally called the ilium is
+placed horizontally and is attached at its extreme anterior end to the
+sacrum. The ischium is ossified and distinct. Ventrally in front of
+the ischium there is a tract of unossified cartilage which is often
+regarded as the pubis. In _Xenopus_, however, the bone corresponding
+to the ilium of the Frog is seen to ossify from two centres, one
+forming the ilium, the other, which lies at the symphysis, being
+apparently the pubis. This makes it probable that the so-called ilium
+of the Frog is really to be regarded as an ilio-pubis, and renders the
+homology of the cartilaginous part uncertain, but it probably
+corresponds to the acetabular bone of mammals. In _Xenopus_ also there
+is a minute epipubis similar to that of Urodeles.
+
+
+POSTERIOR LIMB.
+
+In Urodela the posterior limb (fig. 31, B) closely resembles the
+anterior limb, but is even less removed from the primitive condition
+of the higher vertebrates in the fact that all five digits are
+commonly present. The tibia and fibula are short bones approximately
+equal in size. In some cases the number of digits is reduced. Thus in
+_Menobranchus_ the pes has four digits, in _Proteus_ it has two, and
+in _Amphiuma_ two or three, while in _Siren_ the posterior limbs have
+atrophied.
+
+In correlation with their habits of jumping, the posterior limbs in
+Anura are much lengthened and considerably modified. The tibia and
+fibula are completely fused. The intermedium is absent, while the
+tibiale and fibulare are greatly elongated. Tarsalia 4 and 5 are
+absent. Five digits are always present, and there is a prehallux
+formed of two or more segments.
+
+In general the posterior limbs in Labyrinthodontia bear the closest
+resemblance to the anterior limbs; in some cases three centralia are
+found.
+
+In Ichthyoidea, and in most Labyrinthodontia, the cartilages of the
+carpus and tarsus remain unossified; in Salamandrina and in Anura they
+are generally ossified.
+
+
+FOOTNOTES:
+
+[59] O. Hertwig. Ueber das Zahnsystem der Amphibien. _Arch. mikr.
+Anat._ supplem. Bd. XI. 1875.
+
+[60] G.A. Boulenger, _P.Z.S._ 1890, p. 664.
+
+[61] See p. 14.
+
+[62] See many papers by W.K. Parker published in the _Phil. Trans._ of
+the Royal Soc.
+
+[63] Perhaps this bone includes supra-orbital and postorbital
+elements.
+
+[64] The first digit present is sometimes regarded as the pollex, but
+from analogy with Anura it is probable that the pollex is the missing
+digit.
+
+
+
+
+CHAPTER XIII.
+
+SAUROPSIDA.
+
+
+This great group includes the Reptiles and Birds and forms the second
+of the three into which the Gnathostomata may be divided. There is
+nearly always a strongly-developed epiblastic exoskeleton which has
+the form of scales or feathers, and in some cases a dermal exoskeleton
+is also well developed. In living forms the notochord never persists,
+being replaced by vertebrae, but in some extinct forms the centra are
+notochordal. The vertebral centra are ossified, and only in
+exceptionally rare cases have terminal epiphyses. The skull is well
+ossified and has membrane bones incorporated in its walls.
+
+The occipital segment is completely ossified, and an interorbital
+septum or bony partition separating the two orbits is usually
+developed to a greater or less extent. The skull generally articulates
+with the vertebral column by a single occipital condyle into the
+composition of which the exoccipitals and basi-occipital enter in
+varying proportions. The pro-otic ossifies, and either remains
+distinct from the epi-otic[65] and opisthotic throughout life, or
+unites with them only after they have fused with the adjacent bones.
+The hyoid and branchial arches are much reduced; and the
+representative of the hyomandibular is connected with the auditory
+apparatus, forming the auditory ossicles[66]. Each ramus of the
+mandible always consists of a cartilage bone, the articular, and
+several membrane bones. The mandible articulates with the cranium by
+means of a quadrate. The ribs in Birds and some Reptiles bear
+_uncinate processes_, i.e. small, flat, bony or cartilaginous plates
+projecting backwards from their posterior borders. The sternum is not
+transversely segmented as in mammals, and there are commonly distinct
+cervical ribs. The ankle joint is intertarsal, or situated between the
+proximal and distal row of tarsal bones, not cruro-tarsal as in
+Mammalia.
+
+
+CLASS I. REPTILIA[67].
+
+The axial skeleton is generally long, and that of the limbs frequently
+comparatively short, or sometimes absent.
+
+The exoskeleton generally has the form of epidermal scales, which are
+often combined with underlying bony dermal plates or scutes and may
+sometimes form a continuous armour. Neither feathers nor true hairs
+are ever present. The vertebral column is generally divisible into the
+five usual regions. The centra of the vertebrae vary enormously, and
+may be amphicoelous, procoelous, opisthocoelous or flat, but they
+never have saddle-shaped articulating surfaces. The quadrate is always
+large, and is sometimes fixed, sometimes movable. A transpalatine bone
+uniting the pterygoid and maxillae is generally present.
+
+Free ribs are often borne along almost the whole length of the trunk
+and tail, and often occur attached to the cervical vertebrae. The
+sacrum is generally composed of two vertebrae which are united with
+the ilia by means of expanded ribs. The sternum is rhomboidal, and may
+either be cartilaginous or formed of cartilage bone, but never of
+membrane bone; it differs from that of birds also in the fact that it
+does not ossify from two or more centres. An interclavicle is
+generally present. There are always more than three digits in the
+manus, and never less than three in the pes. In all living reptiles
+the ilia are prolonged further behind the acetabula than in front of
+them, and the bones of the pelvis remain as a rule, distinct from one
+another throughout life.
+
+The pubes (pre-pubes) and ischia both commonly meet in ventral
+symphysis, and the acetabula are wholly or almost wholly ossified. The
+metatarsals are not ankylosed together.
+
+
+_Order 1._ THEROMORPHA[68].
+
+This order includes a number of mainly terrestrial, extinct reptiles,
+which differ much from one another, and show remarkable points of
+affinity on the one hand with the Labyrinthodont Amphibia, and on the
+other with the Mammalia. The vertebrae are nearly always amphicoelous
+and sometimes have notochordal centra. The skull is short and has the
+quadrate immovably fixed. There is an interparietal foramen, and
+generally large supratemporal fossae bounded by supratemporal[2]
+arcades, but with no infratemporal[69] arcades; _Elginia_ however has
+the whole of the temporal region completely roofed over.
+
+The teeth are placed in distinct sockets and are very variable in
+form, the dentition sometimes resembling the heterodont dentition of
+mammals. The humerus has distinct condyles and an ent-epicondylar
+foramen[70] as in many mammals.
+
+The pubis is fused with the ischium, and both pectoral and pelvic
+girdles are remarkably solid. The obturator foramen is remarkably
+small or even absent. The anterior ribs have two articulating
+surfaces, and each articulates by its tuberculum with the transverse
+process, and by its capitulum with the centrum as in mammals.
+
+These reptiles occur chiefly in deposits of Triassic and Permian age.
+Some of the best known genera are _Dicynodon_, _Udenodon_, _Placodus_,
+_Pariasaurus_ and _Galesaurus_. They will be noticed in the general
+account of the skeleton in reptiles.
+
+
+_Order 2._ SAUROPTERYGIA.
+
+This order includes a number of extinct marine reptiles, devoid of an
+exoskeleton. The tail is short, the trunk long, and the neck in the
+most typical forms extremely long. The vertebrae have slightly
+biconcave, or nearly flat centra. The skull is relatively small and
+has large supratemporal fossae. The teeth are placed in distinct
+sockets, and are generally confined to the margins of the jaws; they
+are sharp and curved and are coated with grooved enamel. The
+premaxillae are large, and there is an interparietal foramen. The
+quadrate is firmly united to the cranium. The anterior nares are
+separate and are placed somewhat close to the orbits. There is no
+ossified sclerotic ring. The palatines and pterygoids meet the vomers,
+and more or less completely close the palate, and in some forms, e.g.
+_Plesiosaurus_, there is a distinct parasphenoid. Thoracic ribs are
+strongly developed and each articulates with its vertebra by a single
+head. The cervical vertebrae have well-marked ribs, which articulate
+only with the centra, in this respect differing from those of
+Crocodiles. The caudal vertebrae bear both ribs and chevron bones, and
+abdominal splint-ribs are largely developed.
+
+In the shoulder-girdle the coracoids are large and meet in a ventral
+symphysis; precoracoids and a sternum are apparently absent, but parts
+generally regarded[71] as the clavicles and interclavicle are well
+developed. In the pelvis, the pubes and ischia meet in a long
+symphysis. The limbs are pentedactylate, and in the best known forms,
+the Plesiosauridae, form swimming paddles.
+
+The Sauropterygia occur in beds of Secondary age, and some of the best
+known genera are _Plesiosaurus_, _Pliosaurus_ and _Nothosaurus_.
+
+
+_Order 3._ CHELONIA.
+
+In the Tortoises and Turtles the body is enclosed in a bony box,
+formed of the dorsal carapace, and a flat ventral buckler, the
+plastron. Except in _Dermochelys_ the carapace is partly formed from
+the vertebral column and ribs, partly from dermal bones. Both carapace
+and plastron are, except in _Dermochelys_, _Trionyx_ and their allies,
+covered with an epidermal exoskeleton of horny plates, which are
+regularly arranged, though their outlines do not coincide with those
+of the underlying bones. The thoracic vertebrae have no transverse
+processes, and are quite immovably fixed, but the cervical and caudal
+vertebrae are very freely movable. There are no lumbar vertebrae. The
+skull is extremely solid, and frequently has a very complete false
+roof. Teeth have been detected in embryos of _Trionyx_ but with this
+exception the jaws are toothless, and are encased in horny beaks. The
+quadrate is firmly fixed. The facial part of the skull is very short,
+and the alisphenoidal and orbitosphenoidal regions are unossified. In
+living forms there are no separate nasal bones, while large
+prefrontals and postfrontals are developed. There is a comparatively
+complete bony palate chiefly formed of the palatines and pterygoids.
+The anterior nares are united and placed at the anterior end of the
+skull, and the premaxillae are very small. There is no transpalatine
+bone and the vomer is unpaired. The dentaries are generally fused
+together.
+
+There are ten pairs of ribs, and each rib has only a single head and
+is partially attached to two vertebrae; there are no cervical or
+sternal ribs. There is no true sternum.
+
+The three anterior elements of the plastron are respectively
+homologous with the interclavicle and two clavicles of other reptiles,
+while the remaining elements of the plastron are probably homologous
+with the abdominal ribs of Crocodiles. The pectoral girdle lies within
+the ribs, and the precoracoids and coracoids do not meet in ventral
+symphyses. The scapula and precoracoid are ossified continuously. The
+pubis probably corresponds with the pre-pubis of Dinosaurs. There are
+four limbs each with five digits.
+
+The order includes three suborders:--
+
+
+_Suborder (1)._ TRIONYCHIA.
+
+The carapace and plastron have a rough granular surface covered with
+skin and without any horny shields.
+
+The plastron is imperfectly ossified, and marginal bones may be
+absent, or if present are confined to the posterior portion of the
+carapace. The pelvis is not united to the plastron. The cranium has
+not a complete false roof and the head can be drawn back under the
+carapace.
+
+The first three digits of both manus and pes bear claws, and the
+fourth digit in each case has more than three phalanges. The most
+important genus is _Trionyx_.
+
+
+_Suborder (2)._ CRYPTODIRA.
+
+The carapace and plastron vary in the extent to which they are
+ossified, and except in _Dermochelys_[72] and its allies are covered
+by horny plates. Marginal bones are always present. The head can
+generally be drawn back under the carapace. The pelvis is not firmly
+united to the plastron. The cranium often has a complete false roof,
+and in the mandibular articulation the cup is borne by the cranium,
+and the knob by the mandible. Among the more important genera are
+_Dermochelys_, _Chelone_, and _Testudo_.
+
+
+_Suborder (3)._ PLEURODIRA.
+
+The carapace and plastron are strongly ossified, and firmly united to
+the pelvis. The head and neck can be folded laterally under the
+carapace, but cannot be drawn back under it. The cranium has a more or
+less complete false roof, and in the mandibular articulation the knob
+is borne by the cranium, and the cup by the mandible. _Chelys_ is a
+well-known genus.
+
+
+_Order 4._ ICHTHYOSAURIA[73].
+
+The order includes a number of large extinct marine reptiles whose
+general shape is similar to that of the Cetacea. The skull is
+enormously large, and the neck short. The tail is very long, and is
+terminated by a large vertically-placed bilobed fin, the vertebral
+column running along the lower lobe. The very numerous vertebrae are
+short and deeply biconcave. The vertebral column can be divided into
+caudal and precaudal regions only, as the ribs which begin at the
+anterior part of the neck are continued to the posterior end of the
+trunk without being connected with any sternum or sacrum. The precaudal
+vertebrae bear two surfaces for the articulation of the ribs, while
+in the caudal vertebrae the two surfaces have coalesced. The caudal
+region is also distinguished by its chevron bones. The vertebrae have
+no transverse processes, and the neural arches are not firmly united
+to the centra, and have only traces of zygapophyses. The atlas and
+axis are similar to the other vertebrae, but there is a wedge-shaped
+intercentrum between the atlas and the skull, and another between
+the atlas and the axis. The skull is greatly elongated (fig. 32) and
+pointed, mainly owing to the length of the premaxillae. The orbits
+are enormous, and there is a ring of bones in the sclerotic (fig. 32,
+15). The anterior nares are very small; and are placed far back just
+in front of the orbits. There is an interparietal foramen, and the
+supratemporal fossae (fig. 32, 9) are very large, while there are no
+infratemporal fossae. An epipterygoid occurs. The quadrate is firmly
+fixed to the cranium, and there is a large parasphenoid. There are
+large prefrontals, but the frontals are very small. The very numerous
+teeth are large and conical, and are placed in continuous grooves
+without being ankylosed to the bone. They are confined to the jaw-bones.
+
+[Illustration FIG. 32. LATERAL (BELOW) AND DORSAL (ABOVE) VIEWS OF THE
+SKULL OF AN _Ichthyosaurus_. (Modified from Deslongchamps.)
+
+ 1. premaxillae.
+ 2. maxillae.
+ 3. nasal.
+ 4. prefrontal[1].
+ 5. frontal.
+ 6. postfrontal[74].
+ 7. anterior nares.
+ 8. orbit.
+ 9. supratemporal fossa.
+ 10. interparietal foramen.
+ 11. parietal.
+ 12. squamosal.
+ 13. supratemporal.
+ 14. quadratojugal.
+ 15. sclerotic ring.
+ 16. postorbital.
+ 17. jugal.
+ 18. lachrymal.
+ 19. dentary.
+ 20. articular.
+ 21. angular.]
+
+The ribs are long, and the anterior ones have capitula and tubercula.
+There is no sternum, but the ventral body wall is strengthened by a
+complex system of abdominal splint ribs.
+
+The pectoral girdle is strongly developed, the scapulae are narrow,
+the coracoids broad, and meet ventrally without overlapping. There are
+probably no precoracoids, but clavicles and a T-shaped interclavicle
+are well developed.
+
+The limbs are very short, and completely modified into swimming
+paddles. The humerus and femur are both short, while the radius and
+ulna, tibia and fibula are generally still further reduced to the form
+of short polygonal bones.
+
+The digits are formed of longitudinal series of very numerous small
+bones. The number of digits is five, but there sometimes appear to be
+more owing to the bifurcation of certain of them, or to the addition
+of marginal bones, either to the radial or ulnar side of the limb. The
+humerus has no foramen, and both humerus and femur are unique in that
+they are distally terminated by concave surfaces instead of by convex
+condyles. The pelvic limb is much smaller than the pectoral. The
+pelvis has no bony connection with the vertebral column, and all the
+component bones are small and rod-like.
+
+The Ichthyosauria are confined to beds of Secondary age and by far the
+best known genus is _Ichthyosaurus_.
+
+
+_Order 5._ RHYNCHOCEPHALIA.
+
+This order includes the living _Sphenodon_ (_Hatteria_) and various
+extinct forms. The general shape of these animals is lizard-like and
+the tail is long.
+
+The vertebrae are amphicoelous or sometimes nearly flat, and the
+notochord sometimes persists to some extent. _Proterosaurus_ differs
+from the other members of the order in having opisthocoelous cervical
+vertebrae.
+
+The sacrum is composed of two vertebrae. Ossified inter centra
+(interdorsalia) generally occur in the cervical and caudal regions,
+and sometimes throughout the whole vertebral column. In the skull the
+quadrate is immovably fixed and united to the pterygoid. The palate is
+well ossified, while the premaxillae which are often beak-like are
+never ankylosed together. The jaws may be toothless or may be provided
+with teeth which are usually acrodont (see p. 199). The palatines
+frequently bear teeth, and in _Proterosaurus_ teeth occur also on the
+pterygoids and vomers. The rami of the mandible are united by ligament
+at the symphysis except in the Rhynchosauridae, in which the union is
+bony. Superior and inferior temporal arcades occur.
+
+The ribs have capitula and tubercula, and often uncinate processes
+(see p. 190) as in birds. A pectoral girdle and sternum, with
+clavicles and a T-shaped interclavicle are developed, and abdominal
+ribs are always found. The precoracoid is however absent. The limbs
+are pentedactylate.
+
+_Sphenodon_[75] (Hatteria) now living in some of the islands of the
+New Zealand group, is certainly the most generalised of all living
+reptiles. Though lizard-like in form it differs from all living
+lizards in the possession of two temporal arcades, abdominal ribs and
+a fixed quadrate; and is often considered to be nearly allied in many
+respects to the type of reptile from which all the others took their
+origin.
+
+Among the better known extinct forms are _Proterosaurus_ of Permian
+and _Hyperodapedon_ of Triassic age.
+
+
+_Order 6._ SQUAMATA.
+
+This order includes the extinct Mosasaurians, and the lizards and
+snakes which form the vast majority of living reptiles. The trunk may
+be moderately elongated and provided with four short limbs as in
+lizards, or it may be limbless, extremely elongated, and passing
+imperceptibly into the tail. The surface is generally completely
+covered with overlapping horny epidermal scales, below which bony
+dermal scutes may be developed.
+
+The vertebrae are procoelous, rarely amphicoelous. There are no inter
+centra, and the neural arches are firmly united to the centra.
+Additional articulating surfaces, the zygosphenes and zygantra, are
+often developed[76]. The sacrum is formed of two or rarely three
+vertebrae, or may be wanting as in Ophidia. In the skull an
+infratemporal arcade forming the lower boundary of the infratemporal
+fossa is absent, and the quadrate, except in the Chamaeleons, is
+movably articulated to the squamosal. The palatal vacuities are large
+and the nares are separate. There is often a distinct parasphenoid.
+The teeth are either _acrodont_ (i.e. ankylosed to the summit of the
+jaw), or _pleurodont_, i.e. ankylosed to the inner side of the jaw.
+The thoracic ribs each have a single head which articulates with the
+centrum of the vertebra; while uncinate processes and abdominal ribs
+never occur.
+
+A pectoral girdle and sternum may be present, or may be completely
+absent as in snakes. Except in snakes there are generally four
+pentedactylate limbs which may either form paddles or be adapted for
+walking.
+
+
+_Suborder (1)._ LACERTILIA[77].
+
+The body is elongated, and as a rule four short pentedactylate limbs
+are present, but sometimes limbs are vestigial or absent. The
+exoskeleton generally has the form of horny plates, spines, or scales;
+while sometimes as in the Chamaeleons and Amphisbaenians it is absent.
+In other forms such as _Tiliqua_ and _Scincus_, the body has a
+complete armour of bony scutes, whose shape corresponds with that of
+the overlying horny scales.
+
+The vertebrae are procoelous, rarely as in the Geckos amphicoelous;
+they are usually without zygosphenes and zygantra, but these
+structures occur in the Iguanidae. The sacral vertebrae of living
+forms are not ankylosed together, and the caudal vertebrae usually
+have well-developed chevron bones.
+
+In the skull[78] the orbits are separated from one another, only by an
+imperfectly developed interorbital septum, the cranial cavity not
+extending forwards between them, while the alisphenoidal region is
+unossified. The premaxillae may be paired or united (Amphisbaenidae),
+and there is usually an interparietal foramen. There may be a complete
+supratemporal[79] arcade bounding the lower margin of the
+supratemporal fossa, or the supratemporal fossa may be open below. The
+quadratojugal is not ossified, and the quadrate articulates with the
+exoccipital. There is no infratemporal arcade. There is commonly a
+rod-like epipterygoid[80] (fig. 33, 14) connecting the pterygoid and
+parietal.
+
+Teeth are always present, and may be confined to the jaws or may be
+developed also on the pterygoids and rarely on the palatines; they are
+either acrodont or pleurodont. The rami of the mandible are suturally
+united.
+
+A pectoral girdle is always present, and generally also a sternum.
+Clavicles and a T-shaped interclavicle are commonly present, but are
+absent in the Chamaeleons.
+
+[Illustration FIG. 33. A, LATERAL VIEW, AND B, LONGITUDINAL SECTION OF
+THE SKULL OF A LIZARD (_Varanus varius_). × 3/5. (Brit. Mus.)
+
+ 1. premaxillae.
+ 2. maxillae.
+ 3. nasal.
+ 4. lateral ethmoid.
+ 5. supra-orbital.
+ 6. lachrymal.
+ 7. frontal.
+ 8. postfrontal.
+ 9. prefrontal.
+ 10. basisphenoid.
+ 11. pro-otic.
+ 12. epi-otic.
+ 13. pterygoid.
+ 14. epipterygoid (columella cranii).
+ 15. jugal.
+ 16. transpalatine.
+ 17. parasphenoid.
+ 18. quadrate.
+ 19. parietal.
+ 20. squamosal.
+ 21. supratemporal.
+ 22. exoccipital.
+ 23. dentary.
+ 24. splenial.
+ 25. supra-angular.
+ 26. angular.
+ 27. coronoid.
+ 28. articular.
+ 29. vomer.
+ 30. basi-occipital.
+ 31. orbitosphenoid.]
+
+There is no separate precoracoid but a precoracoidal process (fig. 34,
+7) of the coracoid is generally prominent.
+
+[Illustration FIG. 34. LATERAL VIEW OF THE SHOULDER-GIRDLE OF
+_Varanus._ × 3/5.
+
+(Brit. Mus.).
+
+ 1. suprascapula.
+ 2. scapula.
+ 3. glenoid cavity.
+ 4. coracoid.
+ 5. clavicle.
+ 6. interclavicle.
+ 7. precoracoidal process.]
+
+Sternal ribs are present in chamaeleons and scinks. The limbs are in
+the great majority of cases pentedactylate and the digits are clawed.
+The phalanges articulate by means of condyles. Sometimes one or both
+pairs of limbs are absent. When the posterior limbs are absent the
+pelvis is also wanting, though the loss of the anterior limbs does not
+lead to a corresponding loss of the pectoral girdle.
+
+The pubis corresponds to the pre-pubis of Dinosaurs, and both pubes
+and ischia meet in ventral symphyses.
+
+The suborder includes the Lizards, Chamaeleons and Amphisbaenians.
+
+
+_Suborder (2)._ OPHIDIA[81].
+
+The Ophidia or snakes are characterised by their greatly elongated
+body and want of limbs. The body is covered with overlapping horny
+scales and bony dermal scutes are never present. The vertebrae are
+procoelous, and are distinguishable into two groups only, precaudal or
+rib-bearing, and caudal or ribless. The atlas vertebra is also
+ribless. The neural arches are always provided with zygosphenes and
+zygantra. Many of the vertebrae have strong hypapophyses, and the
+caudal vertebrae are without chevron bones.
+
+In the skull the cranial cavity extends forwards between the orbits,
+and is closed in front by downgrowths from the frontals and parietals
+which meet the well-ossified alisphenoids and orbitosphenoids[82]. The
+cranium is strongly ossified, and there are no parotic processes or
+interparietal foramen. There are no temporal arcades and no
+epipterygoid. The premaxillae if present are very small (fig. 51, 1)
+and usually toothless. The quadrates articulate with the squamosals,
+and do not as in Lacertilia meet the exoccipitals. The palatines do
+not unite directly with the vomers or with the base of the cranium,
+and the whole palato-maxillary apparatus is more loosely connected
+with the cranium than it is in Lacertilia. The pterygoids, and in most
+cases also the palatines, bear teeth. The dentition is acrodont, and
+the rami of the mandible are united only by an elastic ligament--an
+important point serving to distinguish the Ophidia from the
+Lacertilia. There is an imperfectly developed interorbital septum, the
+ventral part of which is formed by the parasphenoid. The postfrontal
+is generally well developed, while the jugals and quadratojugals are
+absent. There are never any traces of the anterior limbs or pectoral
+girdle, but occasionally there are vestiges of a pelvis and posterior
+limbs.
+
+
+_Suborder (3)._ PYTHONOMORPHA[83].
+
+This suborder includes _Mosasaurus_ and its allies, a group of
+enormous extinct marine reptiles found in beds of Cretaceous age.
+
+The skin is in most forms at any rate unprovided with dermal scutes.
+The vertebrae may be with or without zygosphenes and zygantra. The
+skull resembles that of lizards, having an interparietal foramen, and
+a cranial cavity open in front. The squamosal takes part in the
+formation of the cranial wall, and the quadrate articulates with the
+squamosal, not as in Lacertilia with the exoccipital. There are large
+supratemporal fossae, bounded below by supratemporal arcades. The
+teeth are large and acrodont, and occur on the pterygoids as well as
+on the jaws. The two rami of the mandible are united by ligament only.
+Pectoral and pelvic girdles are present, but clavicles are wanting,
+and the pelvis is not as a rule united to any sacrum.
+
+The limbs are pentedactylate, and are adapted for swimming, while all
+the limb bones except the phalanges are relatively very short. The
+number of phalanges is not increased beyond the normal, and they
+articulate with one another by flat surfaces. The terminal phalanges
+are without claws.
+
+
+_Order 7._ DINOSAURIA[84].
+
+The extinct reptiles comprising this order were all terrestrial, and
+include the largest terrestrial animals known. They vary greatly in
+size and in the structure of the limbs, some approach close to the
+type of structure met with in birds, others are allied to crocodiles.
+
+Passing to the more detailed characters:--there is sometimes a
+well-developed exoskeleton having the form of bony plates or spines.
+The vertebrae may be solid or their centra may be hollowed internally;
+their surfaces may be flat, biconcave or opisthocoelous. The sacrum is
+composed of from two to six vertebrae.
+
+As regards the skull, the quadrate is large and fixed, and
+supratemporal and infratemporal fossae bounded by bone occur. The
+teeth are more or less laterally compressed, and often have serrated
+edges; they may be placed in distinct sockets or in a continuous
+groove. The ribs have capitula and tubercula, and sternal ribs often
+occur. The scapula is very large, the coracoid small, and there is no
+precoracoid, or T-shaped interclavicle. Clavicles are only known in a
+few cases. In the pelvis the ilium is elongated both in front of, and
+behind, the acetabulum, sometimes the pre-pubis, sometimes the
+post-pubis is the better developed. The anterior limbs are shorter
+than the posterior, and the long bones are sometimes solid, sometimes
+hollow.
+
+There are three well-marked suborders of the Dinosauria.
+
+
+_Suborder (1)._ SAUROPODA[85].
+
+The reptiles belonging to this group were probably quadrupedal and
+herbivorous.
+
+They have the cervical and anterior trunk vertebrae opisthocoelous,
+while the posterior vertebrae are biconcave; all the presacral, and
+sometimes the sacral vertebrae are hollowed internally. The teeth are
+spatulate and without serrated edges, they are always planted in
+distinct sockets, and some of them are borne by the premaxillae.
+
+[Illustration FIG. 35. RESTORED SKELETON OF _Ceratosaurus nasicornis_.
+× 1/30. (After Marsh.)
+
+ 1. anterior nares.
+ 2. prominence on the nasal bones which probably carried a horn.
+ 3. pre-orbital vacuity.
+ 4. orbit.
+ 5. scapula.
+ 6. coracoid.
+ 7. ilium.
+ 8. pubis (pre-pubis).
+ 9. ischium.]
+
+The nares have the form of long slits and there are large pre-orbital
+vacuities.
+
+The limb bones are solid, and the anterior limbs are not much shorter
+than the posterior ones. All the limbs are plantigrade and
+pentedactylate, and the digits of the pes are clawed. There is a large
+pre-pubis directed downwards and forwards, meeting its fellow in a
+ventral symphysis, but there is no post-pubis.
+
+The Sauropoda are found in the secondary rocks of Europe and N.
+America and include the largest land animals that are known to have
+existed. Many of the best known forms such as _Brontosaurus_ and
+_Morosaurus_ are North American.
+
+
+_Suborder (2)._ THEROPODA.
+
+The members of this suborder were all carnivorous, and from the small
+comparative size of the anterior limbs many of them were probably
+bipedal.
+
+The vertebrae are opisthocoelous or amphicoelous, their neural arches
+are provided with zygosphenes and zygantra, and their centra are
+frequently hollowed internally; the limb bones are also hollow, and in
+fact the whole skeleton is extremely light. The tail is of great
+length. The teeth are pointed and recurved, and have one or both
+borders serrated; they are always planted in distinct sockets, and
+some of them are borne by the premaxillae. There are large pre-orbital
+vacuities. The digits of both manus and pes are terminated by pointed
+ungual phalanges which must have borne claws. In the pelvis the
+pre-pubes and ischia are slender bones, the former meeting in a
+ventral symphysis. The ilia are very deep vertically and there are no
+post-pubes. The astragalus is closely applied to the tibia, in front
+of which it sends an ascending process, sometimes the two bones appear
+to have been ankylosed together, as in birds. The metatarsals are
+elongated and the feet digitigrade.
+
+The Theropoda vary greatly in size, one of the best known genera
+_Compsognathus_ was about as large as a cat, another, _Megalosaurus_,
+perhaps as large as an elephant. _Ceratosaurus_ is the name of a
+well-known North American form regarded by many authorities as
+identical with _Megalosaurus_.
+
+
+_Suborder (3)._ ORTHOPODA.
+
+This suborder includes the most specialised of the Dinosaurs, certain
+of which resemble the Theropoda in being bipedal. In some of them such
+as _Stegosaurus_ the exoskeleton is strongly developed, in others such
+as _Iguanodon_ it is absent.
+
+The vertebrae are solid and may be opisthocoelous, biconcave, or flat.
+The teeth are compressed and serrated, often irregularly, and are
+frequently not set in distinct sockets. The anterior part of the
+premaxillae is without teeth, and a toothless predentary or
+mento-meckelian bone is present. The pre-orbital vacuities are small
+or absent, and the nares are large and placed far forwards.
+
+The most characteristic features of the group are found in the pelvis
+which, except in the Ceratopsia, bears a striking resemblance to that
+of birds. The ischium and post-pubis are long slender bones directed
+backwards parallel to one another, and the pre-pubis is also well
+developed. The ischium has an obturator process. The limb bones are
+sometimes hollow, sometimes solid. The anterior limbs are much shorter
+than the posterior, pointing to a bipedal method of progression. The
+pes is digitigrade or plantigrade, and has three, rarely four,
+digits.
+
+The suborder Orthopoda may be further subdivided into three
+sections:--
+
+
+A. STEGOSAURIA.
+
+A dermal exoskeleton is strongly developed. The vertebral centra are
+flat or biconcave, and neither they nor the limb bones are hollowed
+out by internal cavities. The limbs are plantigrade, the anterior ones
+short, the posterior ones very large and strong. The post-pubis is
+well developed;
+
+e.g. _Stegosaurus_ from the Upper Jurassic of Colorado.
+
+
+B. CERATOPSIA.
+
+There is sometimes a well-developed dermal exoskeleton formed of small
+granules and plates of bone. The bones are solid, and the vertebral
+centra flat. The cranium bears a pair of enormous pointed frontal
+horns, and the parietal is greatly expanded and elevated behind,
+forming with the squamosals a shield which overhangs the anterior
+cervical vertebrae. The premaxillae are united, and in front of them is
+a pointed beak-like bone which bites upon a toothless predentary
+ossification of the mandible. The teeth have two roots. The anterior
+limbs are but little shorter than the posterior ones. There is no
+post-pubis;
+
+e.g. _Polyonax_ from the uppermost Cretaceous of Montana.
+
+
+C. ORNITHOPODA[86].
+
+There is no dermal exoskeleton. The cervical vertebrae are
+opisthocoelous, and so are sometimes the thoracic. The limb bones are
+hollow and the anterior limbs are much shorter than the posterior
+ones. The feet are digitigrade and provided with long pointed claws.
+The post-pubis is long and slender and directed back parallel to the
+ischium;
+
+e.g. _Iguanodon_ from the European Cretaceous.
+
+
+_Order 8._ CROCODILIA[87].
+
+This order includes the Crocodiles, Alligators and Garials and various
+extinct forms, some of which are closely allied to the early
+Dinosaurs.
+
+There is always a more or less complete exoskeleton formed of bony
+scutes overlain by epidermal scales; these bony scutes are specially
+well developed on the dorsal surface but may occur also on the
+ventral. The vertebral column is divisible into the five regions
+commonly distinguishable. In all living forms the vertebrae, with the
+exception of the atlas and axis, the two sacrals, and first caudal,
+are procoelous, but in many extinct forms they are amphicoelous. The
+atlas (fig. 71) is remarkable, consisting of four pieces, and the
+first caudal is biconvex.
+
+The teeth are, in the adult, planted in separate deep sockets. The
+skull is very dense and solid, and all the component bones including
+the quadrate are firmly united. The dorsal surface of the skull is
+generally characteristically sculptured. There is an interorbital
+septum, and the orbitosphenoidal and presphenoidal regions are
+imperfectly ossified. Supratemporal, infratemporal, and post-temporal
+fossae occur, but no interparietal foramen. In living genera there is
+a long secondary palate formed by the meeting in the middle line of
+the palatines, pterygoids and maxillae (fig. 43, A).
+
+Cervical ribs (fig. 41, 8 and 9) are well developed, and articulate
+with rather prominent surfaces borne on the neural arches and centra
+respectively. The thoracic ribs articulate with the long transverse
+processes, and sternal ribs and abdominal splint ribs (fig. 46, 4)
+occur. The sternum is cartilaginous, and both it and the
+shoulder-girdle are very simple. The precoracoid is represented by
+merely a small process on the coracoid, while the clavicles are
+absent, except in the Parasuchia. In the pelvis (fig. 49) there is a
+large ilium, and an ischium meeting its fellow in a ventral symphysis;
+these two bones form almost the whole of the acetabulum. In front of
+the acetabulum, in the Eusuchia, projects a bone which is generally
+called the pubis, but is in reality rather an epipubis (fig. 49, 4),
+the true pubis being probably represented by a fourth element which
+remains cartilaginous for some time, and later on ossifies and
+attaches itself to the ischium. The limbs are small in proportion to
+the size of the body, and are adapted for swimming or for shuffling
+along the ground; they are plantigrade and the bones are all solid. In
+living forms the anterior limbs have five digits and the posterior
+four, the fifth being represented only by a short metatarsal. The
+first three digits in each case are clawed. The calcaneum has a large
+backwardly-projecting process.
+
+The order Crocodilia may be subdivided into two suborders.
+
+
+_Suborder (1)._ PARASUCHIA.
+
+The vertebral centra are flat or biconcave. The premaxillae are very
+large, and the nares are separated, and placed far back. The posterior
+narial openings lie comparatively far forward between the anterior
+extremities of the palatines.
+
+The palatines and pterygoids do not form a secondary palate. The
+supratemporal fossae are small, and open posteriorly, the lateral
+temporal fossae are very large. The parietals and frontals are paired.
+Clavicles are present. The best known and most important genus of
+these extinct crocodiles is _Belodon_.
+
+
+_Suborder_ (2). EUSUCHIA.
+
+The vertebrae are either biconcave or procoelous. The premaxillae are
+small, and the anterior nares are united and placed far forwards. The
+posterior nares lie far back, the palatines and in living genera the
+pterygoids, meeting in the middle line, and giving rise to a closed
+palate. The supratemporal fossae are surrounded by bone on all sides,
+and the parietals, and often also the frontals are united. There are
+no clavicles. The suborder includes the genera _Crocodilus_,
+_Alligator_, _Garialis_ and others living and extinct.
+
+
+_Order 9._ PTEROSAURIA[88].
+
+These animals, called also the pterodactyles or Ornithosauria, are a
+group of extinct reptiles, whose structure has been greatly modified
+from the ordinary reptilian type for the purpose of flight.
+
+The skin was naked and they vary greatly in size and in the length of
+the tail. The vertebrae and limb bones are pneumatic just as in birds.
+The presacral vertebrae are procoelous and have their neural arches
+firmly united to the centra. The neck is long, the caudal vertebrae
+are amphicoelous, and from three to five vertebrae are fused together
+in the sacral region. The skull is large and somewhat bird-like, the
+facial portion being much drawn out anteriorly, and the sutures being
+obliterated. It resembles that of other reptiles in having large
+supratemporal fossae; large pre-orbital vacuities also occur. The jaws
+may be toothed or toothless, and the teeth, when present, are imbedded
+in separate sockets. The premaxillae are large, and the quadrate is
+firmly attached to the skull. The rami of the mandible are united at
+the symphysis, and there is an ossified ring in the sclerotic. The
+occurrence of a postfrontal and its union with the jugal behind the
+orbit, are characteristic reptilian features.
+
+The ribs have capitula and tubercula, and sternal and abdominal ribs
+occur. The sternum has a well-developed keel, and the scapula and
+coracoid are large and bird-like. There are no clavicles or
+interclavicle.
+
+The anterior limbs are modified to form wings by the great elongation
+of the fifth digit, to which a membrane was attached. The second,
+third and fourth digits are clawed and are not elongated in the way
+that they are in bats. The pollex, if present at all, is quite
+vestigial.
+
+The pelvis is weak and small, and though the ilia are produced both in
+front of and behind the acetabula, in other features the pelvis is not
+bird-like. The ischia are short and wide, and the pubes are
+represented only by the pre-pubes. The posterior limbs are small and
+the fibula is much reduced. The pes is quite reptilian in type, and
+has five separate slender metatarsals. The two best known genera are
+_Pterodactylus_, in which the tail is short, and _Rhamphorhynchus_, in
+which it is long. The Pterosauria are found throughout the Jurassic
+and Cretaceous formations in both Europe and North America.
+
+
+FOOTNOTES:
+
+[65] According to Baur a distinct epi-otic is not recognisable in the
+reptilian skull.
+
+[66] H. Gadow, _Phil. Trans._, vol. 179, 1888.
+
+[67] See G. Baur, _J. Morph._, vol. I., 1887. R. Lydekker, _Catalogue
+of the Fossil Reptilia and Amphibia in the British Museum_, Parts I. &
+II. C.K. Hoffmann, _Reptilien_, in Bronn's _Classen und Ordnungen des
+Thierreichs_, Bd. VI., 3 abth. 1879-90.
+
+[68] T.H. Huxley, _Quart. J. Geol. Soc._, vol. XV. p. 649, 1859. R.
+Owen, _Catalogue of Fossil Reptiles of S. Africa in the British
+Museum_, London, 1876. H.G. Seeley, various papers published in the
+_P.R. Soc. London_, and _Phil. Trans._
+
+[69] See pp. 281-283.
+
+[70] An ent-epicondylar foramen is one piercing the humerus on its
+inner side just above the condyle.
+
+[71] According to Hulke they should be regarded as the
+omosternum,--the clavicles and interclavicle being wanting.
+
+[72] See p. 272.
+
+[73] R. Lydekker, _Nat. Sci._ vol. I. p. 514, 1892. Further references
+are there given.
+
+[74] The exact position of the suture between the prefrontal and
+postfrontal is not known.
+
+[75] A. Günther, On the Anatomy of Hatteria, _Phil. Trans_, vol. 157,
+1867, p. 595.
+
+[76] =Zygosphenes= are extra articulating surfaces borne upon the
+anterior face of the neural arch; they fit into corresponding
+structures, the =zygantra=, which are borne on the posterior surface
+of the neural arch of the preceding vertebra. Ordinary zygapophyses
+always accompany them.
+
+[77] See E.D. Cope, _P. Amer. Phil. Soc_. vol. xxx. p. 185.
+
+[78] See W.K. Parker, _Phil. Trans._ vol. 170, 1879, p. 595.
+
+[79] See p. 281.
+
+[80] Often called the columella cranii.
+
+[81] See C.K. Hoffmann, in Bronn's _Klassen und Ordnungen des
+Thierreichs_, Bd. VI., 3 abth. 1885-90.
+
+[82] Some anatomists consider that the closing in of the brain case in
+front is entirely due to the frontals and parietals.
+
+[83] E.D. Cope, _Rep. U.S. Geol. Surv._, 1875, vol. II., The
+vertebrata of the Cretaceous formations of the west. E.D. Cope, _P.
+Boston Soc._ 1862, XII. p. 250. O.C. Marsh, _Amer. J. Sci._, 1872,
+vol. 3. R. Owen, _Quart. J. Geol. Soc._, 1877, and 1878.
+
+[84] J.W. Hulke, _Presidential address to the Geol. Soc. of London_,
+1883 and 1884. O.C. Marsh, many papers in the _Amer. J. Sci._ from
+1878 onwards, also in the _Geol. Mag._ R. Owen, _History of British
+fossil reptiles: Dinosauria_ (Palaeont. Soc.).
+
+[85] The diagnostic characters of the different groups of Dinosaurs
+are in the main those given by von Zittel.
+
+[86] See O.C. Marsh, _Amer. J. Sci._ (3), vol. 48, 1894, p. 85.
+
+[87] See C.B. Brühl, _Das Skelet der Krokodiliden_, Wien, 1862. C.K.
+Hoffmann in Bronn's _Klassen und Ordnungen des Thierreichs_, Bd. VI.
+Abth. III. 1881-85. T.H. Huxley, _Proc. Linn. Soc._ (Zoology) 1860
+vol. IV. p. 1. R. Owen, _History of British fossil Reptiles_.
+_Crocodilia_ (Palaeont. Soc.). A. Smith Woodward, _Geol. Mag._ 1885,
+3rd dec. II. p. 496. A. Smith Woodward, _Proc. of Geologists' Assoc._
+vol. IX. p. 288, 1886.
+
+[88] See H.G. Seeley, On the Organisation of the Ornithosauria,
+_Journ. Linn. Soc._ (Zoology) vol. XIII. p. 84. K.A. Zittel, Ueber
+Flugsaurier aus dem lithographischen schiefer, _Palaeontograph._ XXIX.
+p. 49.
+
+
+
+
+CHAPTER XIV.
+
+THE SKELETON OF THE GREEN TURTLE.
+
+(_Chelone midas._)
+
+
+The most striking feature as regards the skeleton of the Turtle is
+that the trunk is enveloped in a bony box, the dorsal portion of which
+is called the =carapace=, while the ventral portion is the =plastron=.
+
+
+I. EXOSKELETON.
+
+_a._ The =epidermal exoskeleton= in the Green Turtle as in all other
+Chelonia except _Dermochelys_, _Trionyx_ and their allies is strongly
+developed, its most important part consisting of a series of horny
+=shields= which cover over the bony plates of the carapace and
+plastron but do not at all correspond to them in size and arrangement.
+
+The shields covering over the =carapace= consist of three rows of
+larger central shields,--five (=vertebral=) shields being included in
+the middle row and four (=costal=) in each lateral row,--and of a
+number of smaller =marginal= shields.
+
+Of the marginal shields, that lying immediately in front of the first
+vertebral is termed the =nuchal=, while the two succeeding the last
+vertebral are called sometimes =pygal=, sometimes =supracaudal=; the
+remainder are the marginal shields proper.
+
+The epidermal covering of the =plastron= consists principally of six
+pairs of symmetrically arranged shields, called respectively the
+=gular=, =humeral=, =pectoral=, =abdominal=, =femoral=, and =anal=,
+the gular being the most anterior. In front of the gular shields is an
+unpaired =intergular=, and the shields of the plastron are connected
+laterally with those of the carapace, by five or six pairs of rather
+irregular =infra-marginal= shields. Smaller horny plates occur on
+other parts of the body, especially on the limbs and head.
+
+Two other sets of structures belong also to the epidermal exoskeleton,
+viz. (_a_) horny =beaks= with denticulated edges which ensheath both
+upper and lower jaws, (_b_) =claws=, which as a rule are borne only by
+the first digit of each limb. Sometimes in young individuals the
+second digit is also clawed.
+
+_b._ The =dermal exoskeleton= is strongly developed, and is combined
+with endoskeletal structures derived from the ribs and vertebrae to
+form the carapace.
+
+The =Carapace= (fig. 36) consists of a number of plates firmly united
+to one another by sutures. They have a very definite arrangement and
+include:
+
+(_a_) the =nuchal= plate (fig. 36, 1), a wide plate forming the whole
+of the anterior margin of the carapace. It is succeeded by three
+series of plates, eight in each series, which together make up the
+main part of the carapace. Of these the small
+
+(_b_) =neural plates=[1] (fig. 36, A, 2) form the middle series. They
+are closely united with the neural arches of the underlying vertebrae;
+
+(_c_) the =costal plates=[89] (fig. 36, A, 3) are broad arched plates
+united to one another by long straight sutures. They are united at
+their inner extremities with the neural plates, but the boundaries of
+the two sets of plates do not regularly correspond. Each is united
+ventrally with a rib which projects beyond it laterally for some
+distance; (_d_) the =marginal plates= (fig. 36, 4) are twenty-three
+in number, eleven lying on each side, while an unpaired one lies in
+the middle line posteriorly. Many of them are marked by slight
+depressions into which the ends of the ribs fit;
+
+(e) the =pygal= plates (fig. 36, 5) are two unpaired plates lying
+immediately posterior to the last neural.
+
+[Illustration Fig. 36. A, DORSAL AND B, VENTRAL VIEW OF THE CARAPACE
+OF A LOGGERHEAD TURTLE (_Thalassochelys caretta_), (after Owen).
+
+ 1. nuchal plate.
+ 2. first neural plate.
+ 3. second costal plate.
+ 4. marginal plate.
+ 5. pygal plate.
+ 6. rib.
+ 7. thoracic vertebra.
+ 8. first vertebral shield.
+ 9. costal shield.]
+
+The sculpturing due to the epidermal shields is very obvious on the
+carapace.
+
+The =plastron= (fig. 37) consists of one unpaired ossification, the
+=entoplastron=, and four pairs of ossifications called respectively
+the =epiplastra=, =hyoplastra=, =hypoplastra=, and =xiphiplastra=.
+
+The =epiplastra= (fig. 37, 1) are the most anterior, they are expanded
+and united to one another in the middle line in front, while behind
+each tapers to a point which lies external to a process projecting
+forwards from the hyoplastron. They are homologous with the
+_clavicles_ of other vertebrates.
+
+The =entoplastron= or =episternum= (fig. 37, 2) which is homologous
+with the _interclavicle_ of other reptiles, is expanded at its
+anterior end and attached to the symphysis of the epiplastra, while
+behind it tapers to a point and ends freely.
+
+The =hyoplastra= are large irregular bones each closely united
+posteriorly with the corresponding hypoplastron, and drawn out
+anteriorly into a process which lies internal to that projecting
+backwards from the epiplastron. Each gives off on its inner surface a
+slender process which nearly meets its fellow, while the anterior half
+of the outer surface is drawn out into several diverging processes.
+
+The =hypoplastra= (fig. 37, 4) are flattened bones resembling the
+hyoplastra, with which they are united by long sutures; the posterior
+half of both outer and inner surfaces is drawn out into a number of
+pointed processes.
+
+The =xiphiplastra= are small flattened elongated bones meeting one
+another in the middle line posteriorly. In front they are notched and
+each interlocks with a process from the hypoplastron of its side. The
+hyoplastra, hypoplastra and xiphiplastra are homologous with the
+abdominal ribs of Crocodiles.
+
+[Illustration FIG. 37. THE PLASTRON OF A GREEN TURTLE (_Chelone
+midas_). × 1/7. (Camb. Mus.)
+
+ 1. epiplastron (clavicle).
+ 2. entoplastron (interclavicle).
+ 3. hyoplastron.
+ 4. hypoplastron.
+ 5. xiphiplastron.]
+
+
+II. ENDOSKELETON.
+
+1. THE AXIAL SKELETON.
+
+The axial skeleton includes the vertebral column, the ribs, and the
+skull.
+
+A. THE VERTEBRAL COLUMN AND RIBS.
+
+The number of vertebrae in the Green Turtle is thirty-eight, not a
+great number as compared with that in many reptiles, and of these
+eighteen are caudal.
+
+The vertebral column is divisible into four regions only--=cervical=,
+=thoracic=, =sacral=, and =caudal=.
+
+
+THE CERVICAL VERTEBRAE.
+
+These are eight in number, and are chiefly remarkable for the great
+variety of articulating surfaces which their centra present, and for
+their mobility upon one another.
+
+The first or =atlas= vertebra differs much from all the others and
+consists of the following parts:--
+
+_a._ the =neural arch=, formed of two separate ossifications united in
+the mid-dorsal line;
+
+_b._ the =inferior arch=;
+
+_c._ the =centrum=, which is detached from the rest and forms the
+odontoid process of the second vertebra.
+
+Each half of the =neural arch= consists of a ventral portion, the
+=pedicel=, which lies more or less vertically and is united ventrally
+to the inferior arch, and of a dorsal portion, the =lamina=, which
+lies more or less horizontally and meets its fellow in the middle line
+in front, partially roofing over the neural canal. Each pedicel bears
+a facet on its anterior surface, which, with a corresponding one on
+the inferior arch, articulates with the occipital condyle of the
+skull. Three similar facets occur also on the posterior surface of the
+pedicel and inferior arch, and articulate with the odontoid process.
+The laminae meet one another in front, but do not fuse, while behind
+they are separated by a wide triangular space. They bear a pair of
+small downwardly-directed facets, the =postzygapophyses=, for
+articulation with the prezygapophyses of the second vertebra.
+
+The =inferior arch= is a short irregular bone bearing two converging
+facets for articulation with the occipital condyle and odontoid
+process respectively.
+
+The =centrum= or =odontoid process= has a convex anterior surface for
+articulation with the neural and inferior arches, and a concave
+posterior surface by which it is united with the centrum of the second
+or axis vertebra. It bears posteriorly a small epiphysis which is
+really a detached portion of the inferior arch.
+
+The second or =axis= and following five cervical vertebrae, though
+showing distinct differences, resemble one another considerably, each
+having a fairly elongated centrum with a keel-like =hypapophysis=,
+each having also a neural arch with prominent articulating surfaces,
+the anterior of which, or =prezygapophyses=, look upwards and inwards,
+while the posterior ones, the =postzygapophyses=, look downwards and
+outwards. They however, as was previously mentioned, differ very
+remarkably in the character of the articulating surfaces of the
+centra. Thus the second and third vertebrae are convex in front and
+concave behind, the fourth is biconvex, the fifth is concave in front
+and convex behind. The sixth is concave in front and attached to the
+seventh by a flat surface behind, the seventh has a flat anterior face
+and two slightly convex facets behind. The vertebrae all have short
+blunt transverse processes and the second has a prominent =neural
+spine=.
+
+The =eighth cervical vertebra= is curiously modified, the centrum is
+very short, has a rather prominent hypapophysis, and is convex behind,
+while in front it articulates with the preceding centrum by two
+concave surfaces. The neural arch is deeply notched in front and bears
+two upwardly-directed prezygapophyses, while behind it is very massive
+and is drawn out far beyond the centrum, bearing a pair of flat
+postzygapophyses. The top of the neural arch almost or quite meets a
+blunt outgrowth from the nuchal plate.
+
+
+THE THORACIC VERTEBRAE.
+
+These are ten in number and are all firmly united with the ribs and
+elements forming the carapace.
+
+The first thoracic vertebra differs from the others, the centrum is
+short and has a concave anterior surface articulating with the centrum
+of the last cervical vertebra, and a pair of prezygapophyses borne on
+long outgrowths. The neural spine arises only from the anterior half
+of the centrum, and is not fused to the carapace. Arising laterally
+from the anterior part of the centrum are a small pair of ribs each of
+which is connected with a process arising from the rib of the
+succeeding vertebra.
+
+The next seven thoracic vertebrae are all very similar, each has a
+long cylindrical centrum, expanded at the ends, and firmly united to
+the preceding and succeeding vertebrae. The neural arches are
+flattened and expanded dorsally, and are united to one another and to
+the overlying neural plates; each arises only from the anterior half
+of its respective centrum, and overlaps the centrum of the vertebra in
+front of it. Between the base of the neural arch and its successor is
+a small foramen for the exit of the spinal nerve. There are no
+transverse processes or zygapophyses.
+
+To each thoracic vertebra from the second to ninth inclusive, there
+corresponds a pair of =ribs= (fig. 36, 6) of a rather special
+character. Each is suturally united with the anterior half of the edge
+of its own vertebra, and overlaps on to the posterior half of the edge
+of the next preceding vertebra. The ribs are much flattened, and each
+is fused with the corresponding costal plate, beyond which it projects
+to fit into a pit in one of the marginal plates.
+
+The tenth thoracic vertebra is smaller than the others, and its neural
+arch does not overlap the preceding vertebra, it bears a pair of small
+ribs which are without costal plates, but meet those of the ninth
+vertebra.
+
+There are no =lumbar= vertebrae.
+
+
+THE SACRAL VERTEBRAE.
+
+The =sacral vertebrae= are two in number, they are short and wide,
+their centra are ankylosed together, and their neural arches are not
+united to the carapace.
+
+The first has the anterior face of the centrum concave and the
+posterior flat, while both faces of the second are flat. Each bears a
+pair of short ribs which meet the ilia, but are not completely
+ankylosed either with them or the centra.
+
+
+THE CAUDAL VERTEBRAE.
+
+The =caudal vertebrae= are eighteen in number. The centrum of the
+first is flat in front and is ankylosed to the second sacral; behind
+it is convex. The others are all very similar to one another, and
+decrease gradually in size when followed back. Each has a moderately
+long centrum, concave in front and convex behind, both terminations
+being formed by epiphyses. The neural arch arises only from the
+anterior half of the vertebra; it bears a blunt truncated neural spine
+and prominent pre- and post-zygapophyses. The first seven caudal
+vertebrae bear short ribs attached to their lateral margins, the
+similar outgrowths on the succeeding vertebrae do not ossify from
+distinct centres, and are transverse processes rather than ribs.
+
+
+B. THE SKULL.
+
+The skull of the Turtle is divisible into the following three parts:--
+
+(1) the cranium;
+
+(2) the lower jaw or mandible;
+
+(3) the hyoid.
+
+(1) THE CRANIUM.
+
+The =cranium= is a very compact bony box, containing a cavity in which
+the brain lies, and which is a direct continuation of the neural canal
+of the vertebrae.
+
+[Illustration FIG. 38. THE SKULL OF THE GREEN TURTLE (_Chelone
+midas_). × 1/2. A, POSTERIOR HALF, B, ANTERIOR HALF. (Brit. Mus.)
+
+ 1. parietal.
+ 2. squamosal.
+ 3. quadrate.
+ 4. basisphenoid.
+ 5. basi-occipital.
+ 6. quadratojugal.
+ 7. opisthotic.
+ 8. exoccipital.
+ 9. foramen magnum.
+ 10. splenial.
+ 11. articular.
+ 12. dentary.
+ 13. angular.
+ 14. supra-angular.
+ 15. premaxilla.
+ 16. maxilla.
+ 17. jugal.
+ 18. postfrontal.
+ 19. vomer.
+ 20. prefrontal.
+ 21. frontal.
+ 22. external auditory meatus leading into tympanic cavity.]
+
+Like those of the skull as a whole its component bones may be
+subdivided into three sets:--
+
+1. those forming the brain-case or =cranium proper=;
+
+2. those developed in connection with the special sense organs;
+
+3. those forming the upper jaw and suspensorial apparatus.
+
+Both cartilage and membrane bones take part in the formation of the
+skull, and a considerable amount of cartilage remains unossified,
+especially in the ethmoidal and sphenoidal regions.
+
+
+1. THE CRANIUM PROPER OR BRAIN-CASE.
+
+The cartilage and membrane bones of the brain-case when taken together
+can be seen to be more or less arranged in three rings or segments,
+called respectively the occipital, parietal, and frontal segments.
+
+The =occipital segment= is the most posterior of these, and consists
+of four cartilage bones, the =basi-occipital=, the two =exoccipitals=
+and the =supra-occipital=; these bound the =foramen magnum=.
+
+The =basi-occipital= (figs. 38 and 39, 5) lies ventral to the foramen
+magnum and only bounds a very small part of it; it forms one-third of
+the =occipital condyle= by which the skull articulates with the atlas
+vertebra. It unites dorsally with the exoccipitals and anteriorly with
+the basisphenoid.
+
+The =exoccipitals= are rather small bones, which form the sides and
+the greater part of the floor of the foramen magnum, and two-thirds of
+the occipital condyle. Laterally each is united with the pterygoid and
+opisthotic of its side. At the sides of the occipital condyle each
+exoccipital is pierced by a pair of foramina, the more dorsal and
+posterior of which transmits the hypoglossal nerve.
+
+The =supra-occipital= (fig. 39, 14) is a larger bone than the others
+of the occipital segment. It forms the upper border of the foramen
+magnum and is drawn out dorsally into a large crest which extends back
+far beyond the occipital condyle. In the adult the supra-occipital is
+completely ankylosed with the epi-otics.
+
+
+The =Parietal segment=.
+
+The ventral portion of the parietal segment is formed by the
+=basisphenoid= (figs. 38 and 39, 4) which lies immediately in front of
+the basi-occipital. A triangular portion of it is seen in a ventral
+view of the skull, but it is quickly overlapped by the pterygoids. It
+gives off dorsally a pair of short processes which meet the pro-otics.
+
+The alisphenoidal region is unossified and the only other constituents
+of the parietal segment are the _parietals_ (fig. 39, 1). These are
+large bones which, after roofing over the cranial cavity, extend
+upwards and become expanded into a pair of broad plates which unite
+with the squamosal and bones of the frontal segment to form a wide,
+solid, false roof to the skull. Each also sends ventralwards a plate
+which meets an upgrowth from the pterygoid and acts as an alisphenoid.
+
+
+The =Frontal segment=.
+
+Of the frontal segment the basal or presphenoidal and lateral or
+orbitosphenoidal portions do not become ossified, the dorsal portion
+however includes three pairs of membrane bones, the _frontals_,
+_prefrontals_ and _postfrontals_.
+
+The _frontals_ are a pair of small bones lying immediately in front of
+the parietals, and in front of them are the _prefrontals_ (figs. 38
+and 39, 20), a pair of similar but still smaller bones, which are
+produced ventrally to meet the vomer and palatines. They form also the
+dorsal boundary of the anterior nares. The _postfrontals_ (figs. 38
+and 39, 18) are larger bones, united dorsally to the frontals and
+parietals, posteriorly to the squamosals, and ventrally to the jugals
+and quadratojugals. All three pairs of frontal bones, especially the
+postfrontals, take part in the bounding of the orbits.
+
+[Illustration FIG. 39. LONGITUDINAL VERTICAL SECTION THROUGH THE
+CRANIUM OF A GREEN TURTLE (_Chelone midas_). × 2/3. (Camb. Mus.)
+
+ 1. parietal.
+ 2. squamosal.
+ 3. quadrate.
+ 4. basisphenoid.
+ 5. basi-occipital.
+ 6. quadratojugal.
+ 7. pro-otic.
+ 8. opisthotic.
+ 9. pterygoid.
+ 10. palatine.
+ 11. rod passed into narial passage.
+ 12. exoccipital.
+ 13. epi-otic fused to supra-occipital.
+ 14. supra-occipital.
+ 15. premaxillae.
+ 16. maxillae.
+ 17. jugal.
+ 18. postfrontal.
+ 19. vomer.
+ 20. prefrontal.
+ 21. frontal.
+ V, VII, VIII, IX, X, XI, XII,
+ foramina for the exit of
+ cranial nerves.]
+
+
+2. THE SENSE CAPSULES.
+
+Skeletal structures occur in connection with each of the three special
+sense organs of hearing, sight, and smell.
+
+
+The =Auditory capsules=.
+
+The auditory or periotic capsule of the turtle is rather large and its
+walls are well ossified, epi-otic, pro-otic and opisthotic bones being
+present.
+
+The =epi-otic= (fig. 39, 13) is the more dorsal of the three bones,
+and in the adult is completely ankylosed with the supra-occipital.
+
+The =opisthotic= (fig. 39, 8) is the ventral posterior element. On its
+inner side it is united to the supra-occipital above, and to the
+exoccipital below; it sometimes becomes completely fused with the
+exoccipital. In front it meets the pro-otic, and on its outer side the
+squamosal and quadrate. Its anterior portion is hollowed out by the
+cavity in which the auditory organ lies, it gives off also a process
+which is separated from the exoccipital by an oval foramen through
+which the glossopharyngeal, pneumogastric, and spinal accessory nerves
+leave the cranial cavity.
+
+The =pro-otic= is the anterior element; it meets the supra-occipital
+and opisthotic posteriorly, while anteriorly it is separated from the
+alisphenoidal plate of the parietal and pterygoid by a large oval
+foramen through which the maxillary and mandibular branches of the
+trigeminal nerve pass out (fig. 39, V 1 & 2). It is hollowed out
+posteriorly by the cavity in which the auditory organ lies, and its
+inner wall as seen in longitudinal section is pierced by a foramen
+through which the external carotid artery and facial nerve leave the
+cranial cavity,--the nerve finally leaving the skull through a small
+oval foramen on the anterior face of the pro-otic near its junction
+with the quadrate.
+
+Between the pro-otic and opisthotic as seen in a longitudinal section
+of the skull is a large opening constricted in the middle. This is
+the =internal auditory meatus= (fig. 39, VIII.). Through it the
+auditory nerve leaves the cranial cavity and enters the ear. The ramus
+vestibularis leaves through the dorsal part of the hole, the ramus
+cochlearis through the ventral.
+
+The cavity of the auditory or periotic capsule communicates with the
+exterior by a fairly large hole, the =fenestra ovalis=, which lies
+between the opisthotic and pro-otic, and opens into a deep depression,
+the =tympanic cavity=, which is seen in a posterior view of the skull
+lying just external to the exoccipital. The cavity communicates with
+the exterior by a large opening, the =external auditory meatus= (fig.
+38, 22).
+
+Several other openings are seen in the tympanic cavity; through one at
+the extreme posterior end the pneumogastric and spinal accessory
+nerves leave the skull, and through another, a little further
+forwards, the glossopharyngeal.
+
+The auditory ossicles consist of a long bony =columella=, whose inner
+end fits into the fenestra ovalis, while the outer end is attached to
+a small cartilaginous plate, the =extra-columella=, which is united to
+the tympanum.
+
+
+The =Optic capsules=.
+
+The skeletal structures developed in connection with the optic capsule
+do not become united to the skull. They consist of:--
+
+(_a_) the =sclerotic=, a cartilaginous sheath investing the eye and
+bearing
+
+(_b_) a ring of ten small bony scales.
+
+There is no _lachrymal_ bone.
+
+
+The =Olfactory= or =Nasal capsules=.
+
+The basicranial axis in front of the basisphenoid remains
+cartilaginous, neither presphenoid nor mesethmoid bones are developed,
+and the orbits in a dry skull communicate by a wide space through
+which the second, third, fourth, and sixth cranial nerves pass out.
+Separate _nasal_ bones also do not occur, the large prefrontals
+extending over the area usually occupied by both nasals and
+lachrymals.
+
+The only bone developed in connection with the nasal capsules is the
+_vomer_ (fig. 39, 19), an unpaired bone lying ventral to the
+mesethmoid cartilage, and in contact laterally with the maxillae,
+premaxillae and palatines.
+
+
+3. THE UPPER JAW AND SUSPENSORIAL APPARATUS.
+
+A number of pairs of bones are developed in connection with the upper
+jaw and suspensorial apparatus, one pair, the =quadrates=, being
+cartilage bones, while the rest are all membrane bones.
+
+The _squamosals_ (fig. 38, 2) are large bones which, lying external to
+the auditory bones, extend dorsalwards to meet the parietals and
+postfrontals, and form a large part of the false roof of the skull.
+They are united ventrally with the quadrates and quadratojugals.
+
+Each =quadrate= (fig. 38, 3) forms the outer boundary of the tympanic
+cavity, and is firmly united on its inner side with the opisthotic,
+exoccipital, and pterygoid. Dorsally it is fixed to the squamosal and
+anteriorly to the quadratojugal. Its outer surface is marked by a deep
+recess, and it ends below in a strong condyle with which the mandible
+articulates. In front of the quadrates are a pair of thin plate-like
+bones, the _quadratojugals_ which are united in front to the jugals or
+malars.
+
+The _jugals_ (fig. 38, 17) are also thin plate-like bones, and form
+part of the posterior boundary of the orbit. They are attached
+dorsally to the postfrontals, and anteriorly to the maxillae, while
+each also sends inwards a horizontal process which meets the pterygoid
+and palatine.
+
+The _maxillae_ (figs. 38 and 39, 16) are a pair of large
+vertically-placed bones, each drawn out ventrally into a straight,
+sharp, cutting edge. They form the lateral boundaries of the anterior
+nares, and each sends dorsalwards a process which meets the
+postfrontal. Each also sends inwards a horizontal =palatine process=,
+which meets the palatine and vomer, and also forms much of the floor
+of the narial passage.
+
+The _premaxillae_ (figs. 38 and 39, 15) are a pair of very small bones
+forming the floor of the anterior narial opening, they are wedged in
+between the two maxillae, and send back processes which meet the vomer
+and palatines.
+
+The _palatines_ (fig. 39, 10) are a pair of small bones firmly united
+with the pterygoids behind, with the maxillae and jugals externally,
+and with the vomer in the middle line. Each also gives off a palatine
+plate which unites with the expanded lower edge of the vomer, and
+forms the ventral boundary of the posterior nares. Anteriorly the
+palatines form the posterior boundary of a large foramen through which
+the ophthalmic branches of the fifth and seventh nerves pass to the
+olfactory organs.
+
+The _pterygoids_ (fig. 39, 9) are a pair of large bones which unite
+with one another by a long median suture. They are united also with
+the palatines in front, and with the quadrate, basisphenoid,
+basi-occipital, and exoccipitals behind. Each also sends dorsalwards a
+short =alisphenoid plate= which meets that from the parietal.
+
+Piercing the posterior end of the _pterygoid_ is the prominent opening
+of the carotid canal; a bristle passed into this hole emerges through
+a foramen lying between the pro-otic and the alisphenoid process of
+the pterygoid.
+
+
+(2) THE LOWER JAW OR MANDIBLE.
+
+The =mandible= consists of one unpaired bone, formed by the fusion of
+the two _dentaries_, and five pairs of bones, called respectively the
+=articular=, _angular_, _supra-angular_, _splenial_ and _coronoid_.
+
+The fused _dentaries_ (fig. 38, 12) form by far the largest of the
+bones; they constitute the flattened anterior part of the mandible,
+and extend back below the other bones almost to the end of the jaw.
+
+The _coronoid_ is the most anterior of the paired bones, it forms a
+prominent process to which the muscles for closing the jaw are
+attached.
+
+The =articular= (fig. 38, 11) is expanded, and with the
+_supra-angular_ forms the concave articulating surface for the
+quadrate.
+
+The _splenial_ (fig. 38, 10) is a thin plate applied to the inner
+surface of the posterior part of the mandible.
+
+The _angular_ (fig. 38, 13) is a slender plate of bone lying below the
+supra-angular and splenial.
+
+
+(3) THE HYOID.
+
+The hyoid apparatus is well developed, parts of the first two
+branchial arches being found, as well as of the hyoid proper. It
+consists of a more or less oblong flattened =basilingual plate= or
+=body of the hyoid= which represents the fused ventral ends of the
+hyoid and branchial arches of the embryo, and is drawn out into a
+point anteriorly. The greater part is formed of unossified cartilage,
+but at the posterior end it is bilobed, and a pair of ossified tracts
+occur. To its sides are attached three pairs of structures, which are
+portions of the hyoid and first and second branchial arches
+respectively.
+
+The free part of the =hyoid= consists of a small piece of cartilage
+attached to the anterior part of the basilingual plate at its widest
+portion (fig. 53, 2).
+
+The =anterior cornu= or free part of the =first branchial arch= is
+much the largest of the three structures. Its proximal portion
+adjoining the basilingual plate is cartilaginous, as is its distal
+end; the main part is however ossified.
+
+The =posterior cornu= or free part of the =second branchial arch=
+(fig. 53, 4) consists of a short flattened cartilaginous bar arising
+from the bilobed posterior end of the basilingual plate.
+
+The hyoid apparatus has no skeletal connection with the rest of the
+skull.
+
+
+2. THE APPENDICULAR SKELETON.
+
+This includes the skeleton of the two pairs of limbs and their
+girdles.
+
+
+THE PECTORAL GIRDLE.
+
+The pectoral girdle has an anomalous position, being situated
+internal or ventral to the ribs. It consists of three bones, a dorsal
+bone, the =scapula=, an anterior ventral bone, the =precoracoid=, and
+a posterior ventral bone, the =coracoid=.
+
+The =scapula= is a small somewhat rod-shaped bone forming about
+two-thirds of the glenoid cavity. At its proximal end it is closely
+united with the precoracoid, the two bones ossifying continuously. It
+tapers away distally, and is directed dorsalwards towards the
+carapace.
+
+The =precoracoid= forms an angle of about 130° with the scapula, with
+which it is completely fused at its proximal end. Its distal end is
+somewhat expanded and flattened, and is terminated by a
+fibrocartilaginous =epiprecoracoid= which meets its fellow. It takes
+no part in the formation of the glenoid cavity.
+
+The =coracoid= is a large flattened blade-shaped bone forming about
+one-third of the glenoid cavity. It does not meet its fellow in a
+ventral symphysis, and is terminated by a cartilaginous =epicoracoid=.
+The glenoid articulating surfaces of both scapula and coracoid are
+lined by a thick pad of cartilage.
+
+
+THE ANTERIOR LIMB.
+
+This is divisible into three portions, the =upper arm=, =fore-arm= and
+=manus=.
+
+The =upper arm= contains a single bone, the =humerus=.
+
+The =humerus= (fig. 40, A, 1) is a stout, nearly straight, somewhat
+flattened bone widely expanded at both ends. At the proximal end is
+the large hemispherical =head=, which articulates with the glenoid
+cavity. Behind the head the bone is drawn out into another large
+rounded process. Below the head the shaft bears a small outgrowth
+which is continuous with a larger one on the flexor surface (see p.
+29). The bone is terminated distally by the =trochlea=, consisting of
+three partially distinct convex surfaces which articulate with the
+bones of the fore-arm.
+
+The =fore-arm= includes two bones, the =radius= and =ulna=; both these
+are small bones, and are immovably fixed to one another proximally and
+distally.
+
+The =radius= or pre-axial bone is the larger of the two, and is a
+rod-like bone terminated at either end by an epiphysis. It articulates
+at its proximal end with the humerus, and at its distal end with the
+radiale or scaphoid bone of the carpus.
+
+The =ulna= (fig. 40, A, 3) or postaxial bone is shorter than the
+radius, and more expanded at its proximal end, where it articulates
+with the humerus. It articulates distally with the intermedium (lunar)
+and the ulnare (cuneiform) bones of the carpus. All three bones of the
+arm have their terminations formed by epiphyses which ossify from
+centres distinct from those forming the shafts.
+
+The =Manus= consists of the =carpus= or =wrist= and the =hand= which
+includes the metacarpals and phalanges.
+
+The =carpus= consists of a series of ten small bones, one of which,
+the =pisiform= (fig. 40, A, 10), differs from the others in being
+merely an ossification in the tendon of a muscle. The remaining nine
+bones are arranged in a proximal row of three, the =ulnare= (fig. 40,
+A, 6), =intermedium=, and =radiale=, and a distal row of five
+(carpalia 1-5), each of which supports one of the metacarpals. A ninth
+bone, the =centrale= (fig. 40, A, 7), is wedged in between the two
+rows. The ulnare, intermedium and pisiform are comparatively large
+flattened bones, the others are small and cubical.
+
+The =hand=. This is composed of five digits, each of which consists of
+a metacarpal and of a varying number of phalanges.
+
+The =metacarpals=. The first metacarpal (fig. 40, A, 11) is a short
+flattened bone, the others are all elongated and cylindrical, and are
+terminated proximally by slightly concave surfaces, and distally by
+slightly convex ones.
+
+The =phalanges=. The first and fifth digits both have two phalanges,
+the second, third, and fourth have each three. The distal phalanx of
+the first digit is stout and curved, and bears a horny claw; those of
+the other digits are flattened and more or less pointed.
+
+[Illustration FIG. 40. A. ANTERIOR LIMB OF A YOUNG HAWKSBILL TURTLE
+(_Chelone imbricata_) × 1/4 (Brit. Mus.). B. POSTERIOR LIMB OF A LARGE
+GREEN TURTLE (_Chelone midas_) × 1/8 (Camb. Mus.).
+
+ 1. humerus.
+ 2. radius (almost hidden by the ulna).
+ 3. ulna.
+ 4. radiale.
+ 5. intermedium.
+ 6. ulnare.
+ 7. centrale.
+ 8. carpale I.
+ 9. carpale IV.
+ 10. pisiform.
+ 11. first metacarpal.
+ 12. femur.
+ 13. tibia.
+ 14. fibula.
+ 15. tibiale intermedium and centrale fused.
+ 16. fibulare.
+ 17. tarsale 1.
+ 18. tarsale 2.
+ 19. tarsalia 4 and 5 fused.
+ 20. first metatarsal.
+ 21. fifth metatarsal.
+ I, II, III, IV, V, digits.]
+
+
+THE PELVIC GIRDLE.
+
+The pelvic girdle consists of three bones; a dorsal bone, the =ilium=,
+an anterior ventral bone, the =pubis=, and a posterior ventral bone,
+the =ischium=. All three bones contribute largely to the formation of
+the =acetabulum=, with which the head of the femur articulates.
+
+The =ilium= is a small slightly curved bone, which unites ventrally
+with the pubis and ischium, and extends dorsalwards and backwards to
+meet the distal ends of the sacral ribs.
+
+The =pubis= is the largest bone of the three; its distal end forms a
+wide bilobed plate, the inner lobe meeting its fellow in a median
+symphysis, while the other lobe or lateral process extends outwards.
+Attached to the symphysis in front is a cartilaginous =epipubis=,
+while behind, the two pubes are terminated by a wide rounded
+cartilaginous area.
+
+The =ischium=, the smallest bone of the three, is flattened and like
+the pubis meets its fellow in a median symphysis. A narrow band of
+cartilage connects the symphysis pubis with the symphysis ischii, and
+separates the two =obturator foramina= from one another.
+
+
+THE POSTERIOR LIMB.
+
+This is divisible into three portions, the =thigh=, the =crus= or
+=shin=, and the =pes=.
+
+The =thigh= includes a single bone, the =femur=.
+
+The =femur= (fig. 40, B, 12) is a short thick bone, with a prominent
+rounded =head= articulating with the acetabulum. Behind this head is a
+deep pit, beyond which is a roughened area corresponding with the
+great trochanter of mammals. The distal end is expanded and somewhat
+convex.
+
+The bones of the =crus= or =shin= are the =tibia= and =fibula=. These
+are both straight rod-like bones with expanded terminations which
+closely approach one another, while elsewhere the bones diverge
+considerably.
+
+The terminations of all three of the leg bones are formed by
+epiphyses.
+
+The =Pes= consists of the =tarsus= or =ankle=, and the =foot=, which
+is made up of five digits.
+
+The =tarsus=. The tarsal bones of the Turtle do not retain their
+primitive arrangement to such an extent as do the carpals. They are
+arranged in a proximal row of two and a distal row of four. Of the
+bones in the proximal row the postaxial one is much the smaller and is
+the =fibulare=; the larger pre-axial one (fig. 40, B, 15) represents
+the =tibiale=, =intermedium=, and =centrale= fused, and articulates
+with both tibia and fibula. The first three distal tarsalia are all
+small bones and are very similar in size, and each articulates
+regularly with the corresponding metatarsal. The fourth bone (fig. 40,
+B, 19) is much larger, and represents tarsalia 4 and 5 fused. The
+first two distal tarsalia articulate with the pre-axial tarsal of the
+proximal row, the third only with its neighbours the second, and the
+fused fourth and fifth. The latter articulates with both bones of the
+proximal row.
+
+Each =digit= consists of a metatarsal and of a varying number of
+phalanges.
+
+The =metatarsals=. The first metatarsal (fig. 40, B, 20) is broad and
+flattened, the second, third and fourth, are all elongated bones with
+nearly flat terminations formed by small epiphyses. The fifth is large
+and flattened, and the articular surface for the phalanx is situated
+somewhat laterally.
+
+The =phalanges=. The first digit has two phalanges and is the stoutest
+of them all; its distal phalanx is sheathed in a large horny claw. The
+other digits, of which the third is the longest, have each three
+phalanges. The distal phalanges of the second and third digits are
+flattened and pointed and bear small horny claws.
+
+
+FOOTNOTES:
+
+[89] Another view commonly held is that the neural and costal plates
+are respectively formed by the expanded neural arches and ribs.
+
+
+
+
+CHAPTER XV.
+
+THE SKELETON OF THE CROCODILE.
+
+
+The species chosen for description is _C. palustris_, a form occurring
+throughout the Oriental region, but the description would apply almost
+equally well to any of the other species of the genus _Crocodilus_,
+and with comparatively unimportant modifications to any of the living
+Crocodilia.
+
+
+I. EXOSKELETON.
+
+The exoskeleton of the Crocodile is strongly developed and includes
+elements of both epidermal and dermal origin.
+
+_a._ The =epidermal exoskeleton= is formed of a number of horny
+=scales= or plates of variable size covering the whole surface of the
+body. Those covering the dorsal and ventral surfaces are oblong in
+shape, and are arranged in regular rows running transversely across
+the body. The scales covering the limbs and head are mostly smaller
+and less regularly arranged, and are frequently raised into a more or
+less obvious keel. Those covering the dorsal surface of the tail are
+very prominently keeled.
+
+The epidermal exoskeleton also includes the horny =claws= borne by the
+first three digits of both manus and pes.
+
+_b._ The =dermal exoskeleton=. This has the form of bony =scutes=
+which underlie the epidermal scales along the dorsal surface of the
+trunk and anterior part of the tail. Except in very young individuals
+the epidermal scales are rubbed off from these scutes, which
+consequently come to project freely on the surface of the body. Each
+scute is a nearly square bony plate, deeply pitted or sculptured, and
+marked by a strong ridge on its dorsal surface, while its ventral
+surface is smooth. Contiguous scutes are united to one another by
+interlocking sutures.
+
+The scutes are arranged in two distinct areas, viz. (1) a small
+anterior =nuchal shield= which lies just behind the head and is formed
+of six large scutes more or less firmly united together, and (2) a
+larger posterior =dorsal shield= covering the whole of the back and
+anterior part of the tail, and formed of smaller scutes, which are
+arranged in regular transverse rows, and progressively diminish in
+size when followed back.
+
+The =teeth= are exoskeletal structures, partly of dermal, partly of
+epidermal origin. They lie along the margins of the jaws and are
+confined to the premaxillae, maxillae and dentaries. They are simple
+conical structures, without roots; each is in the adult placed in a
+separate socket, and is replaced by another which as it grows comes to
+occupy the pulp cavity of its predecessor. In the young animal the
+teeth are not placed in separate sockets but in a continuous groove.
+This feature is met with also in the Ichthyosauria. The groove
+gradually becomes converted into a series of sockets by the ingrowth
+of transverse bars of bone. The anterior teeth are sharply pointed and
+slightly recurved, the posterior ones are more blunt.
+
+The upper jaw bears about nineteen pairs of teeth, the lower jaw about
+fifteen pairs. The largest tooth in the upper jaw is the tenth, and in
+the lower jaw the fourth.
+
+The three living families of Crocodilia, the Crocodiles, Alligators
+and Garials, can be readily distinguished by the characters of the
+first and fourth lower teeth. In Alligators both first and fourth
+lower teeth bite into pits in the upper jaw; in Garials they both bite
+into notches or grooves in the upper jaw. In Crocodiles the first
+tooth bites into a pit, the fourth into a notch in the upper jaw.
+
+
+II. ENDOSKELETON.
+
+1. THE AXIAL SKELETON.
+
+This includes the vertebral column, the skull, and the ribs and
+sternum.
+
+A. THE VERTEBRAL COLUMN.
+
+The vertebral column is very long, consisting of some sixty vertebrae.
+It can be divided into the usual five regions, the cervical, thoracic,
+lumbar, sacral, and caudal regions.
+
+[Illustration FIG. 41. FIRST FOUR CERVICAL VERTEBRAE OF A CROCODILE
+(_C. vulgaris_). (Partly after VON ZITTEL.)
+
+ 1. pro-atlas.
+ 2. lateral portion of atlas.
+ 3. odontoid process.
+ 4. ventral portion of atlas.
+ 5. neural spine of axis.
+ 6. postzygapophysis of fourth vertebra.
+ 7. tubercular portion of fourth cervical rib.
+ 8. first cervical rib.
+ 9. second cervical rib.
+ 10. convex posterior surface of centrum of fourth vertebra.]
+
+THE CERVICAL VERTEBRAE.
+
+Counting as cervical all those vertebrae which are anterior to the
+first one whose ribs meet the sternum, there are nine cervical
+vertebrae, all of which bear ribs.
+
+As a type of the cervical vertebrae the fifth may be taken. It has a
+short cylindrical =centrum= deeply concave in front and convex behind.
+From the anterior part of the ventral surface of the centrum arises a
+short =hypapophysis=, and on each side is a facet with which the lower
+limb (=capitulum=) of the cervical rib articulates. The =neural arch=
+is strongly developed and drawn out dorsally into a long =neural
+spine=, in front of which are a pair of upstanding processes bearing
+the prominent upwardly and inwardly directed =prezygapophyses=. At the
+sides and slightly behind the neural spine are a corresponding pair of
+processes bearing the =postzygapophyses=, which look downwards and
+outwards. At the point where it joins the centrum the neural arch is
+drawn out into a short blunt =transverse process= with which the upper
+limb (=tuberculum=) of the cervical rib articulates. The sides of the
+neural arch are slightly notched behind for the exit of the spinal
+nerves.
+
+The first or =atlas= vertebra differs much from any of the others, and
+consists of four quite detached portions, a ventral arch, with two
+lateral portions and one dorsal. The =ventral arch= (fig. 41, 4) is
+flat below and slightly concave in front, forming together with two
+flattened surfaces on the lateral portions a large articulating
+surface for the occipital condyle of the skull. Its posterior face is
+bevelled off and forms with a second pair of facets on the lateral
+portions a surface with which the odontoid process of the second
+vertebra articulates. The postero-lateral surfaces of the ventral arch
+also bear a pair of little facets with which the cervical ribs
+articulate. The lateral portions are somewhat flattened and expanded,
+and bear in addition to those previously mentioned a pair of small
+downwardly directed facets, the postzygapophyses, which articulate
+with the prezygapophyses of the second vertebra. The dorsal portion
+(fig. 41, 1) is somewhat triangular in shape, and overhangs the
+occipital condyle. It is often regarded as the neural arch of a
+vertebra in front of the atlas and is called the _pro-atlas_; but as
+it is a membrane bone it is not properly a vertebral element.
+
+The second or =axis vertebra= also differs a good deal from the other
+cervicals. The centrum is massive, and is terminated in front by a
+very large slightly concave articulating surface formed by the
+=odontoid process= (fig. 41, 3) which is united with the centrum by
+suture only, and is really the detached centrum of the first vertebra.
+The cervical rib (fig. 41, 9) articulates with two little
+irregularities on the odontoid process. The posterior surface of the
+centrum is convex. The neural arch is strongly developed and
+terminated dorsally by a long neural spine (fig. 41, 5), its sides are
+notched, slightly in front and more prominently behind for the exit of
+the spinal nerves. It is drawn out in front into two little processes
+bearing a pair of upwardly and outwardly directed prezygapophyses,
+while the postzygapophyses are similar to those of the other cervical
+vertebrae.
+
+The last two cervical vertebrae resemble the succeeding thoracic
+vertebrae, in the increased length of the transverse processes, and
+the shifting dorsalwards of the facet with which the capitulum of the
+rib articulates.
+
+THE THORACIC VERTEBRAE.
+
+The thoracic vertebrae commence with the first of those that bears
+ribs reaching the sternum. They are ten in number, and the first eight
+are directly connected with the sternum by ribs.
+
+The =third= of them may be taken as a type. It has a thick cylindrical
+centrum, concave in front and convex behind, there is a slight
+hypapophysis, and the centrum is suturally united with a strong neural
+arch enclosing a narrow neural canal. The neural arch is drawn out
+dorsally into a wide truncated neural spine, and laterally into two
+prominent transverse processes, with the ends of which the tubercula
+of the ribs articulate, while the capitulum articulates in each case
+with a step-like facet (fig. 42, A, 3) on the anterior face of the
+transverse process. The prezygapophyses (fig. 42, A, 2) are borne on
+outgrowths from the bases of the transverse processes, and the
+postzygapophyses on outgrowths at the base of the neural spine.
+
+The thoracic vertebrae behind the third have no hypapophyses, and the
+capitular facets gradually come to be placed nearer and nearer the
+ends of the transverse processes, at the same time becoming less
+prominent; otherwise these vertebrae are just like the third.
+
+[Illustration FIG. 42. ANTERIOR VIEW OF A, A LATE THORACIC AND B, THE
+FIRST SACRAL VERTEBRA OF A YOUNG CROCODILE (_C. palustris_). × 1/3.
+
+ 1. neural spine.
+ 2. process bearing prezygapophysis.
+ 3. facet for articulation with the capitulum of the rib.
+ 4. sacral rib.
+ 5. surface which is united with the ilium.
+ 6. concave anterior face of centrum.]
+
+In the first and second thoracic vertebrae the capitulum of the rib
+articulates, not with a facet on the transverse process, but with a
+little elevation borne at the line of junction of the centrum and
+neural arch.
+
+THE LUMBAR VERTEBRAE.
+
+These are five in number, and are precisely like the posterior
+thoracic vertebrae, except in the fact that the transverse processes
+have no facets for the articulation of ribs.
+
+THE SACRAL VERTEBRAE.
+
+These are two in number, and while the centrum of the first is concave
+in front (fig. 42, B, 6) and nearly flat behind, that of the second is
+flat in front and concave behind. Each has a pair of strong =ribs=
+(fig. 42, B, 4) firmly ankylosed in the adult with a wide surface
+furnished partly by the centrum, partly by the neural arch. The distal
+ends of these ribs are united with the ilia. The character of the
+neural spines and zygapophyses is the same as in the thoracic
+vertebrae.
+
+THE CAUDAL VERTEBRAE.
+
+These are very numerous, about thirty-four in number. The first
+differs from all the other vertebrae of the body in having a biconvex
+centrum. The succeeding ones are procoelous and are very much like the
+posterior thoracic and lumbar vertebrae, having high neural spines and
+prominent straight transverse processes. They differ however in having
+the neural spines less strongly truncated above, and the transverse
+processes arise from the centra and not from the neural arches. When
+followed further back the centra and neural spines gradually lengthen
+while the transverse processes become reduced, and after the twelfth
+vertebra disappear. Further back still the neural spines and
+zygapophyses gradually become reduced and disappear, as finally the
+neural arch does also, so that the last few vertebrae consist simply
+of cylindrical centra.
+
+Each caudal vertebra, except the first and the last eleven or so, has
+a =V=-shaped =chevron bone= attached to the postero-ventral edge of
+its centrum. The anterior ones are the largest and they gradually
+decrease in size till they disappear.
+
+B. THE SKULL[90].
+
+The skull of the Crocodile is a massive depressed structure
+presenting a number of striking characteristics, some of the more
+important of which are:--
+
+1. All the bones except the mandible, hyoid, and columella are firmly
+united by interlocking sutures. In spite of this, however, growth of
+the whole skull and of the component bones goes on continuously
+throughout life, this growth being especially marked in the case of
+the facial as opposed to the cranial part of the skull.
+
+2. All the bones appearing on the dorsal surface are remarkable for
+their curious roughened and pitted character; this feature is
+prominent also in many Labyrinthodonts.
+
+3. The size of the jaws and teeth is very great.
+
+4. The mandibular condyle is carried back to some distance behind the
+occipital condyle.
+
+5. The occipital plane (see p. 386) of the skull is vertical.
+
+6. The length of the secondary palate is remarkably great, and the
+vomer takes no part in its formation.
+
+7. The posterior nares are placed very far back, the nasal passages
+being as in mammals separated from the mouth by the long secondary
+palate.
+
+8. There is a complicated system of Eustachian passages communicating
+at one end with the tympanic cavity and at the other end with the
+mouth cavity.
+
+9. The interorbital septum is mainly cartilaginous, the presphenoidal
+and orbitosphenoidal regions remaining unossified.
+
+The =skull= is divisible into three parts:--
+
+(1) the cranium, (2) the lower jaw, (3) the hyoid.
+
+The =cranium= may again for purposes of description be divided into:--
+
+1. the cranium proper or brain case;
+
+2. the bones connected with the several special sense organs;
+
+3. the bones of the upper jaw, and suspensorial apparatus.
+
+
+1. THE CRANIUM PROPER OR BRAIN CASE.
+
+[Illustration FIG. 43. PALATAL ASPECT A, OF THE CRANIUM, B, OF THE
+MANDIBLE OF AN ALLIGATOR (_Caiman latirostris_). × 1/3. (Brit. Mus.)
+
+ 1. premaxillae.
+ 2. maxillae.
+ 3. palatine.
+ 4. pterygoid.
+ 5. posterior nares.
+ 6. transpalatine.
+ 7. posterior palatine vacuity.
+ 8. anterior palatine vacuity.
+ 9. basi-occipital.
+ 10. opening of median Eustachian canal.
+ 11. jugal.
+ 12. quadratojugal.
+ 13. quadrate.
+ 14. dentary.
+ 15. splenial.
+ 16. coronoid.
+ 17. supra-angular.
+ 18. angular.
+ 19. articular.
+ 20. lateral temporal fossa.
+ 21. openings of vascular canals leading into alveolar sinus.]
+
+The cartilage and membrane bones of the cranium proper when taken
+together can in most vertebrates be seen to be more or less arranged
+in three rings or segments called respectively the =occipital=,
+=parietal= and =frontal= segments; in the Crocodile however only the
+occipital and parietal segments are clearly seen.
+
+The =occipital segment= consists of four cartilage bones, three of
+which together surround the =foramen magnum=.
+
+The most ventral of these, the =basi-occipital= (figs. 43 and 45, 9),
+forms the single convex =occipital condyle= for articulation with the
+atlas, bounds the base of the foramen magnum, and is continuous
+laterally with two larger bones, the =exoccipitals= (fig. 45, 24),
+which meet one another dorsally and form the remainder of the boundary
+of the foramen magnum. Each is drawn out externally into a strong
+process, which is united below with the quadrate, and above with the
+squamosal by a surface seen in a disarticulated skull to be very rough
+and splintered. In a longitudinal section the anterior face of the
+exoccipital is seen to be closely united with the opisthotic.
+
+The exoccipital is pierced by a number of foramina, four lying on the
+posterior surface. Just external to the foramen magnum is a small
+foramen for the exit of the hypoglossal nerve (figs. 44 and 45, XII).
+External to this is the foramen for the pneumogastric (fig. 44, X),
+while more ventrally still is the foramen (fig. 44, 15) through which
+the internal carotid artery enters the skull. Some distance further to
+the side, and more dorsally, is a larger foramen which gives passage
+to the facial nerve and certain blood-vessels.
+
+In a median longitudinal section of the skull the hypoglossal foramen
+is seen, and just in front of it a small foramen for a vein. Further
+forwards the long slit-like opening between the exoccipital and
+opisthotic is the =internal auditory meatus= (fig. 45, VIII) through
+which the auditory nerve leaves the cranial cavity and enters the
+internal ear.
+
+The =supra-occipital= (fig. 45, 5) is a small bone which takes no part
+in the formation of the foramen magnum, and is closely united in front
+with the epi-otic. It is characteristic of Crocodiles that all the
+bones of the occipital segment have their longer axes placed
+vertically, and that they scarcely if at all appear on the dorsal
+surface.
+
+In front of the occipital segment is the =parietal segment=. The
+dorsal and ventral portions of the two segments are in contact with
+one another, but the lateral portions are widely separated by the
+interposition of the =auditory= and =suspensorial bones=.
+
+The =basisphenoid= (fig. 45, 12) is an unpaired wedge-shaped bone,
+united along a deep vertical suture with the basi-occipital. The two
+bones are, however, partially separated in the mid-ventral line by a
+foramen, the opening of the =median Eustachian canal=, which leads
+into a complicated system of Eustachian passages ultimately
+communicating with the tympanic cavity.
+
+The dorsal surface of the basisphenoid is well seen in a section of
+the skull, but owing to the way it tapers ventrally, it appears on the
+ventral surface only as a very narrow strip of bone wedged in between
+the basi-occipital and pterygoids. In a lateral view it is seen to be
+drawn out in front into an abruptly truncated process, the =rostrum=,
+which forms part of the =interorbital septum.= On the anterior part of
+the dorsal surface is a deep pit, the =pituitary fossa= or =sella
+turcica=, at the base of which are a pair of foramina, through which
+the carotid arteries pass. Dorsolaterally the basisphenoid articulates
+with the =alisphenoids=.
+
+[Illustration FIG. 44. LATERAL VIEW OF THE SKULL OF AN ALLIGATOR
+(_Caiman latirostris_). × 1/3. (Brit. Mus.)
+
+ 1. premaxillae.
+ 2. maxillae.
+ 3. lachrymal.
+ 4. prefrontal.
+ 5. jugal.
+ 6. postfrontal.
+ 7. squamosal.
+ 8. quadrate.
+ 9. palatine.
+ 10. pterygoid.
+ 11. transpalatine.
+ 12. quadratojugal.
+ 13. exoccipital.
+ 14. basi-occipital.
+ 15. foramen by which carotid artery enters skull.
+ 16. external auditory meatus.
+ 17. frontal.
+ 18. supra-angular.
+ 19. articular.
+ 20. dentary.
+ 21. coronoid.
+ 22. angular.
+ III, VI, opening for exit of oculomotor and abducens nerves.
+ V, foramen ovale.
+ X, pneumogastric foramen.
+ XII, hypoglossal foramen.]
+
+The =alisphenoids= (fig. 45, 13) are a pair of irregular bones which
+arise from the basisphenoid antero-laterally, and are united dorsally
+with the parietal, frontal, and postfrontals. They bound most of the
+anterior part of the brain case, and each presents on its inner face a
+deep concavity which lodges the cerebral hemisphere of its side.
+Viewed from the ventral side the two alisphenoids are seen to almost
+or quite meet one another immediately below the frontal, and then to
+diverge, forming an irregular opening--partially closed by cartilage
+in the fresh specimen,--through which the optic nerves leave the
+cranial cavity. Further back the alisphenoids meet one another for a
+narrow area, and then diverge again, so that between each and the
+rostrum of the basisphenoid there appears an opening (fig. 44, III,
+VI) through which the oculomotor and abducens nerves leave the
+cranium. Further back still each is united for a short space with the
+basisphenoid, pterygoid and quadrate, and then becomes separated from
+the quadrate by a large foramen, the =foramen ovale= (fig. 44, V),
+through which the whole of the trigeminal nerve passes out.
+
+The dorsal portion of the parietal segment is formed by the _parietal_
+(fig. 45, 4), which though double in the embryo, early comes to form a
+single bone. It extends over the posterior part of the cranial cavity,
+and is continuous in front with the frontal, behind with the
+supra-occipital, and laterally with the postfrontals, squamosals,
+alisphenoids, pro-otics and epi-otics. It forms the inner boundary of
+a large rounded vacuity on the roof of the skull, the =supratemporal
+fossa=.
+
+The =frontal segment= is very imperfectly ossified, there being no
+certain representatives of either the ventral member, the presphenoid,
+or the lateral members, the orbitosphenoids. On the dorsal side there
+is, however, a large development of membrane bones. There is a large
+_frontal_ (fig. 45, 3), unpaired, except in the embryo, united behind
+with the parietal and postfrontal, and drawn out in front into a long
+process which is overlapped by the prefrontals and posterior part of
+the nasals. The frontal ends off freely below, owing to the
+orbitosphenoidal region being unossified, it forms a considerable part
+of the roof of the cranial cavity, but takes no part in the formation
+of the wall.
+
+Each _prefrontal_ (fig. 45, 14) forms part of the inner wall of the
+orbit and sends ventralwards a process which meets the palatine.
+
+The _postfrontals_ (fig. 44, 6) are small bones lying at the sides of
+the posterior part of the frontal. Each is united with a number of
+bones, on its inner side with the frontal and parietal, behind with
+the squamosal, and ventrally with the alisphenoid. It also unites by
+means of a strong descending process with an upgrowth from the jugal,
+and thus forms a =postorbital bar= separating the orbit from the
+lateral temporal fossa. The postfrontal forms also part of the outer
+boundary of the supratemporal fossa.
+
+
+2. THE SENSE CAPSULES.
+
+Skeletal capsules occur in connection with each of the three special
+sense organs of sight, of hearing and of smell.
+
+The =Auditory capsules= and associated bones.
+
+Three bones, the =epi-otic=, =opisthotic= and =pro-otic=, together
+form the auditory or =periotic= capsule of each side. They are wedged
+in between the lateral portions of the occipital and parietal segments
+and complete the cranial wall in this region. Their relations to the
+surrounding structures are very complicated, and many points can be
+made out only in sections of the skull passing right through the
+periotic capsule. The relative position of the three bones is,
+however, well seen in a median longitudinal section. The =opisthotic=
+early becomes united with the exoccipital, while the =epi-otic=
+similarly becomes united with the supra-occipital, the =pro-otic=
+(fig. 45, 7),--seen in longitudinal section to be pierced by the
+prominent =trigeminal foramen=--alone remaining distinct throughout
+life. The three bones together surround the essential organ of hearing
+which communicates laterally with the deep tympanic cavity by the
+=fenestra ovalis=.
+
+The =tympanic cavity=, leading to the exterior by the =external
+auditory meatus= (fig. 44, 16), is well seen in a side-view of the
+skull; it is bounded on its inner side by the periotic bones,
+posteriorly in part by the exoccipital, and elsewhere mainly by the
+quadrate. A large number of canals and passages open into it. On its
+inner side opening ventro-anteriorly is the =fenestra ovalis=, opening
+ventro-posteriorly the =internal auditory meatus= (fig. 45, VIII),
+while dorsally there is a wide opening which forms a communication
+through the roof of the brain-case with the tympanic cavity of the
+other side. On its posterior wall is the prominent foramen through
+which the facial nerve passes on its way to its final exit from the
+skull through the exoccipital, this foramen is bounded by the
+quadrate, squamosal, and exoccipital.
+
+The opening of the fenestra ovalis is in the fresh skull occupied by
+the expanded end of the auditory ossicle, the =columella=, whose outer
+end articulates by a concave facet with a trifid =extra-columellar=
+cartilage which reaches the tympanic membrane. The lower process of
+this extra-columella passes into a cartilaginous rod which lies in a
+canal in the quadrate and is during life continuous with Meckel's
+cartilage within the articular bone of the mandible.
+
+The columella and extra-columella are together homologous with the
+chain of mammalian auditory ossicles.
+
+ * * * * *
+
+The =Optic capsules= and associated bones.
+
+Two pairs of bones are associated with the optic capsules, viz. the
+_lachrymals_ and the _supra-orbitals_. The _lachrymal_ (fig. 44, 3) is
+a fairly large flattened bone lying wedged in between the maxillae,
+nasal, jugal, and prefrontal. It forms a considerable part of the
+anterior boundary of the orbit, and is pierced by two foramina. On the
+orbital edge is a large hole leading into a cavity within the bone
+which lodges the naso-lachrymal sac, and communicates with the narial
+passage by a wide second foramen near the anterior end of the bone.
+The _supra-orbital_ is a very small loose bone lying in the eyelid
+close to the junction of the frontal and prefrontal.
+
+ * * * * *
+
+The =Olfactory capsules= and associated bones.
+
+Two pairs of membrane bones, the _vomers_ and _nasals_, are developed
+in association with the olfactory organ, but the =mesethmoid= is not
+ossified.
+
+The _vomers_ form a pair of delicate bones, each consisting of a
+vertical plate (fig. 45, 15), which with its fellow separates the two
+narial passages, and of a horizontal plate which forms much of their
+roof. The vomers articulate with one another and with the pterygoids,
+palatines, and maxillae.
+
+The _nasals_ (fig. 45, 2) are very long narrow bones extending along
+the middle line from the frontal almost to the anterior nares. They
+are continuous laterally with the premaxillae, maxillae, lachrymals and
+prefrontals. They form the roof of the narial passages.
+
+3. THE UPPER JAW AND SUSPENSORIAL APPARATUS.
+
+These are enormously developed in the Crocodile and are firmly united
+to the cranium. It will be most convenient to begin by describing the
+bones at the anterior end of the jaw and to work back thence towards
+the brain-case. The most anterior bones are the _premaxillae_. The
+_premaxillae_ (figs. 44 and 45, 1) are small bones, each bearing five
+pairs of teeth, set in separate sockets in their alveolar borders.
+They constitute almost the whole of the boundary of the =anterior
+nares=, which are confluent with one another and form a large
+semicircular opening in the roof of the skull, leading into the wide
+narial passage. They are also partially separated from one another in
+the ventral middle line, by the small =anterior palatine vacuity=
+(fig. 43, A, 8). They form the anterior part of the broad =palate=.
+The alveolar border on each side between certain of the teeth is
+marked by pits which receive the points of the teeth of the other jaw.
+The first pair of these pits in the premaxillae are often so deep as to
+be converted into perforations. Pits of the same character occur
+between the maxillary and mandibular teeth.
+
+[Illustration FIG. 45. LONGITUDINAL SECTION THROUGH THE SKULL OF AN
+ALLIGATOR (_Caiman latirostris_). × 1/3. (Brit. Mus.)
+
+ 1. premaxilla.
+ 2. nasal.
+ 3. frontal.
+ 4. parietal.
+ 5. supra-occipital.
+ 6. epi-otic.
+ 7. pro-otic.
+ immediately in front of the figure 7 is the prominent foramen
+ for the trigeminal nerve.
+ 8. opisthotic.
+ 9. basi-occipital.
+ 10. quadrate.
+ 11. pterygoid.
+ 12. basisphenoid.
+ 13. alisphenoid.
+ 14. prefrontal.
+ 15. vomer.
+ 16. maxilla.
+ 17. palatine.
+ 18. dentary.
+ 19. splenial.
+ 20. angular.
+ 21. supra-angular.
+ 22. articular.
+ 23. coronoid.
+ 24. exoccipital.
+ 25. squamosal.
+ 26. jugal.
+ 27. external mandibular foramen.
+ 28. internal mandibular foramen.
+ VIII. internal auditory meatus.
+ XII. hypoglossal foramen.]
+
+The _maxillae_ (figs. 43, A, 2 and 44, 2) are a pair of very large
+bones and bear the remaining teeth of the upper jaw, set in sockets
+along their alveolar borders. On the dorsal side each maxillae is
+continuous with the premaxillae, nasal, lachrymal, and jugal, while
+ventrally it meets its fellow in a long straight suture and forms the
+greater part of the long bony palate. The maxillae are separated in the
+middle line posteriorly by processes from the palatines, while further
+back they meet the transpalatines. The internal or nasal surface, like
+that of the premaxillae, is excavated by a deep longitudinal groove,
+the =narial passage=. In a ventral view of the skull a number of small
+openings (fig. 43, A, 21) are seen close to the alveolar border, these
+are the openings of small vascular canals which lead into the
+=alveolar sinus=, a passage traversing the maxillae, and transmitting
+the superior maxillary branch of the trigeminal nerve and certain
+blood-vessels. This alveolar sinus opens posteriorly by the more
+external of the two large holes in the maxillae, which lie close to the
+anterior edge of the posterior palatine vacuity, to be described
+immediately. The more internal of these holes, on the other hand,
+leads into a cavity lodging the nasal sac. Behind the maxillae the
+completeness of the palate is broken up by the large oval =posterior
+palatine vacuities= (fig. 43, A, 7); these are separated from one
+another in the middle line by the palatines, and are bounded elsewhere
+by the maxillae, transpalatines, and pterygoids.
+
+The _palatines_ (fig. 43, A, 3) are long and rather narrow bones
+interposed between the maxillae in front and pterygoids behind. They
+meet one another in a long suture and form much of the posterior part
+of the palate, while the whole length of their dorsal surface
+contributes to the floor of the narial passage. The dorsal surface of
+each bone is also drawn out on its outer side into a prominent ridge
+which forms much of the side and roof of the narial passage, being in
+contact with the vomer and pterygoid, and at one point by means of a
+short ascending process with the descending process of the
+prefrontal.
+
+The _pterygoids_ (figs. 43, A, 4, and 45, 11) are a pair of large
+bones, each consisting of a median more or less vertical part, which
+becomes ankylosed to its fellow in the middle line early in life, and
+of a wide horizontal part which meets the transpalatine. They
+completely surround the posterior nares (fig. 43, A, 5) and their
+median portions form the whole boundary of the posterior part of the
+narial passage, and assist the palatines and vomers in bounding the
+middle part. The horizontal parts form the posterior part of the
+secondary palate, while the dorsal surface of each looks into the
+=pterygoid fossa=, a large cavity lying below the quadrate and
+quadratojugal at the side of the skull. The lateral margin adjoining
+the transpalatine is in the fresh skull terminated by a plate of
+cartilage against which the mandible plays. Dorsally the pterygoid
+articulates with the basisphenoid, quadrate, and alisphenoid.
+
+The _transpalatines_ (fig. 44, 11) connect the pterygoids with the
+jugals and maxillae, articulating with each of the three bones by a
+long pointed process. The jugal process meets also a down-growth from
+the postfrontal.
+
+The _jugals_ or _malars_ (fig. 44, 5) are long somewhat flattened
+bones which are united to the lachrymals and maxillae in front, while
+passing backwards each is united behind to the _quadratojugal_ (fig.
+44, 12), the two forming the =infratemporal arcade= which constitutes
+the external boundary of the orbit and lateral temporal fossa. The
+jugal is united below to the transpalatine, and the two bones together
+form an outgrowth, which meeting that from the postfrontal forms the
+=postorbital bar=, and separates the orbit from the lateral temporal
+fossa. The quadratojugals are small bones and are united behind with
+the quadrates.
+
+The =quadrate= (figs. 43, A, 13 and 44, 8) of each side is a large
+somewhat flattened bone firmly fixed in among the other bones of the
+skull. It is terminated posteriorly by an elongated slightly convex
+surface, coated with cartilage in the fresh skull, by which the
+mandible articulates with the cranium. The dorsal surface of the
+quadrate is flat behind, further forwards it becomes much roughened
+and articulates with the exoccipital and squamosal; further forwards
+still it becomes marked by a deep groove which forms the floor of the
+external auditory meatus and part of the tympanic cavity. The anterior
+boundary of the quadrate is extremely irregular, it is united dorsally
+with the postfrontal, pro-otic, and squamosal, and more ventrally with
+the alisphenoid. The smooth ventral surface looks into the pterygoid
+fossa. In front the quadrate forms the posterior boundary of the
+supratemporal fossa and foramen ovale, and is continuous with the
+alisphenoid, while it sends down a thin plate meeting the pterygoid
+and basisphenoid. On the inner side of the dorsal surface of the
+quadrate near the condyle, is a small foramen which leads into a tube
+communicating with the tympanic cavity, by a foramen lying in front of
+and ventral to that for the exit of the facial nerve. By this tube air
+can pass from the tympanic cavity into the articular bone of the
+mandible.
+
+The _squamosal_ (fig. 44, 7) meets the quadrate and exoccipital below,
+and forms part of the roof of the external auditory meatus, while
+above it forms part of the roof of the skull and has a pitted
+structure like that of the other bones of the roof. It is continuous
+with the postfrontal in front, forming with it the =supratemporal
+arcade= which constitutes the outer boundary of the supratemporal
+fossa. It meets also the parietal on its inner side, forming the
+=post-temporal bar=, the posterior boundary of the supratemporal
+fossa.
+
+It may be useful to recapitulate the large vacuities in the surface of
+the Crocodile's cranium.
+
+_Dorsal surface._
+
+1. =The Supratemporal fossae=. Each is bounded internally by the
+parietal, behind by the =post-temporal bar= formed by the parietal and
+squamosal, and externally by the =supratemporal arcade= formed by the
+squamosal and postfrontal. The postfrontal meets the parietal in front
+and forms the anterior boundary of the supratemporal fossa.
+
+2. The =Lateral temporal= or =infratemporal fossae=. These lie below
+and to the outer side of the supratemporal fossae. Each is bounded
+dorso-internally by the supratemporal arcade; and behind by a
+continuation of the post-temporal bar formed by the quadrate and
+quadratojugal. The external boundary is the =infratemporal arcade=
+formed of the quadratojugal and jugal, while in front the fossa is
+separated from the orbit by the =postorbital bar= formed by the
+junction of outgrowths from the postfrontal and jugal.
+
+3. The =Orbits=. Each is bounded behind by the postorbital bar,
+externally by the jugal forming a continuation of the infratemporal
+arcade, in front by the lachrymal, and internally by the frontal and
+prefrontal.
+
+4. The =Anterior nares=. These form an unpaired opening bounded by the
+premaxillae.
+
+_Posterior surface._
+
+5. The =Foramen magnum=. The exoccipitals form the chief part of its
+boundary, but part of the ventral boundary is formed by the
+basi-occipital.
+
+6. The =Pterygoid fossae=. These form a pair of large cavities at the
+sides of the occipital region of the skull. The dorsal boundary is
+formed by the quadrate and quadratojugal, the ventral by the
+pterygoid, the internal chiefly by the quadrate, pterygoid,
+alisphenoid, and basisphenoid. The transpalatine forms a small part of
+the external boundary which is incomplete.
+
+_Ventral surface._
+
+7. The =Posterior nares=. These form a median unpaired opening (fig.
+43, A, 5) bounded by the pterygoids.
+
+8. The =Posterior palatine vacuities=. Each is bounded by the maxillae
+in front, the maxillae and transpalatine externally, the transpalatine
+and pterygoid behind, and the palatine on the inner side (fig. 43, A,
+7).
+
+9. The =Anterior palatine vacuity=. This is unpaired and is bounded by
+the premaxillae (fig. 43, A, 8).
+
+
+(_b_) THE LOWER JAW OR MANDIBLE.
+
+The mandible is a strong compact bony structure formed of two halves
+or =rami=, which are suturally united at the symphysis in the middle
+line in front. Each ramus is formed of six separate bones.
+
+The most anterior and largest of these is the _dentary_ (figs. 44, 20,
+and 45, 18), which forms the symphysis, and greater part of the
+anterior half of the jaw, and bears along the outer part of its dorsal
+border a number of sockets or =alveoli= in which the teeth are placed.
+Lying along the inner side of the dentary is a large splint-like bone,
+the _splenial_ (fig. 45, 19), which does not extend so far forwards as
+the symphysis, and is separated from the dentary posteriorly by a
+large cavity. Forming the lower part of all the posterior half of the
+jaw is the large _angular_ (figs. 44, 22, and 45, 20), which underlies
+the posterior part of the dentary in front and sends a long process
+below that bone to the splenial. On the inner side of the jaw there is
+an oval vacuity, the =internal mandibular foramen= (fig. 45, 28),
+between the angular and the splenial; through this pass blood-vessels
+and branches of the inferior dental nerve. Lying dorsal to the angular
+is another large bone, the _supra-angular_ (figs. 44, 18, and 45, 21).
+It extends back as far as the posterior end of the jaw and forwards
+for some distance dorsal to the dentary and splenial. It forms part of
+the posterior margin of a large vacuity, the =external mandibular
+foramen=, which is bordered above and in front by the dentary and
+below by the angular; it gives passage to the cutaneous branch of the
+inferior dental nerve. The concave surface for articulation with the
+mandible and much of the posterior end of the jaw is formed by a short
+but solid bone, the =articular= (fig. 45, 22), which in young skulls
+rather readily becomes detached. The remaining mandibular bone is the
+_coronoid_ (fig. 45, 23), a very small bone of irregular shape
+attached to the angular below, and to the supra-angular and splenial
+above.
+
+(_c_) THE HYOID.
+
+The hyoid of the Crocodile consists of a wide flattened plate of
+cartilage, the =basilingual plate= or =body of the hyoid=, and a pair
+of =cornua=.
+
+The =basilingual plate= (fig. 53, 1) is rounded anteriorly and marked
+by a deep notch posteriorly. The =cornua= (fig. 53, 3), which are
+attached at a pair of notches near the middle of the outer border of
+the basilingual plate, are partly ossified, but their expanded ends
+are formed of cartilage. They pass at first backwards and then upwards
+and inwards. They are homologous with part of the first branchial
+arches of Selachians.
+
+The columella and extra-columella have been already described (p.
+251).
+
+C. THE RIBS AND STERNUM.
+
+=Thoracic ribs.=
+
+The Crocodile has ten pairs of =thoracic ribs=, all except the last
+one or two of which consist of three parts,--a vertebral rib, an
+intermediate rib and a sternal rib.
+
+Of the =vertebral ribs= the third may be taken as a type, it consists
+of a curved bony rod which articulates proximally with the transverse
+process of the vertebra by two facets. The terminal one of these, the
+=capitulum= or =head=, articulates with a notch on the side of the
+transverse process; the other, the =tuberculum=, which lies on the
+dorsal surface a short distance behind the head, articulates with the
+end of the transverse process. From near the distal end an imperfectly
+ossified uncinate =process= (see p. 190) projects backwards.
+
+The =intermediate ribs= are short and imperfectly ossified; they are
+united with the =sternal ribs= (fig. 46, 3), which are large,
+flattened, likewise imperfectly ossified structures, and articulate at
+their distal ends with a pair of long divergent =xiphisternal horns=
+(fig. 46, 5), which arise from the posterior end of the sternum
+proper. The last pair of sternal ribs are attached to the preceding
+pair, not to the xiphisternal horns.
+
+The first and second vertebral ribs differ from the others in the fact
+that the tuberculum forms a fairly long outstanding process.
+
+
+=Cervical ribs.=
+
+Movable ribs are attached to all the cervical as well as to the
+thoracic vertebrae. Those borne by the atlas and axis are long, narrow
+structures attached by a fairly broad base, and tapering gradually.
+The ribs borne by the third to seventh cervical vertebrae are shaped
+like a =T= with a double base, one limb of which, corresponding to the
+tuberculum (fig. 41, 7), articulates with a short transverse process
+arising from the neural arch, while the other, corresponding to the
+capitulum, articulates with a surface on the centrum. The ribs
+attached to the eighth and ninth cervical vertebrae are intermediate
+in character between the =T=-shaped ribs and the ordinary thoracic ribs.
+The anterior limb of the =T= is shortened, the posterior one is drawn
+out, forming the shaft of the rib. The distal portion of the rib of
+the ninth cervical vertebra is unossified.
+
+The =Sacral ribs= have been described in connection with the sacral
+vertebrae.
+
+
+THE STERNUM.
+
+The =sternum= of Crocodiles is a very simple structure, consisting of
+a plate of cartilage (fig. 46, 2) lying immediately dorsal to the
+interclavicle, and drawn out posteriorly into a pair of long
+=xiphisternal horns= (fig. 46, 5).
+
+
+THE ABDOMINAL SPLINT RIBS.
+
+Lying superficially to the recti muscles of the ventral body-wall,
+behind the sternal ribs, are seven or eight series of slender curved
+bones, the _abdominal ribs_ (fig. 46, 4). Each series consists of four
+or more bones, arranged in a =V=-like form with the angle of the =V=
+directed forwards. They show a considerable amount of variability in
+number and character. They are really membrane bones, and are in no
+way homologous with true ribs, but correspond rather with the more
+posterior of the bones constituting the plastron of Chelonia.
+
+[Illustration FIG. 46. STERNUM AND ASSOCIATED MEMBRANE BONES OF A
+CROCODILE (_C. palustris_) × 1/3. (Brit. Mus.)
+
+The last pair of abdominal ribs which are united with the epipubes by
+a plate of cartilage have been omitted.
+
+ 1. interclavicle.
+ 2. sternum.
+ 3. sternal rib.
+ 4. abdominal splint rib.
+ 5. xiphisternal horn.]
+
+
+2. THE APPENDICULAR SKELETON.
+
+This includes the skeleton of the two pairs of limbs and their
+respective girdles.
+
+[Illustration: FIG. 47. LEFT HALF OF THE PECTORAL GIRDLE OF AN
+ALLIGATOR (_Caiman latirostris_) × 2/3. (Brit. Mus.)
+
+ 1. scapula.
+ 2. coracoid.
+ 3. interclavicle.
+ 4. glenoid cavity.]
+
+
+THE PECTORAL GIRDLE.
+
+The pectoral girdle of the Crocodile is less complete than is that of
+most reptiles. It consists of a dorsal bone, the =scapula=, and a
+ventral bone, the =coracoid=, with a median unpaired element, the
+_interclavicle_; but there is no separate representative either of the
+clavicle or precoracoid.
+
+The =scapula= (fig. 47, 1) is a large bone, flattened and expanded
+above where it is terminated by an unossified margin, the
+=suprascapula=, and thickened below where it meets the coracoid. The
+scapula forms about half the =glenoid cavity= (fig. 47, 4) for
+articulation with the humerus, and has the lower part of its anterior
+border drawn out into a roughened ridge.
+
+The =coracoid= (fig. 47, 2) is a flattened bone, much expanded at
+either end; it bears on its upper posterior border a flattened surface
+which forms half the glenoid cavity, and is firmly united to the
+scapula at its dorsal end. Its ventral end meets the sternum.
+
+The _interclavicle_ (figs. 46, 1, and 47, 3) is a long narrow
+blade-shaped bone lying along the ventral side of the sternum; about a
+third of its length projects beyond the sternum in front.
+
+
+THE ANTERIOR LIMB.
+
+This is as usual divisible into three portions, the upper arm,
+fore-arm and manus.
+
+The =upper arm= or =brachium= contains one bone, the =humerus.=
+
+The =humerus= (fig. 48, A, 1) is a fairly long stout bone,
+considerably expanded at either end. The proximal end or head is
+evenly rounded and is formed by an epiphysis ossifying from a centre
+different from that forming the shaft. It articulates with the glenoid
+cavity. The shaft bears on the flexor surface, at some little distance
+behind the head, a prominent rounded protuberance, the =deltoid
+ridge.= The distal end or trochlea is also formed by an epiphysis and
+is partially divided by a groove into two convex surfaces; it
+articulates with the two bones of the fore-arm, the radius and ulna.
+
+[Illustration FIG. 48. A, RIGHT ANTERIOR, AND B, RIGHT POSTERIOR LIMB
+OF A YOUNG ALLIGATOR (_Caiman latirostris_). (Brit. Mus.)
+
+A × 1/2. B × about 1/3.]
+
+ 1. humerus.
+ 2. radius.
+ 3. ulna.
+ 4. radiale.
+ 5. ulnare.
+ 6. pisiform.
+ 7. patch of cartilage representing carpalia 1 and 2; between it
+ and the radiale should be another flattened patch, the centrale.
+ 8. carpalia 3, 4, and 5 (fused).
+ 9. first metacarpal.
+ 10. proximal phalanx of second digit.
+ 11. second phalanx of fifth digit.
+ 12. femur.
+ 13. tibia.
+ 14. fibula.
+ 15. tibiale, intermedium and centrale (fused).
+ 16. fibulare.
+ 17. tarsalia 1, 2, and 3 (fused).
+ 18. tarsalia 4 and 5 (fused).
+ 19. first metatarsal.
+ 20. ungual phalanx of second digit.
+ 21. fifth metatarsal.
+
+The =radius= and =ulna= are nearly equal in size and each consists of
+a long shaft terminated at either end by an epiphysis.
+
+The =radius= (fig. 48, A, 2) or pre-axial bone is slightly the smaller
+of the two. It has a straight cylindrical shaft and is slightly and
+nearly evenly expanded at either end. The proximal end which
+articulates with the humerus is flat or slightly concave, the distal
+end which articulates with the carpus is slightly convex.
+
+The =ulna= (fig. 48, A, 3) or postaxial bone is a curved bone rather
+larger than the radius. Its proximal end is large and convex, but is
+not drawn out into an olecranon process.
+
+The =Manus= consists of the =carpus= or =wrist=, and the =hand=.
+
+The =Carpus=. This differs considerably from the more primitive type
+met with in the Turtle. It consists of six elements arranged in a
+proximal row of three and a distal row of two, with one intervening.
+The bones of the proximal row are the radiale, the ulnare, and the
+pisiform. The =radiale= (fig. 48, A, 4) is the largest bone of the
+carpus: it is a somewhat hour-glass shaped bone, with its ends formed
+by flattened epiphyses. It articulates by its proximal end with the
+whole of the radius, and partly also with the ulna, and by its distal
+end with the centrale.
+
+The =ulnare= (fig. 48, A, 5) is a smaller bone, also somewhat
+hour-glass shaped; it articulates proximally with the pisiform and
+radiale, not quite reaching the ulna. The third bone of the proximal
+row is the =pisiform= (fig. 48, A, 6), an irregular bone, articulating
+with the ulna, radiale, and fifth metacarpal. The =centrale= is a
+flattened cartilaginous element applied to the distal surface of the
+radiale.
+
+The distal row of carpals consists of two small structures. The first
+of these forms a small cartilaginous patch, which is wedged in between
+the first and second metacarpals, the centrale and the bone
+representing carpalia 3, 4 and 5; this cartilaginous patch represents
+=carpalia 1 and 2= (fig. 48, A, 7). The bone representing =carpalia 3,
+4 and 5= is a good deal larger, rounded, and well-ossified; it
+articulates with the ulnare, the pisiform, and the third, fourth, and
+fifth metacarpals.
+
+The =hand=. Each of the five digits consists of an elongated
+metacarpal, terminated at each end by an epiphysis, and of a varying
+number of phalanges. The terminal phalanx of each digit has an
+epiphysis only at its proximal end, the others have them at both ends.
+
+The first digit, or =pollex=, is the stoutest, and has two phalanges,
+the second has three, the third four, the fourth three, and the fifth
+two. The terminal phalanx of each of the first three digits is pointed
+and sheathed in a horny claw; and is also marked by a pair of
+prominent lateral grooves.
+
+THE PELVIC GIRDLE.
+
+The pelvic girdle of the Crocodile consists of four parts, a dorsal
+element, the =ilium=, an anterior ventral element, the =pubis=, a
+posterior ventral element, the =ischium=, and an accessory anterior
+ventral element, the =epipubis=. All except the epipubis take part in
+the formation of the =acetabulum=, which is perforated by a prominent
+hole.
+
+The =ilium= (fig. 49, 1) is a thick strong bone, firmly united on its
+inner side with the two sacral ribs. Its dorsal border is rounded, its
+ventral border bears posteriorly two irregular surfaces, completed by
+epiphyses, which are united respectively with the ischium and pubis.
+
+The =ischium= (fig. 49, 2)--the largest bone of the pelvis, is
+somewhat contracted in the middle and expanded at either end. Its
+proximal end, which is formed by an epiphysis, bears two surfaces, one
+of which is united to the ilium, while the other forms part of the
+acetabulum. The anterior border is also drawn out dorsally into a
+strong process, which is terminated by a convex epiphysis, and is
+united to the pubis. The ventral end of the ischium forms a flattened
+blade, meeting its fellow in a median symphysis.
+
+The =pubis= (fig. 49, 3) is much smaller than either the ilium or
+ischium; it forms a small patch of unossified cartilage, interposed
+between the anterior parts of the ilium and ischium.
+
+[Illustration FIG. 49. PELVIS AND SACRUM OF AN ALLIGATOR (_Caiman
+latirostris_) × 1/2. (Brit. Mus.)
+
+ 1. ilium.
+ 2. ischium.
+ 3. true pubis.
+ 4. epipubis (so-called pubis).
+ 5. acetabular foramen.
+ 6. neural spines of sacral vertebrae.
+ 7. symphysis ischii.
+ 8. process bearing prezygapophysis.]
+
+The =epipubis= (fig. 49, 4) is a large bone with a thickened proximal
+end, which is loosely articulated to the ischium, and a flattened
+expanded distal end, which is united with its fellow, and with the
+last pair of abdominal ribs by a large plate of cartilage. This bone
+is generally described as the pubis.
+
+
+THE POSTERIOR LIMB.
+
+This is as usual divisible into three portions, the =thigh=, the
+=crus= or =shin=, and the =pes=.
+
+The =thigh= is formed by the =femur= (fig. 48, B, 12), a moderately
+long stout bone, not unlike the humerus; it articulates with the
+acetabulum by a fairly prominent rounded =head=. The distal end
+articulating with the tibia and fibula is also expanded, and is
+partially divided into equal parts by anterior and posterior grooves.
+The flexor surface bears a fairly prominent trochanteric ridge. Each
+end of the femur is formed by an epiphysis.
+
+The =crus= or =shin= includes two bones, the =tibia= and =fibula=.
+Both are well developed, but the tibia is considerably the larger of
+the two.
+
+The =tibia= (fig. 48, B, 13) is a strong bone with a flattened
+expanded proximal end articulating with almost the whole of the end of
+the femur, and a similarly expanded distal end articulating with a
+bone representing the fused astragalus and centrale.
+
+The =fibula= (fig. 48, B, 14) is flattened proximally, and articulates
+with only quite a small part of the femur, while distally it is more
+expanded, and articulates with the fibulare (calcaneum) and with a
+facet on the side of the fused astragalus and centrale.
+
+The =Pes= consists of the =tarsus= or =ankle=, and the =foot=.
+
+The =Tarsus=. This, like the carpus, is much reduced and modified from
+the primitive condition. It consists of only four bones, arranged in
+two rows of two each. The two bones of the proximal row are much
+larger than are those of the distal row. The pre-axial of them (fig.
+48, B, 15) representing the fused =astragalus= (tibiale and
+intermedium) and =centrale=, articulates proximally with the tibia and
+fibula, and distally with the first metatarsal, and a small bone
+representing the first three tarsalia. The postaxial bone, the
+=calcaneum= (fibulare) (fig. 48, B, 16), is drawn out into a
+prominent posterior process forming a heel such as is almost unknown
+elsewhere except in mammals. It articulates with the fibula, the
+tibiale-centrale, and distally with a bone representing the fourth and
+fifth tarsalia, and with the fifth metatarsal.
+
+The two bones forming the distal row of tarsals are both small and
+rounded; one represents the first three tarsalia fused together, the
+other tarsalia 4 and 5.
+
+The =Foot=. The =foot= has five digits, but the fifth is much reduced,
+consisting only of a short metatarsal. The first four =metatarsals=
+are all long bones, slightly expanded at each end, and terminated by
+small epiphyses. The first digit has two phalanges, the second three,
+the third four, and the fourth five. The terminal or =ungual phalanx=
+in each instance is grooved and pointed, and in the case of the first
+three digits bears a horny claw. The ungual phalanx progressively
+decreases in size from the first to the fourth. The fifth digit
+consists only of a small, somewhat square metatarsal (fig. 48, B, 21),
+attached to the bone representing the fused fourth and fifth
+tarsalia.
+
+
+FOOTNOTES:
+
+[90] Free use has been made of L.C. Miall's _Studies in Comparative
+Anatomy_, I., _The Skull in Crocodilia_, London, 1878. See also W.K.
+Parker, _Tr. Z.S._, vol. XI. 1885, p. 263.
+
+
+
+
+CHAPTER XVI.
+
+GENERAL ACCOUNT OF THE SKELETON IN REPTILES.
+
+
+EXOSKELETON.
+
+The exoskeleton both epidermal and dermal is exceedingly well
+developed in reptiles.
+
+
+EPIDERMAL EXOSKELETON.
+
+This generally has the form of overlapping horny =scales= which invest
+outgrowths of the dermis, and are found covering the whole body in
+most Rhynchocephalia, Ophidia, and Lacertilia, and many Crocodilia. In
+the Ophidia the ventral surface of the tail is commonly covered by a
+double row of broad scales, while the ventral surface of the precaudal
+part of the body is covered by a single row. In the burrowing snakes
+(Typhlopidae) and some sea snakes (Hydrophidae) these broad scales do
+not occur, the scales of the ventral surface being similar to those of
+the dorsal.
+
+In the Chelonia with the exception of _Dermochelys_, _Trionyx_ and
+their allies there is a well-developed system of horny shields having
+a regular arrangement which has been described in the account of the
+Turtle's skeleton[91].
+
+The =rattle of the rattlesnake= is an epidermal structure formed of
+several loosely articulated horny rings, produced by the modification
+of the epidermal covering of the end of the tail, which instead of
+being cast off when the rest of the outer skin is shed is retained
+loosely interlocked with the adjoining ring or joint. New rings are
+thus periodically added to the base of the rattle, and in old animals
+the terminal ones wear away and are lost.
+
+Horny claws occur on the ends of some or all of the digits in most
+living reptiles.
+
+Owen's Chameleon bears three epidermal horns, one arising from the
+nasal and two from the frontal region.
+
+In the Chelonia, some of the Theromorpha such as _Udenodon_ and
+_Dicynodon_, probably also in the Pterosauria and _Polyonax_ among the
+Dinosaurs, the jaws are more or less cased in horny beaks. The horny
+beaks of Chelonia are variable; sometimes they have cutting edges,
+sometimes they are denticulated, sometimes they are adapted for
+crushing.
+
+
+DERMAL EXOSKELETON.
+
+Nearly all Crocodilia, many Dinosauria, some Rhynchocephalia and
+Pythonomorpha, and some Lacertilia such as _Tiliqua_, _Scincus_ and
+_Anguis_ have a dermal exoskeleton of bony scutes, developed below and
+corresponding in shape to the epidermal scales. Sometimes as in
+_Caiman sclerops_, _Jacare_ and _Teleosaurus_, the scutes completely
+invest the body, being so arranged as to form a dorsal and a ventral
+shield, and a continuous series of rings round the tail. In
+_Crocodilus_ they are confined to the dorsal surface, and in
+_Alligator_ to the dorsal and ventral surfaces. The scutes of some
+extinct forms articulate with one another by a peg and socket
+arrangement as in some Ganoid fish.
+
+The =carapace= of most Chelonia is a compound structure, being partly
+endoskeletal and formed from the ribs and vertebrae, partly from
+plates derived from the dermal exoskeleton. The common arrangement is
+seen in fig. 36. All the surface plates are probably exoskeletal in
+origin, but united with the ventral surfaces of the costal and neural
+plates respectively are the expanded ribs and neural arches of the
+vertebrae.
+
+The =plastron= in the common genus _Chelone_ (fig. 37) includes nine
+plates of bone, one unpaired and four pairs; they will be referred to
+in connection with the ribs and pectoral girdle.
+
+In the Leathery Turtle (_Dermochelys_) the carapace and plastron
+differ completely from those of any other living form. The carapace
+consists of a number of polygonal ossifications fitting closely
+together and altogether distinct from the vertebrae and ribs. The
+plastron is imperfectly ossified, and not united with the pelvis, and
+the whole surface of both carapace and plastron is covered with a
+tough leathery skin, without horny shields.
+
+Some of the extinct Dinosauria have an enormously developed dermal
+exoskeleton. Thus in _Stegosaurus_ and _Omosaurus_ the dorsal surface
+is provided with flattened plates or with spines reaching a length of
+upwards of two feet. In _Polacanthus_ the posterior part of the body
+is protected by a bony shield somewhat recalling that of the little
+armadillo _Chlamydophorus_. No exoskeleton is known in Ichthyosauria,
+Sauropterygia, Pterosauria, many Dinosauria and Theromorpha, and some
+Lacertilia, such as _Chamaeleon_ and _Amphisbaena_.
+
+
+TEETH.
+
+The teeth of reptiles are generally well developed, and in the great
+majority of forms are simple conical structures, uniform in character,
+generally somewhat recurved, and often with serrated edges. Another
+common type of tooth is that with a laterally compressed triangular
+crown provided with a double cutting edge which may or may not be
+serrated. The teeth are mainly formed of dentine, with usually an
+external layer of enamel, and often a coating of cement on the root.
+Vasodentine is found below the dentine in _Iguanodon_. The teeth of
+reptiles never have the enamel deeply infolded, nor do they have
+double roots.
+
+Teeth may be present not only on the jaw-bones, but as in many
+_Squamata_, also on the palatines, pterygoids or vomers. The method
+by which they are attached to the bones varies much. Sometimes as in
+_Iguana_ and some other lizards, they are pleurodont[1], sometimes
+they are acrodont[92], as in the Rhynchocephalia, Pythonomorpha,
+Ophidia and some Lacertilia such as _Agama_. Again they may be set in
+a continuous groove as in the Ichthyosauria and young Crocodilia.
+Finally the teeth may be _thecodont_ or placed in distinct sockets as
+in the Theromorpha, Sauropterygia, adult Crocodilia, Sauropoda and
+Theropoda. In _Iguanodon_ the teeth are set in shallow sockets in a
+groove one side of which is higher than the other; the method of
+attachment thus shows points of resemblance to the thecodont
+condition, the pleurodont condition, and that met with in the
+Ichthyosauria.
+
+In _Ichthyosaurus_ the teeth are marked by a number of vertical
+furrows, and it is from a furrow of this nature greatly enlarged and
+converted into a tube that the channel down which flows the poison of
+venomous snakes is derived.
+
+In most reptiles the dentition is more or less homodont. The only
+reptiles in which a definite heterodont dentition is known are the
+extinct Theromorpha, and in them the teeth vary greatly. Thus
+_Udenodon_ is toothless, the jaws having been probably cased in a
+horny beak. In _Dicynodon_ the jaws are likewise toothless with the
+exception of a pair of permanently growing tusks borne by the maxillae.
+_Dicynodon_ is the only known reptile whose teeth have permanently
+growing pulps. In _Pariasaurus_ the teeth are uniform and very
+numerous, and though placed in distinct sockets are ankylosed to the
+jaw. In _Galesaurus_ and _Cynognathus_ three kinds of teeth can be
+distinguished, slender conical incisor-like teeth, large canine-like
+teeth, and cheek teeth with two or three cusps. The teeth in
+_Galesaurus_ are confined to the jaws, in _Placodus_ and its allies,
+however, large flat crushing teeth are attached to the palatines as
+well as to the jaw-bones, and in _Pariasaurus_ the vomer, palatine and
+pterygoid all bear teeth as well as the jaw bones. The upper jaw of
+_Sphenodon_ and other Rhynchocephalia is provided with two parallel
+rows of teeth, one borne on the maxillae and one on the palatines, the
+mandibular teeth bite in a groove between these two rows. The bone of
+the jaws in _Sphenodon_ is so hard that when the teeth get worn away,
+it can act as a substitute. In the young _Sphenodon_ the vomers bear
+teeth, as they do also in _Proterosaurus_.
+
+There is generally a continuous succession of teeth throughout life,
+the new tooth coming up below, or partly at the side of the one in
+use, and causing the absorption of part of its wall or base. In this
+way the new tooth comes to lie in the pulp cavity of the old one. This
+method of succession is well seen in the Crocodilia.
+
+[Illustration FIG. 50. PREPARATION OF PART OF THE RIGHT MANDIBULAR
+RAMUS OF _Crocodilus palustris_ × 1/2. (Brit. Mus.)
+
+ 1. tooth in use.
+ 2. fairly old germ of future
+ tooth.
+ 3. symphysial surface of the
+ mandible.]
+
+Teeth have been detected in embryos of _Trionyx_, but otherwise no
+teeth are known to occur in Chelonia, or in _Pteranodon_
+(Pterosauria), while the anterior part of the jaw is edentulous in
+_Iguanodon_, _Polyonax_ and some other Dinosaurs, and in
+_Rhamphorhynchus_.
+
+
+ENDOSKELETON.
+
+
+VERTEBRAL COLUMN.
+
+The vertebral column is commonly divisible into the usual five
+regions, but in the Ophidia, Ichthyosauria, and Amphisbaenidae among
+Lacertilia, only into caudal and precaudal regions. In the Chelonia
+there are no lumbar vertebrae.
+
+The form of the vertebral centra is very variable. A large proportion
+of extinct reptiles,--several entire orders,--and the earlier and more
+primitive forms in some of the other groups have amphicoelous
+vertebrae. Vertebrae of this type occur in the Theromorpha,
+Ichthyosauria, most Sauropterygia and Rhynchocephalia, and many
+Dinosauria, also in some of the early Crocodilia such as _Belodon_,
+_Teleosaurus_ and _Goniopholis_, and the Geckonidae among Lacertilia.
+
+The majority of living reptiles have procoelous vertebrae. Thus they
+occur in the Lacertilia (excluding the Geckos), the Ophidia, and the
+Crocodilia, also among extinct forms in the Pterosauria and many
+Dinosauria. On the other hand some Dinosauria such as _Iguanodon_ have
+opisthocoelous cervical vertebrae, while others have opisthocoelous
+thoracic vertebrae. The vertebrae of the Ceratopsidae and some
+Sauropterygia, the thoracic vertebrae of _Iguanodon_, and the sacral
+vertebrae of Crocodilia have flat centra. The first caudal vertebra of
+modern Crocodilia is biconvex, and in the Chelonia all types of
+vertebral centra are found. The cervical vertebrae of _Sphenodon_ are
+noticeable for the occurrence of a small pro-atlas, which may
+represent the neural arch of a vertebra in front of the atlas.
+
+In most reptiles the vertebrae are fully ossified, but in some of the
+more primitive forms the notochord persists in the centre of the
+vertebra (i.e. intervertebrally), this is the case for instance in
+many of the Theromorpha and Rhynchocephalia, and also in the Geckos.
+In other reptiles it persists longest intravertebrally.
+
+The centrum of each of the caudal vertebrae of most Lacertilia is
+traversed by an unossified septum along which it readily breaks.
+
+Chevron bones occur below the caudal vertebrae in Lacertilia,
+Chelonia, Ichthyosauria, many Dinosauria, and _Sphenodon_,
+articulating with quite the posterior part of the centrum which bears
+them. In Lacertilia and Crocodilia (fig. 41, 3) the axis has a
+well-marked odontoid process. The ventral portions of the
+intervertebral discs are sometimes ossified, forming wedge-shaped
+inter centra, as in Geckos, and the cervical vertebrae of _Sphenodon_.
+
+In snakes, Theropod Dinosaurs, and the iguanas among lizards, the
+neural arches are provided with _zygosphenes_, and _zygantra_.
+
+The neural arches are usually firmly ankylosed to the centra, but in
+the Crocodilia and some Chelonia, Sauropterygia, and Dinosauria, the
+suture between the centrum and neural arch persists at any rate till
+late in life. In the Ichthyosauria the neural arches were united to
+the centra by cartilage only.
+
+The thoracic vertebrae of some of the Theromorpha (_Dimetrodon_) are
+remarkable for the extraordinary development of the neural spine, and
+those of Chelonia for the absence of transverse processes.
+
+In living reptiles the number of sacral vertebrae is nearly always
+two, but in the Theromorpha, Dinosauria, and Pterosauria, as many as
+five or six bones may be ankylosed together in the sacral region. In
+Crocodiles the two halves of the pelvis sometimes articulate with
+different vertebrae. The vertebrae of some of the great Sauropoda are
+remarkably hollowed out, having a large vacuity on each side of the
+centrum communicating with a series of internal cavities. The whole
+structure of these vertebrae shows a combination of great strength
+with lightness.
+
+
+THE SKULL.
+
+The reptilian skull is well ossified and the bones are noticeable for
+their density. The true cranium is often largely concealed by a
+secondary or false roof of membrane bones, which is best seen in the
+Ichthyosauria and some of the Chelonia. In other reptiles the false
+roof is more or less broken up by vacuities exposing the true cranial
+walls. The ethmoidal region is the only one in which much of the
+primordial cartilaginous cranium remains. The lateral parts of the
+sphenoidal region are also as a rule not well ossified.
+
+In some reptiles, such as most Lacertilia and Chelonia, the orbits are
+separated only by the imperfect interorbital septum, while in others,
+such as the Ophidia, Crocodilia and Amphisbaenidae, the cranial cavity
+extends forwards between the orbits.
+
+In the occipital region all four bones are ossified. The great
+majority of reptiles have a single convex occipital condyle, but some
+of the Theromorpha such as _Cynognathus_ have two distinct condyles as
+in mammals. Sometimes, as in Chelonia, Ophidia and Lacertilia, the
+exoccipitals, as well as the basi-occipital, take part in the
+formation of the single condyle; sometimes, as in Crocodiles, it is
+formed by the basi-occipital alone, as in birds. The relations of the
+bones to the foramen magnum vary considerably, in Chelonia the
+basi-occipital generally takes no part in bounding it, and in the
+Theromorpha, Crocodilia, and Ophidia, the supra-occipital is excluded.
+The parietals are paired in Geckos and Chelonia alone among living
+forms, and in the extinct Ichthyosauria and some Theromorpha; in all
+other reptiles they are united.
+
+The frontals are paired in Ichthyosauria (fig. 32, 5), Chelonia,
+Ophidia, _Sphenodon_ (fig. 52, B, 4) and some extinct crocodiles, such
+as _Belodon_. They are completely fused in living Crocodilia and some
+Lacertilia and Dinosauria. In the gigantic _Polyonax_ they are drawn
+out into a pair of enormous horns, and the parietals and squamosals
+are greatly expanded behind.
+
+An interparietal foramen occurs in the Theromorpha, the Ichthyosauria
+(fig. 32, 10), _Sphenodon_, the Sauropterygia and most Lacertilia.
+The posterior part of the skull is curiously modified in some
+Chamaeleons, the parietals and supra-occipitals being drawn out into a
+backwardly-projecting sagittal crest which unites with the two
+prolongations from the squamosals. In other Chamaeleons (_C. bifidus_)
+prolongations of the prefrontals and maxillae form large
+forwardly-projecting bony processes.
+
+The roof of the skull is characterised by the development of
+prefrontals and postfrontals, which lie respectively near the anterior
+and posterior extremity of the orbit. In Theromorpha, Squamata,
+Crocodilia, and some Dinosauria lachrymals are developed. There is a
+ring of bones in the sclerotic in the Ichthyosauria (fig. 32, 15), the
+Metriorhynchidae among Crocodiles and some Rhynchocephalia,
+Dinosauria, and Pterosauria.
+
+The pro-otic lies in front of the exoccipital and together with the
+opisthotic forms the hind border of the fenestra ovalis. In Chelonia
+the opisthotic remains separate, in all other living reptiles it fuses
+with the exoccipital. The epi-otic fuses with the supra-occipital.
+
+The parasphenoid, so important in Ichthyopsids, has very often
+disappeared completely; it is present, however, in the Ichthyosauria,
+the Plesiosauridae, and a number of Squamata, in many Ophidia its
+anterior part forming the base of the interorbital septum.
+
+In the Plesiosauridae and most Lacertilia, but not in the
+Amphisbaenidae, a slender bone, the epipterygoid, occurs uniting the
+parietal or the anterior end of the pro-otic with the pterygoid. A
+homologous arrangement occurs in the Ichthyosauria and some Chelonia.
+
+In most reptiles a transpalatine occurs, connecting the maxillae with
+the pterygoid, but this is absent in the Chelonia, and some
+Dinosauria, and in the Typhlopidae among snakes.
+
+The quadrate is always well developed, and except in the Squamata is
+firmly fixed to the surrounding bones. The Chamaeleons also, among the
+Squamata, have a fixed quadrate, and in them too the quadratojugal is
+absent. Separate nasal bones do not occur in any living Chelonia.
+
+The vomers are generally paired as in Squamata, sometimes unpaired as
+in Chelonia.
+
+[Illustration FIG. 51. DORSAL (TO THE LEFT) AND VENTRAL (TO THE RIGHT)
+VIEWS OF THE SKULL OF THE COMMON SNAKE (_Tropidinotus natrix_). (After
+PARKER.)
+
+ 1. premaxillae (fused).
+ 2. anterior nares.
+ 3. nasal.
+ 4. prefrontal.
+ 5. frontal.
+ 6. parietal.
+ 7. maxillae.
+ 8. transpalatine.
+ 9. palatine.
+ 10. pterygoid.
+ 11. pro-otic.
+ 12. exoccipital.
+ 13. supra-occipital.
+ 14. opisthotic.
+ 15. epi-otic.
+ 16. quadrate.
+ 17. parasphenoid.
+ 18. basisphenoid.
+ 19. basi-occipital.
+ 20. occipital condyle.
+ 21. splenial.
+ 22. dentary.
+ 23. angular.
+ 24. articular.
+ 25. supra-angular.
+ 26. coronoid.
+ 27. vomer.
+ 28. squamosal.
+
+ IX, X foramina for the ninth
+ and tenth cranial nerves.]
+
+The disposition of the bones of the jaws is subject to much
+modification in the Ophidia in order to adapt them for swallowing very
+large prey. The arrangements again differ greatly in the venomous and
+non-venomous snakes. In the non-venomous snakes, such as _Python_ and
+_Tropidonotus_, the palatine is large and is fixed to the pterygoid
+which extends outwards (fig. 51, 10) so as to be united to the
+quadrate, and is at the same time firmly connected by the
+transpalatine with the maxillae. The quadrate is united to the
+squamosal, which is loosely attached to the cranium. The premaxillae is
+moderately developed and bears teeth, and the maxillae forms a long bar
+loosely connected with the rest of the skull. The rami of the mandible
+are united only by an extremely elastic ligament. It is as regards the
+maxillae and premaxillae that the skulls of venomous and non-venomous
+snakes differ most. In the rattlesnake (_Crotalus_) and other venomous
+snakes the premaxillae is extremely small and toothless. The maxillae is
+small and subcylindrical, and is movably articulated to the lachrymal,
+which also is capable of a certain amount of motion on the frontal.
+The maxillae is connected by means of the transpalatine with the
+pterygoid, which in its turn is united to the quadrate. When the mouth
+is shut the quadrate is directed backwards, and carrying back the
+pterygoid and transpalatine pulls at the maxillae and causes its
+palatal face, to which the poison teeth are attached, to lie back
+along the roof of the mouth. When the mouth opens the distal end of
+the quadrate is thrust forward, and this necessitates the pushing
+forward of the pterygoid and transpalatine, causing the tooth-bearing
+surface of the maxillae to look downwards and the tooth to come into
+the position for striking.
+
+The Ophidian skull is also noticeable for the absence of the jugals
+and quadratojugals. In poisonous snakes the place of the jugal is
+taken by the zygomatic ligament which connects the quadrate and
+maxillae.
+
+The extent to which the palate is closed in reptiles varies much. In
+many reptiles, such as the Squamata and Ichthyosauria, the palate is
+not complete, both palatines and pterygoids being widely separated in
+the middle line. In others, such as the Crocodilia, Sauropterygia, and
+Chelonia, there is a more or less complete bony palate. In many
+Chelonia this is chiefly formed of the vomer, palatines, and
+pterygoids, the posterior nares being mainly bounded by the palatines.
+In living Crocodilia, however, outgrowths are formed from the
+pterygoids and palatines which arch round and meet one another
+ventrally, forming a secondary palate (fig. 43, A), which completely
+shuts off the true sphenoidal floor of the skull, and causes the
+posterior nares which are bounded by the pterygoids to open very far
+back. Though this feature is common to all postsecondary crocodiles,
+it is interesting to notice that it is not found in the earlier forms,
+but that its gradual evolution can be traced. In the Triassic
+_Belodon_, for instance, the posterior nares open far forwards, and
+are not surrounded by either the palatines or pterygoids. In the
+Jurassic crocodile, _Teleosaurus_, the posterior nares lie further
+back, being surrounded by the palatines, but the pterygoids do not
+meet them. Finally, in the Tertiary forms the arrangements are as in
+living crocodiles.
+
+A short secondary hard palate is found also in the Theriodontia. The
+palatines of _Ichthyosaurus_ are noticeable for their transverse
+position, which recalls that in the Frog.
+
+The various =fossae= or =vacuities= in the false roof of the skull are
+important, and their relations may best be understood by a description
+of their mode of occurrence in _Sphenodon_, a form in which they are
+very completely developed.
+
+[Illustration FIG. 52. SKULL OF HATTERIA. (_Sphenodon punctatus_). A,
+lateral; B, dorsal; C, ventral; D, posterior. (After VON ZITTEL.)
+
+ 1. premaxillae.
+ 2. nasal.
+ 3. prefrontal.
+ 4. frontal.
+ 5. postfrontal.
+ 6. parietal.
+ 7. squamosal.
+ 8. quadratojugal.
+ 9. quadrate.
+ 10. postorbital.
+ 11. jugal.
+ 12. maxillae.
+ 13. vomer.
+ 14. palatine.
+ 15. pterygoid.
+ 16. transpalatine.
+ 17. exoccipital.
+ 18. epipterygoid.
+ 19. basisphenoid.
+ 20. supratemporal fossa.
+ 21. infratemporal or lateral temporal
+ fossa.
+ 22. orbit.
+ 23. post-temporal fossa.
+ 24. foramen magnum.
+ 25. anterior nares.
+ 26. interparietal foramen.
+ 27. dentary.
+ 28. supra-angular.
+ 29. articular.]
+
+In _Sphenodon_, then, on the dorsal surface of the skull, are the
+large =supratemporal fossae= (fig. 52, 20). Their inner margins are
+separated from one another by the parietal walls of the cranium, while
+externally each is bounded by a bony arch, the =supratemporal arcade=,
+formed of the postfrontal, postorbital, and squamosal. Posteriorly the
+boundary is formed by a =post-temporal bar=, formed by the parietal
+and squamosal. Below the supratemporal arcade is another large
+vacuity, the =infratemporal= or =lateral temporal fossa= (fig. 52,
+21). This is bounded above by the supratemporal arcade, and is
+separated from the orbit in front by the =postorbital bar=, formed by
+the union of outgrowths from the jugals and postorbitals. Behind it is
+bounded by a continuation of the post-temporal bar formed of the
+squamosal and quadratojugal, and below by an =infratemporal arcade=,
+which is chiefly composed of the quadratojugal and jugal.
+
+Below the post-temporal bar is a third vacuity, the =post-temporal
+fossa= (fig. 52, D, 23), bounded above by the post-temporal bar and
+below by the exoccipital and opisthotic.
+
+_Sphenodon_ and the Crocodilia are the only living reptiles with
+complete supratemporal and infratemporal arcades, but they are both
+present in the extinct Pterosauria and some Dinosauria.
+
+Supratemporal fossae, bounded below by supratemporal arcades, occur in
+all reptiles except some Chelonia, the Ophidia, the Geckonidae among
+Lacertilia, and the Pariasauria and others among Theromorpha; they are
+specially large in _Nothosaurus_ among the Sauropterygia, _Dicynodon_
+among the Theromorpha, and many Crocodilia and Pterosauria. In some
+Dinosaurs, such as _Ceratosaurus_, they are very small, while the
+infratemporal fossae are correspondingly large.
+
+In _Elginia_[93] (Theromorpha) and some Chelonia, such as _Chelone_,
+there are no fossae on the surface of the skull, a complete false roof
+being developed; in other Chelonia, such as _Trionyx_, the true
+cranium is freely visible, the only part of the false roof developed
+being the infratemporal arcade.
+
+In many reptiles large =pre-orbital vacuities= occur; they are
+specially large in the Pterosauria and in some of the Crocodilia and
+Dinosauria (fig. 35, 3). In some Pterosauria they are confluent with
+the orbits.
+
+The premaxillae are usually separate, but sometimes, as in some Ophidia
+(fig. 51, 1), Chelonia, Lacertilia (Agamidae), and Dinosaurs
+(Ceratopsia) they are united. In the Dinosaur _Hadrosaurus_ they are
+exceedingly large and spatulate. In the Rhynchocephalian
+_Hyperodapedon_ they are drawn out into a strongly curved beak.
+
+As regards the mandible, sometimes, as in most Rhynchocephalia,
+Ophidia and Pythonomorpha, the rami have only a ligamental union;
+sometimes, as in Crocodilia, the Rhynchosauridae and the majority of
+Lacertilia, they are suturally united. In Chelonia (fig. 28, B, 12),
+and apparently in Pterosauria, the two dentaries are completely fused
+together. The sutures between the various bones of the lower jaw
+usually persist, but in Ophidia those between the angular,
+supra-angular, articular and coronoid are obliterated. There are
+sometimes large vacuities in the mandible, as in Theromorpha,
+Crocodilia, and some Dinosauria. In _Iguanodon_, _Polyonax_,
+_Hypsilophodon_ and _Hadrosaurus_ among Dinosaurs the mandible has a
+predentary or mento-meckelian bone which, in some cases at any rate,
+was probably sheathed in a horny beak.
+
+The principal part of the auditory ossicular chain is formed by a
+rod-like columella. The development of the hyoid apparatus varies, and
+it often happens that the first branchial arch is better developed
+than is the hyoid arch. In the Crocodilia and Chelonia there is a
+large basilingual plate or body of the hyoid (fig. 53, 1); but while
+in the Crocodilia the first branchial forms the only well-developed
+arch, in the Chelonia the first and second branchials are both
+strongly developed, and the hyoid is often fairly large.
+
+
+THE RIBS.
+
+[Illustration FIG. 53. HYOIDS OF AN ALLIGATOR (_Caiman latirostris_)
+(TO THE LEFT) AND OF A GREEN TURTLE (_Chelone midas_) (TO THE RIGHT) ×
+5/8. (Brit. Mus.)
+
+The cartilaginous portions are dotted.
+
+ 1. basilingual plate or body of
+ the hyoid.
+ 2. hyoid arch.
+ 3. first branchial arch (anterior
+ cornu).
+ 4. second branchial arch (posterior
+ cornu).]
+
+Ribs are always present, and may be attached to any of the precaudal
+vertebrae. In most reptiles the posterior cervical vertebrae bear
+ribs, while the atlas and axis are ribless; in Crocodiles and Geckos,
+however, ribs are borne even by the atlas and axis. On the other hand,
+in the Chelonia none of the cervical vertebrae bear obvious ribs. In
+the following groups the thoracic ribs have both capitula and
+tubercula--Theromorpha, Ichthyosauria, Crocodilia, Dinosauria,
+Pterosauria. In the other groups each rib articulates by a single
+head, and the position of the facet is subject to a considerable
+amount of variation, thus in the Squamata it lies on the centrum, and
+in the Sauropterygia on the neural arch, while in the Chelonia the rib
+articulates with the contiguous parts of two centra instead of
+directly with one.
+
+In most reptiles a greater or smaller number of ribs are united
+ventrally with a sternum; but in snakes a continuous series of similar
+ribs, all articulating freely with the vertebral column, extends from
+the third cervical vertebra to the end of the trunk. The number of
+ribs connected with the sternum varies from three or four in Lizards
+to eight or nine in Crocodiles. Those which reach the sternum are
+nearly always divided into vertebral, sternal, and intermediate
+portions, and as a rule only the vertebral portion is completely
+ossified. In Crocodiles a number of sternal ribs are connected with a
+cartilaginous arch, which is attached to the hind end of the sternum,
+and represents the xiphisternum. The sacral ribs connecting the
+vertebral column with the ilia are very distinct in Crocodiles; in
+these animals and _Sphenodon_ the vertebral ribs have backwardly
+projecting uncinate processes as in birds.
+
+In the curious arboreal lizard, _Draco volans_, the posterior ribs are
+long and straight, and support a parachute-like expansion of the
+integument used in its long flight-like leaps. In Chelonia the ribs
+are generally combined with the carapace.
+
+In Ichthyosauria, Sauropterygia, Crocodilia and _Sphenodon_, abdominal
+splint ribs occur; and probably all except the first of the paired
+ossifications forming the plastron of Chelonia are of similar
+character. Abdominal ribs have quite a different origin from true
+ribs, for while true ribs are cartilage bones, abdominal ribs have no
+cartilaginous precursors, but are simply the ossified tendons of the
+rectus abdominalis muscle.
+
+
+THE STERNUM.
+
+A sternum occurs in the following groups of reptiles: Rhynchocephalia,
+nearly all Lacertilia, Pythonomorpha, Crocodilia, and Pterosauria, and
+is generally more or less rhomboidal or shield-shaped. In Pterosauria
+it is keeled and bears some resemblance to that of birds. It may have
+been replaced by membrane bone.
+
+[Illustration FIG. 54. VENTRAL VIEW OF THE SHOULDER-GIRDLE OF STERNUM
+OF A LIZARD (_Loemanctus longipes_) × 2. (After PARKER.)
+
+ 1. interclavicle.
+ 2. clavicle.
+ 3. scapula.
+ 4. coracoid.
+ 5. precoracoidal process.
+ 6. glenoid cavity.
+ 7. sternum.
+ 8. xiphisternum.
+ 9. sternal rib.]
+
+The sternum is absent in Sauropterygia, Ichthyosauria, Chelonia,
+Ophidia, and most of the snake-like Amphisbaenidae among Lacertilia;
+while it is not well known in Theromorpha and Dinosauria. In the
+Sauropod _Brontosaurus_, however, two rounded bones occur near the
+base of the coracoids, and these probably represent ossified patches
+in a sternum, which was mainly cartilaginous; similar structures occur
+in _Iguanodon_.
+
+The sternum frequently remains wholly cartilaginous, especially in
+Lacertilia; sometimes it becomes calcified, but true ossification does
+not as a rule take place.
+
+
+APPENDICULAR SKELETON.
+
+
+THE PECTORAL GIRDLE.
+
+The pectoral girdle is well developed in all groups of reptiles except
+the Ophidia, occurring even in the limbless Amphisbaenidae. It is very
+solid in the Theromorpha. As a rule all three cartilage bones,
+scapula, coracoid, and precoracoid are represented, and frequently
+also the membrane bones,--clavicles, and interclavicle.
+
+The coracoids are generally flat expanded bones, which sometimes, as
+in Sauropterygia and Ichthyosauria, meet in a ventral symphysis;
+sometimes, as in Lacertilia, are united with the sides of the sternum.
+In Chelonia neither the coracoids nor precoracoids meet one another,
+but their free ends are connected by fibrocartilaginous bands. In
+Lacertilia the coracoids are pierced by fenestrae.
+
+The precoracoid is generally represented, but the Theromorpha are the
+only reptiles in which it is separately ossified; it forms a
+well-marked process on the coracoid in Lacertilia (fig. 54, 5). It is
+absent in Ichthyosauria, and Dinosauria, and probably in
+Sauropterygia. In some Lacertilia and Chelonia the sternal ends of the
+coracoids are unossified and form epicoracoids; in some Chelonia there
+are also epiprecoracoids; but neither these nor the epicoracoids
+overlap their fellows of the opposite side as they do in arciferous
+Anura (see p. 185). In some Lacertilia with degenerate limbs the
+pectoral girdle is also much reduced, in _Ophisaurus apus_ the ventral
+borders of the coracoids are widely separated.
+
+A scapula is always present, and is generally expanded distally, but
+in the Chelonia the distal end is cylindrical. In the Theromorpha it
+has an acromial process with which the precoracoid articulates, and it
+is very large in Dinosauria. In the Chelonia the scapula and
+precoracoid are ossified continuously. Among the Pterosauria,
+_Pteranodon_ has an unique pectoral girdle; the scapula and coracoid
+are ankylosed and the scapula articulates with the neural spines of
+several ankylosed vertebrae.
+
+Clavicles occur in some Theromorpha such as _Pariasaurus_, and also in
+the Ichthyosauria, Sauropterygia, Rhynchocephalia, and most
+Lacertilia. They are absent in the Pterosauria, the Chamaeleons among
+Lacertilia, the Ophidia and the Crocodilia. They are wanting too in
+the Chelonia, unless the first pair of ossifications in the plastron
+are to be regarded as clavicles. In the Sauropterygia bones regarded
+as the clavicles and interclavicle are generally well developed. The
+unpaired ossification in the plastron of Chelonia is an interclavicle,
+and a representative of the same bone occurs arising from the sternum
+in Pterosauria. A well developed =T=-shaped interclavicle is found in
+Ichthyosauria, Rhynchocephalia, Lacertilia, and some Theromorpha, such
+as _Pariasaurus_.
+
+
+THE LIMBS.
+
+In most reptiles there are two pairs of pentedactylate limbs provided
+with claws, but in nearly all Ophidia and some Lacertilia
+(_Amphisbaena_, _Lialis_, _Anguis_) the limbs have entirely
+disappeared. In a few Ophidia such as _Python_ traces of the posterior
+limbs occur, and in _Chirotes_ among the Amphisbaenidae there are
+minute anterior limbs. The Lacertilians, _Chalcides_ (_Seps_) and
+_Ophisaurus_ (_Bipes_, _Pseudopus_) have very small posterior limbs.
+
+The limbs are as a rule adapted for walking, but in Ichthyosauria,
+Sauropterygia, Pythonomorpha and some Chelonia, they have the form of
+swimming paddles, the relative size of the manus and pes being
+increased, while that of the proximal and middle portions of the limbs
+is reduced. This reduction is carried to its furthest extent in the
+Ichthyosauria in which radius and ulna, tibia and fibula, have the
+form of short polygonal bones similar to those constituting the manus
+and pes. In the Pythonomorpha the reduction of the limb bones is not
+quite so marked, in the Sauropterygia it is less, and still less in
+the Chelonia. In the earlier Ichthyosauria too, the limb bones are not
+so short as they are in the later forms. The Ichthyosaurian limb is
+also remarkable, firstly for the fact that both humerus and femur are
+terminated by concave articulating surfaces instead of by convex
+condyles, and secondly for the great multiplication of the phalangeal
+bones, each digit being sometimes composed of a series of over twenty.
+Sometimes too the number of series is increased, either by the
+bifurcation of some of the digits or by the development of marginal
+bones. In the Sauropterygia the phalanges are likewise increased above
+the normal but not so much as in Ichthyosauria. The humerus and femur
+of Sauropterygia are noticeable for the enormous size of the terminal
+epiphyses which form in each case by far the greater part of the bone.
+
+
+THE ANTERIOR LIMB.
+
+The anterior limb is usually approximately equal in length to the
+posterior, but in many Dinosauria it is considerably the shorter of
+the two. The humerus is generally without distinct condyles, but they
+are well developed in the Theromorpha, the Lacertilia and _Sphenodon_.
+
+In the Theromorpha, some Rhynchocephalia, and some Sauropterygia, such
+as _Mesosaurus_, the humerus has an ent-epicondylar foramen; in
+Lacertilia, Chelonia and some Dinosauria there is an ect-epicondylar
+foramen or groove; _Sphenodon_ possesses both ent- and ect-epicondylar
+foramina. The radius and ulna are always separate. In some Chelonia,
+such as _Chelydra_, the carpus has a very simple arrangement, namely,
+a proximal row of three bones, the radiale, intermedium and ulnare,
+and a distal row of five carpalia, with one bone, the centrale,
+between the two rows. Many reptiles have a carpus only slightly
+different from this. Thus the carpus in _Sphenodon_ differs mainly in
+having two centralia, that of most Lacertilia, in having the centrale
+and intermedium fused.
+
+Crocodiles have a much reduced carpus with the radiale and ulnare
+considerably elongated. The manus in Chamaeleons is curiously
+modified, having the first three digits arranged in one group and
+turned inwards, and the fourth and fifth in another group turned
+outwards; carpalia 3 and 4 are united.
+
+In the Pterosauria the anterior limbs form wings, the phalanges of the
+fifth digit being very greatly elongated to support the wing membrane.
+The first digit is vestigial and the second, third, and fourth are
+clawed.
+
+
+THE PELVIC GIRDLE.
+
+The pelvic girdle is well developed in all reptiles which have
+posterior limbs, but is absent or quite vestigial in Ophidia and those
+Lacertilia which have no posterior limbs. The ilium and ischium agree
+in their general characters throughout all the various groups of
+reptiles, but that is not the case with the pubis.
+
+In many reptiles such as Chelonia, Ichthyosauria and Lacertilia the
+ilia are small, more or less cylindrical bones either directed
+backwards, or vertically placed as in the Chamaeleons. In the
+Crocodilia they are larger and more expanded, while in Dinosauria and
+Pterosauria they are greatly elongated both in front of, and behind,
+the acetabulum. The ischia are generally strongly developed somewhat
+square bones meeting in a ventral symphysis. In Dinosauria the ischium
+(fig. 35, 9) is a much elongated and backwardly-directed bone,
+bearing a forwardly projecting obturator process. In Pterosauria the
+ischium is fused with the ilium, and in both pterosaurs and crocodiles
+the ilium and ischium are the only bones taking part in the formation
+of the acetabulum. In most Lacertilia there is an unpaired structure,
+the _hypo-ischium_ or _os cloacae_ projecting back from the symphysis
+ischii, which is usually separated from the symphysis pubis by a large
+space, the _foramen cordiforme_. In some Lacertilia and Chelonia there
+is a cartilaginous bar dividing the foramen cordiforme into two
+obturator foramina; in many Chelonia this bar is ossified. Among
+_Ophidia_, _Python_, _Tortrix_, _Typhlops_ and their allies have a
+structure representing a vestigial ischio-pubis: but in most Ophidia
+there is no trace of the pelvis. In some Theromorpha all the bones of
+the pelvis are completely fused, forming an os innominatum as in
+mammals; the pubes and ischia are so completely fused that sometimes
+as in _Pariasaurus_ even the obturator foramina are closed.
+
+Concerning the reptilian pubis there are considerable difficulties.
+Sometimes there is only a single pubic structure present, sometimes
+there are two. The reptilian pubis is best understood by comparing the
+arrangements met with in the various other groups with that in the
+Orthopod Dinosaurs such as _Iguanodon_. In _Iguanodon_ the pubis
+consists of two portions, viz. of a moderately broad pre-pubis
+directed downwards and forwards, and of a narrow greatly elongated
+post-pubis directed backwards parallel to the ischium. The pubis is
+united to both ilium and ischium, the acetabulum has a large
+unossified space, and neither pre-pubes nor post-pubes meet in ventral
+symphyses. The arrangement bears a great resemblance to that of Ratite
+birds. In Lacertilia, Chelonia, Rhynchocephalia and Ichthyosauria
+together with Theropod and Sauropod Dinosaurs the pubis corresponds to
+the pre-pubis of _Iguanodon_ and is a more or less cylindrical bone
+expanded at both ends, meeting its fellow in a ventral symphysis. In
+Chelonia and Lacertilia the pubis bears a lateral process which is
+homologous with the post-pubis of Iguanodon. In Lacertilia and
+sometimes in Chelonia there is a cartilaginous epipubis attached to
+the anterior border of the pubic symphysis; this is well developed in
+the Chamaeleons and Geckos. In Crocodilia there is, forming the
+anterior and ventral portion of the acetabulum, a patch of cartilage
+(fig. 49, 3) which is probably the true pubis homologous with that of
+lizards and with the pre-pubis of _Iguanodon_. The large bone
+generally called the pubis in Crocodiles is probably an epipubis.
+
+
+THE POSTERIOR LIMB.
+
+The posterior limb is entirely absent in some Lacertilia and in most
+Ophidia, though traces occur in _Python_, _Tortrix_ and _Typhlops_. In
+the Ichthyosauria, Sauropterygia and Pythonomorpha the posterior limbs
+form swimming paddles and have been already referred to.
+
+The arrangement of the proximal and middle segments of the limb is
+fairly constant in all reptiles with limbs adapted for walking, and
+the tibia and fibula are always separate. The pes is however subject
+to a considerable amount of variation, especially as regards the
+tarsus. In some Chelonia the tarsus like the carpus has an extremely
+simple arrangement, consisting of a proximal row of three bones, the
+tibiale, intermedium and fibulare, a centrale, and a distal row of
+five tarsalia. In most living reptiles, however, the tibiale and
+intermedium are as in mammals united, forming the astragalus. In
+Crocodiles (fig. 48, B, 15) the centrale is also united with the
+tibiale while the distal tarsalia are very slightly developed. The
+calcaneum in Crocodiles is drawn out into a long process forming a
+heel in a manner almost unique among Sauropsida. In _Sphenodon_ and
+Lacertilia the tibia and fibula articulate with a single large bone
+representing the whole proximal row of tarsalia.
+
+The pes is generally pentedactylate, but in some Crocodiles the fifth
+digit is vestigial (fig. 48, B), and in some Dinosauria (fig. 35)
+there are only three digits. The North American Dinosaurs present a
+continuous series ranging from a pentedactylate plantigrade form like
+_Morosaurus_, to such a form as _Hallopus_ with a highly digitigrade
+and specialised pes reduced to three functional digits, and a
+vestigial fifth metatarsal. The second, third and fourth metatarsals
+in this form are nearly two-thirds as long as the femur, and the
+calcaneum is drawn out into a heel much as it is in most mammals.
+
+In Lacertilia, Orthopoda and many Chelonia, the ankle joint comes to
+lie between the proximal and distal row of tarsals as in birds.
+
+
+FOOTNOTES:
+
+[91] See pp. 214 and 215.
+
+[92] These terms are defined on p. 199.
+
+[93] E.T. Newton, _Phil. Trans._ vol. CLXXXIV, B, p. 431 (1893).
+
+
+
+
+CHAPTER XVII.
+
+CLASS. AVES[94].
+
+
+Birds form a large and extremely homogeneous class of the vertebrata,
+and are readily distinguished from all other animals by the possession
+of an epidermal exoskeleton having the form of feathers. Feathers
+differ from hairs in the fact that they grow from papillae formed of
+both the horny and the Malpighian layer of the epidermis, which
+papillae at first project from the surface, and only subsequently
+become imbedded in pits of the dermis. A dermal exoskeleton does not
+occur in birds.
+
+The endoskeleton is characterised by its lightness, the large bones
+being generally hollow; but the pneumaticity does not vary in
+proportion to the power of flight. The cervical part of the vertebral
+column is very long and flexible, while the post-cervical portion is
+generally very rigid, owing to the fusion of many of the vertebrae,
+especially in the lumbar and sacral regions. The vertebrae are
+generally without epiphyses to their centra. The cervical vertebrae in
+living forms have saddle-shaped articulating surfaces, and many of
+them bear ribs. The thoracic ribs in almost all birds have large
+uncinate processes. The sternum is very large, and the ribs are always
+attached to its sides, not as in many reptiles to anybackwardly
+projecting process or processes. The sternum ossifies from two or more
+centres.
+
+The skull is extremely light, and its component bones show a great
+tendency to fuse together completely. The facial part of the skull is
+prolonged into a beak, chiefly formed of the premaxillae; this beak is
+in all modern birds devoid of teeth, and is coated externally with a
+horny epidermal sheath. The quadrate is large and freely movable. The
+supratemporal arcade[1] is imperfect, while the infratemporal
+arcade[95] is complete. There are no postorbital or postfrontal bones.
+Neither parotic processes nor an interparietal foramen occur. There
+are commonly large pre-orbital vacuities. The palatines and pterygoids
+never form a secondary bony palate as in Crocodiles. Part of the floor
+of the skull is formed by a wide _basitemporal_ (paired in the embryo)
+which is continued in front as a long slender _rostrum_; these
+structures have replaced the parasphenoid of Ichthyopsids. Cartilage
+or bone is always developed in the sclerotic. The first branchial arch
+is well developed, the hyoid arch but slightly. The coracoids are
+large, and the clavicles are nearly always united forming the
+_furcula_. There is no separate interclavicle and hardly any trace of
+a precoracoid.
+
+The anterior limbs form wings, and the manus is in the adult always
+much modified, never having more than three digits. The three bones of
+the pelvis are, except in Archaeornithes, always ankylosed together in
+the adult, and the ilium is greatly prolonged in front of the
+acetabulum, which is perforated. The ilia are not connected with the
+sacrum by ossified sacral ribs. The pubes and ischia are directed
+backwards parallel to one another, and except in a very few forms
+never meet their fellows in ventral symphyses. The fibula is generally
+much reduced. The proximal tarsal bones are always ankylosed to the
+tibia, and the distal tarsals to the metatarsals, so that the ankle
+joint is _intertarsal_. The first metatarsal is nearly always free.
+The pes never has more than four digits in the adult.
+
+The class Aves is most conveniently divided into two subclasses: 1.
+Archaeornithes. 2. Neornithes.
+
+
+Subclass I. ARCHAEORNITHES.
+
+The only form referred to this subclass of extinct birds is
+_Archaeopteryx_[96], the earliest known bird. In this animal the
+skeleton does not seem to be pneumatic. The cervical and trunk
+vertebrae are generally thought to be flat, certainly their
+articulating surfaces are not saddle-shaped. There is no long compound
+sacrum as in modern birds. The tail is longer than the whole body, the
+caudal vertebrae are twenty in number, they gradually taper as traced
+away from the trunk, and each bears a pair of feathers. The posterior
+caudal vertebrae are not united together to form a _pygostyle_. The
+upper jaw bears thirteen pairs of conical teeth, planted in distinct
+sockets in the maxillae and premaxillae, but the mandible has only three
+pairs. The presence of these teeth forms the most essential difference
+between the skull of _Archaeopteryx_ and that of modern birds, and the
+fact that they occur on the premaxillae renders it improbable that a
+horny beak was present. There is a ring of ossifications in the
+sclerotic. The ribs do not show uncinate processes, and articulate
+with the vertebrae by single heads not divided into capitula and
+tubercula. Abdominal ribs appear to have been present. The furcula is
+large, and the scapula has a well developed acromion. The sternum is
+unknown. The radius and ulna are approximately equal in size. In the
+manus the first, second and third digits[97] are present, each
+terminated by a claw. The second digit is considerably the longest,
+while the third includes four phalanges. The three bones of the pelvis
+probably remained distinct throughout life. The tarsals are ankylosed
+respectively to the tibia and metatarsals as in other birds. The
+metatarsals are ankylosed together, and the pes has four digits.
+
+
+_Subclass II._ NEORNITHES.
+
+To this subclass may be referred all known birds except
+_Archaeopteryx_. They all agree in having a short tail whose component
+vertebrae are commonly ankylosed together forming a pygostyle. The
+three metacarpals do not all remain distinct. The bones of the pelvis
+are ankylosed together, and to a large though variable number of
+vertebrae. There are three orders, the Ratitae, Odontolcae, and
+Carinatae.
+
+
+_Order_ 1. RATITAE.
+
+The Ratitae differ from _Archaeopteryx_ and the great majority of
+Carinatae in being flightless. The bones are generally not pneumatic,
+containing marrow instead of air, in the Ostrich however they are very
+pneumatic. The tail is short and the posterior caudal vertebrae are
+generally ankylosed together forming a pygostyle. The pectoral girdle
+has comparatively a much smaller size than in Carinatae, clavicles are
+small or absent, and the scapula and coracoid lie nearly in the same
+straight line. The ilium and ischium do not as in Carinatae unite
+posteriorly, and enclose a foramen except in very old Rheas and Emeus.
+The quadrate articulates with the cranium by a single head. The vomers
+unite and form a broad plate, separating the palatines, pterygoids and
+basisphenoidal rostrum.
+
+The anterior limbs are greatly reduced in size or even absent, while
+the posterior limbs are greatly developed and adapted for running. The
+tibia and fibula are quite distinct.
+
+Many ornithologists agree that the various forms grouped together as
+Ratitae are not all very closely allied to one another, that they
+resemble one another mainly in having lost the power of flight, and do
+not form a natural group.
+
+The Ratitae include the following groups:--
+
+_Æpyornithes_[98], huge extinct birds from Madagascar.
+
+_Apteryges_, including the Apteryx of New Zealand.
+
+_Dinornithes_[99], the Moas, huge extinct birds from New Zealand, and
+some of the neighbouring islands.
+
+_Megistanes_, including the Cassowaries (_Casuarius_) of Australia,
+New Guinea, and some of the neighbouring islands; and the Emeus
+(_Dromaeus_) of Australia.
+
+_Rheornithes_, including the Rheas of S. America.
+
+_Struthiornithes_, including the Ostriches (_Struthio_) now living in
+Africa, and found fossil in N. India and Samos.
+
+
+_Order_ 2. ODONTOLCAE.
+
+This order includes only an extinct N. American bird
+_Hesperornis_[100]. The jaws are provided with a series of sharp teeth
+placed in continuous grooves, but the premaxillae are toothless, and
+were probably sheathed in a horny beak. The rami of the mandible are
+not ankylosed together in front. The skeleton is not pneumatic. The
+cervical vertebrae have saddle-shaped articulating surfaces as in
+ordinary birds, and the thoracic vertebrae are not ankylosed together.
+The tail is comparatively long, and formed of twelve vertebrae with
+only slight indications of a pygostyle. The ribs have uncinate
+processes. The anterior limb is quite vestigial, being reduced to a
+slender humerus. The posterior limb is very powerful and adapted for
+swimming.
+
+
+_Order_ 3. CARINATAE.
+
+This order includes the vast majority of living birds. The cervical
+vertebrae have saddle-shaped articulating surfaces (except in the
+Ichthyornithiformes). The posterior caudal vertebrae are ankylosed
+forming a pygostyle. The quadrate articulates with the cranium by a
+double head. In all except the Tinamidae the vomers are narrow behind
+and not interposed between the palatines, pterygoids and
+basisphenoidal rostrum. The sternum has a median keel, and the
+anterior limbs are in the great majority of cases adapted for flight.
+Clavicles are well developed, and the scapula and coracoid are nearly
+at right angles to one another. The various groups into which the
+Carinatae are divisible are shown in the table on pp. 40-42. Their
+special characters will not be dealt with.
+
+[Illustration FIG. 55. _Gallus bankiva_ var. _domesticus_. THE LEFT
+HALF OF THE SKELETON. The skull, vertebral column, and sternum are
+bisected in the median plane. (After Marshall and Hurst.)
+
+A, acetabulum. B, cerebral fossa. CB, cerebellar fossa. CL, clavicle.
+CO, coracoid. CR, cervical rib. C 1 = one, first cervical vertebra.
+FE, femur. HC, ventral end of clavicle. HU, humerus. HY, hyoid. IF,
+ilio-sciatic foramen. IL, ilium. IS, ischium. L, lachrymal. MC 3,
+postaxial metacarpal. MN, mandible. MS, xiphoid processes. MT,
+tarso-metatarsus. MT 1, first metatarsal. N, nasal. OP, optic foramen.
+P, premaxillae. PB, pubis. PL, palatine. PY, pygostyle. R, radius. RC,
+radial carpal. S, keel of sternum. SC, scapula. T, tibio-tarsus. TH 4,
+fourth thoracic vertebra. U, ulna. UC, ulnar carpal. UP, uncinate
+process. Z, infra-orbital bar. 1, 2, 3, 4, first, second, third and
+fourth digits of pes. 3, pre-axial, 4, middle, and 5, postaxial digit
+of manus.]
+
+
+FOOTNOTES:
+
+[94] M. Fürbringer, _Untersuchungen zur Morphologie und Systematik der
+Vögel_, I. and II. Amsterdam, 1888. Cf. H. Gadow, _Nature_, XXXIX.
+1888, pp. 150 and 177.
+
+T.H. Huxley, "On the classification of birds." _P.Z.S._, London, 1867.
+
+E. Selenka and H. Gadow, _Vögel_ in Bronn's _Classen und Ordnungen des
+Thierreichs_ 1869-1890.
+
+[95] See p. 283.
+
+[96] R. Owen, _Phil. Trans._, vol. CLIII., p. 33; 1863. T.H. Huxley,
+_P. R.S._, vol. XVI., p. 243; 1868. C. Vogt, _Rev. Scient._, ser. 2,
+tom. 9, p. 241; 1879. C.H. Hurst, _Nat. Sci._, vol. III., p. 275;
+1893; vol. VI., pp. 112, 180, 244; 1895. W.P. Pycraft, _Nat. Sci._,
+vol. V., pp. 350 and 437; 1894; and vol. VIII., p. 261; 1896.
+
+[97] According to Hurst the fourth and fifth digits are also present.
+
+[98] See C.W. Andrews, _P.Z.S._, 1894, p. 108.
+
+[99] See T.J. Parker, _Tr. Zool. Soc. London_, vol. XIII., pt. 2,
+1895, and F.W. Hutton, several papers in _Tr. N. Zealand Inst._, 1893
+and 1895.
+
+[100] See O.C. Marsh. _Odontornithes. A monograph of the extinct
+toothed birds of N. America._ New Haven, 1880.
+
+
+
+
+CHAPTER XVIII.
+
+THE SKELETON OF THE WILD DUCK (_Anas boschas_).
+
+
+I. EXOSKELETON.
+
+The exoskeleton of the Duck and indeed of all birds is entirely
+epidermal in origin. Its most important part consists of =feathers=,
+but it includes also the following horny structures:--
+
+(_a_) =scales=, which cover the toes and tarso-metatarsus;
+
+(_b_) =claws=, which are attached to the distal phalanges of the toes
+and of the pollex;
+
+(_c_) the wide =beak=, which sheaths both upper and lower jaws, and
+whose edges are raised into lamellae, which act as strainers.
+
+
+FEATHERS.
+
+A well developed feather, such as one of the large quill feathers of
+the wing or tail, consists of the following parts: A main stem, the
+=scapus=, which forms the axis running along the whole length of the
+feather, and is divided into (1) a proximal hollow cylindrical
+portion, the =calamus= or =quill=, and (2) a distal solid portion, the
+=rachis= or =shaft=, which is square in section, flexible and grooved
+along its ventral surface, and bears a number of lateral processes,
+the =barbs=. The =calamus= which is partly imbedded in a pit in the
+dermis, bears two holes: one, the =inferior umbilicus=, is at its
+proximal end, and into it enters a vascular outgrowth from the dermis;
+the other, the =superior umbilicus=, lies on the ventral surface at
+the junction of the calamus and scapus.
+
+The =barbs= are a series of narrow elastic plates, attached by their
+bases to the rachis, and with their edges looking upwards and
+downwards. The barbs are connected together by a number of smaller
+processes, the =barbules=, which interlock with one another by means
+of hooklets, and bear the same relation to the barbs that the barbs do
+to the rachis. The barbs and barbules, together with the rachis,
+constitute the =vexillum= or =vane= of the feather. Any feather having
+the above type of structure is called a =penna= or a =contour
+feather=, from the fact that it helps to produce the contour of the
+body.
+
+
+VARIETIES OF FEATHERS.
+
+1. =Pennae.= There are two kinds of pennae or contour feathers.
+
+(_a_) The =quills=. These form the large feathers of the wing and
+tail. They are divided into two groups, the =remiges=, or wing quills,
+and the =rectrices=, or tail quills.
+
+The =remiges=[101] include three sets of feathers, the =primaries= or
+=metacarpo-digitals=, which are attached to the bones of the manus,
+the =secondaries= or =cubitals=, which are attached to the ulna, and
+the =humerals=, which are attached to the humerus.
+
+The =primaries= differ from all the other quill feathers in having the
+posterior half of the vane much wider than the anterior half. They are
+ten in number, and of these six, the =metacarpal= quills (fig. 57,
+14), are attached to the second and third metacarpals, one, the
+=ad-digital= (fig. 57, 15), to the phalanx of the third digit, two,
+the =mid-digitals= (fig. 57, 16), to the first phalanx of the second
+digit, and two, the =pre-digitals= (fig. 51, 17), to the second
+phalanx of the second digit. One of the pre-digitals is very small,
+and is called the =remicle= (fig. 57, 11).
+
+[Illustration FIG. 56. THE WING OF A WILD DUCK (_Anas boschas_).
+
+The upper figure shows the dorsal side of a right wing, the lower
+figure the ventral side of a left wing. × 1/3. (Brit. Mus.)
+
+ 1. scapulars.
+ 2. tectrices marginales.
+ 3. tectrices minores.
+ 4. bastard wing.
+ 5. tectrices majores.
+ 6. metacarpo-digitals or primaries.
+ 7. tectrices mediae.
+ 8. cubitals or secondaries.
+ 9. pennae humerales.
+ 10. pennae axillares.]
+
+In addition, a group of three quill feathers is attached to the first
+digit, constituting the =bastard wing= or =ala spuria= (fig. 56, 4).
+
+The =secondaries= or =cubitals= (fig. 56, 8) form a group of seventeen
+feathers, attached to the ulna; they are shorter than the primaries,
+and do not have the posterior half of the vane much wider than the
+anterior half.
+
+The =humerals= (figs. 56, 9 and 57, 12) form a group of eight small
+feathers, of varying length, attached to the anterior half of the
+humerus.
+
+[Illustration FIG. 57. WINGS OF A WILD DUCK WITH THE COVERTS REMOVED
+(_Anas boschas_). × 1/3.
+
+A. Right wing seen from the dorsal side. B. Left wing disarticulated
+and seen from the ventral side. (Brit. Mus.)
+
+ 1. humerus.
+ 2. radius.
+ 3. ulna.
+ 4. radial carpal.
+ 5. ulnar carpal.
+ 6. first phalanx of first digit.
+ 7. second metacarpal.
+ 8. third metacarpal.
+ 9. first phalanx of second digit.
+ 10. second phalanx of second digit.
+ 11. remicle.
+ 12. pennae humerales.
+ 13. cubitals or secondaries.
+ 14. metacarpal quills.
+ 15. ad-digital.
+ 16. mid-digitals.
+ 17. pre-digital.]
+
+(b) The =tectrices= or =coverts= are short feathers, which cover over
+the quills of the rectrices and remiges, and clothe the body
+generally. Their barbules are less developed than is the case with the
+quill feathers, so that the barbs separate readily from one another,
+especially at the base of the vane. The nomenclature of the various
+patches of coverts on the wings is seen in fig. 56. A small patch of
+backwardly-directed feathers surrounding the external auditory opening
+are known as the =auriculars=.
+
+2. The =filoplumes= are rudimentary feathers, consisting of a minute
+stem and slightly developed vane. They are left in the skin after the
+other feathers have been removed.
+
+3. The =plumulae=, or down feathers, have the stem very slightly
+developed, while the barbs are soft and free from one another. They
+are distributed all over the body, not only among the contour
+feathers, but also over the spaces (_apteria_) which bear no contour
+feathers.
+
+In the young bird the rudiments of the new feathers are formed at the
+bases of the embryonic down feathers, and as they grow they push them
+out from the skin. The embryonic down feathers however remain attached
+to the apices of the new feathers till these have reached a length of
+about an inch; they are then shed.
+
+
+II. ENDOSKELETON.
+
+As compared with that of the Turtle or Crocodile, the endoskeleton of
+the Duck is characterised by:
+
+1. The great lightness of the bones, many of which contain air
+cavities.
+
+2. The tendency to become ankylosed together shown by many of the
+bones.
+
+3. The modification of the anterior limbs and girdle for the purpose
+of flight.
+
+
+1. THE AXIAL SKELETON.
+
+This, as in other vertebrates, is divisible into--
+
+ A. The vertebral column.
+
+ B. The skull.
+
+ C. The ribs and sternum.
+
+
+A. THE VERTEBRAL COLUMN.
+
+The vertebral column of the duck, like that of the great majority of
+birds, presents a number of well-marked characteristics, contrasting
+strongly with those of the generality of higher vertebrates. The centra
+are always without epiphyses. The neck is exceedingly long, about as
+long as all the rest of the vertebral column put together, and is
+remarkable for its flexibility. The trunk portion of the vertebral
+column on the other hand is characterised by extreme rigidity, and
+the marked tendency shown by the component vertebrae to fuse together
+into one almost continuous mass. The most rigid part of the vertebral
+column is that to which the pelvis is united, as no less than seventeen
+vertebrae take part in the union. The tail of the duck, like that of
+all living birds, is very short, and the posterior caudal vertebrae are
+united together, forming the =pygostyle=. The vertebral column may be
+divided into cervical, thoracic, lumbar, sacral, and caudal regions,
+but the boundaries between the several regions are ill-defined.
+
+
+THE CERVICAL VERTEBRAE.
+
+All the vertebrae anterior to the first one that bears a rib meeting
+the sternum are regarded as cervical vertebrae. There are therefore
+sixteen cervical vertebrae, the last two of which bear well developed
+ribs. All are freely movable on one another.
+
+As a typical cervical vertebrae, any one from the fifth to the ninth
+may be taken. The vertebra is rather elongated, and is very lightly
+and strongly made, its most characteristic feature being the shape of
+the articulating surfaces of the centra, which are generally described
+as saddle-shaped. The anterior surface is convex from above downward,
+and concave from side to side, while the posterior and more prominent
+surface is concave from above downwards and convex from side to side.
+The neural arch is low, and is drawn out into a slight blade-like
+=neural spine=. Its base is deeply notched on both sides posteriorly
+for the exit of the spinal nerves. Above these notches it is drawn out
+into two rather prominent diverging processes, which bear the
+=postzygapophyses=,--two flattened surfaces which look downwards and
+outwards. The =transverse processes= form irregular outgrowths from
+the anterior two-thirds of the sides of the vertebra; each projects
+for a short distance downwards and outwards, and is terminated
+posteriorly by a short backwardly-projecting spine. The transverse
+processes are shown by development to ossify from separate centres,
+and are therefore to be regarded as cervical ribs, and each is
+perforated at its base by a canal for the passage of the vertebral
+artery. Above the anterior end of the vertebrarterial canal are a pair
+of thickened outgrowths, which bear upwardly and inwardly directed
+=prezygapophyses=. Each transverse process is perforated near its
+middle by a prominent foramen through which passes a vein which is
+connected with the jugular vein.
+
+The third and fourth cervical vertebrae resemble the succeeding ones
+in most respects, but have small =hypapophyses=, and the neural spines
+are less blade-like. The posterior cervical vertebrae (tenth to
+sixteenth) differ somewhat from the middle ones. They are shorter and
+more massive, the neural arch is much shorter, being deeply notched in
+the middle line in front and behind. The transverse processes arise
+from the anterior half of the vertebra only, and in the eleventh
+vertebra each is drawn out below into a pair of rather prominent
+downwardly and inwardly directed processes. In the twelfth vertebra
+these processes have almost coalesced, and in the thirteenth vertebra
+they have coalesced completely, forming a prominent =hypapophysis=. In
+the succeeding vertebrae this hypapophysis rapidly decreases in size.
+
+The fifteenth and sixteenth cervical vertebrae resemble the succeeding
+thoracic vertebrae, having short thick centra and prominent squarely
+truncated neural spines; the sides of the neural arches are very
+deeply notched. The fifteenth vertebra has a short transverse process,
+perforated by a wide vertebrarterial foramen, but this foramen is
+absent in the sixteenth. The transverse processes of the fifteenth
+vertebra bear two facets for the articulation of the capitulum and
+tuberculum of the rib. The sixteenth vertebra has its tubercular facet
+on the transverse process, but the capitular facet is borne on the
+centrum.
+
+The second or =axis= vertebra is small, and has the centrum drawn out
+into a comparatively very large hypapophysis. The posterior
+articulating surface of the centrum is saddle-shaped, the anterior
+nearly flat: above it the centrum is prolonged into the prominent
+=odontoid process=, which is shown by development to be the detached
+centrum of the atlas. The neural arch is deeply notched in the middle
+line in front, and at the sides behind. It is drawn out posteriorly
+into a wide massive outgrowth, which overhangs the third vertebra and
+bears the downwardly-directed postzygapophyses. The prezygapophyses
+are situated at the sides of the anterior end of the neural arch, and
+look directly outwards. The transverse processes are very slightly
+developed, and are pierced by the vertebrarterial canals.
+
+The =atlas= vertebra is a very slight ring-like structure, thickened
+ventrally and bearing in front a prominent concave cavity for
+articulation with the occipital condyle of the skull. Posteriorly it
+bears a more or less flattened surface for articulation with the
+centrum of the axis. It surrounds a large cavity partially divided
+into a larger dorsal portion, which is the neural canal, and a smaller
+ventral portion which lodges the odontoid process. The sides of the
+atlas are pierced by the vertebrarterial canals, above which there are
+two slight backwardly-projecting outgrowths bearing the
+postzygapophyses on their inner faces.
+
+
+THE THORACIC VERTEBRAE.
+
+The thoracic region includes all the vertebrae bearing free ribs,
+except the first two, viz. those whose ribs do not reach the sternum.
+There are seven thoracic vertebrae. The first four have centra with
+saddle-shaped articulating surfaces, but are more or less firmly
+united together by their neural spines; the last two are completely
+ankylosed by their centra to the lumbar vertebrae.
+
+Each of the first five vertebrae has a prominent, vertical, abruptly
+terminated neural spine, and straight transverse processes. The
+zygapophyses and articulating surfaces at the ends of the centra are
+well developed. The third, fourth, fifth, and sixth vertebrae have
+very prominent hypapophyses. The articular facets for the ribs are
+well marked, those for the tubercula lying at the free ends of the
+transverse processes, and those for the capitula at the sides of the
+anterior ends of the centra. The sixth and seventh thoracic vertebrae
+are firmly fused by their centra and neural arches to one another and
+to the lumbar vertebrae behind, and by their transverse processes to
+the ilia. The sixth has its centrum terminated in front by a
+saddle-shaped articulating surface, and bears a pair of prominent
+prezygapophyses. Its transverse processes and centrum bear facets for
+the tubercula and capitula of the ribs respectively. In the seventh
+vertebra the tubercular facet is wanting.
+
+
+THE SACRUM.
+
+[Illustration FIG. 58. A, DORSAL AND B, VENTRAL VIEW OF THE PELVIS AND
+SACRUM OF A DUCK (_Anas boschas_).
+
+ 1. ilium.
+ 2. ischium.
+ 3. pubis.
+ 4. pectineal process.
+ 5. lumbar vertebrae.
+ 6. true sacral vertebrae.]
+
+The =sacrum= generally consists of seventeen vertebrae fused with one
+another and with the ilia. Their number may be reckoned from the
+number of foramina for the exit of spinal nerves. The two most
+anterior of these vertebrae bear ribs and have been already described
+with the other thoracic vertebrae. Their neural spines and those of
+the four succeeding vertebrae are fused together, forming a continuous
+crest of bone completely united laterally with the ilia. The
+transverse processes of all these six vertebrae are well developed,
+but those of the posterior two (fig. 58, B, 5) are much the stoutest.
+The next three vertebrae have broad centra, but their transverse
+processes are very slightly developed and have no ventral elements.
+These seven vertebrae belong to the =lumbar= series. The remaining
+eight vertebrae have well-developed transverse processes, which in the
+case of the first three or four are divisible into dorsal and ventral
+elements. All the dorsal elements are united to form a pair of
+flattened plates, partially separated by a series of foramina from a
+median plate formed by the united neural arches. Laterally they are
+continuous with the ischia. The first two of this series of vertebrae
+are shown by their relation to the nerves to be the true =sacrals=
+(fig. 58, B, 6), the remaining six belonging to the =caudal= series.
+
+Behind them come the six free caudal vertebrae, succeeded by a
+terminal piece, the =pygostyle=, formed of a number of vertebrae fused
+together; this bears the rectrices or tail quills.
+
+[Illustration FIG. 59. SKULL OF A DUCK (_Anas boschas_). × 1.
+
+A. Dorsal view of the cranium. B. Palatal view of the mandible. C. The
+Hyoid.
+
+For numbers see Fig. 60.]
+
+
+B. THE SKULL.
+
+[Illustration FIG. 60. A. Ventral view of the cranium of a Duck (_Anas
+boschas_). B. Cranium and mandible seen from the left side. × 1.
+
+ 1. maxillae.
+ 2. premaxillae.
+ 3. anterior nares.
+ 4. nasal process of premaxillae (fig. 59).
+ 5. nasal.
+ 6. frontal (fig. 59).
+ 7. lachrymal.
+ 8. postfrontal process.
+ 9. parietal (fig. 59).
+ 10. jugal.
+ 11. quadratojugal.
+ 12. quadrate.
+ 13. condyle of mandible.
+ 14. posterior articular process.
+ 15. dentary at symphysis. }
+ 16. basi-hyal. } (fig.
+ 17. uro-hyal. } 59).
+ 18. basibranchial. }
+ 19. vomer.
+ 20. palatine.
+ 21. pterygoid.
+ 22. anterior palatine foramen.
+ 23. basitemporal.
+ 24. foramen leading into tympanic cavity.
+ 25. bristle inserted into posterior opening of carotid canal.
+ 26. bristle inserted into posterior opening of Eustachian canal.
+ 27. bristle emerging through anterior opening of carotid canal.
+ Close by is seen the bristle emerging through the anterior
+ opening of the Eustachian canal.
+ 28. fenestral recess.
+ 29. maxillo-palatine.
+ 30. lambdoidal crest.
+ 31. rostrum.
+
+I. II. IV. V. IX. X. nerve foramina.]
+
+The skull of the duck, like that of birds in general, is characterised
+(1) by its lightness, (2) by the contrast between the bones of the
+cranium proper and those forming the rest of the skull, for the bones
+forming the cranium proper are closely fused together, the sutures
+between them being nearly all completely obliterated in the adult,
+while the bones forming the face are loosely connected with the
+cranium proper; (3) by the prolongation of the face into a long
+toothless beak; (4) by the size of the orbits, and their position
+entirely in front of the cranium, so that they are separated from one
+another only by a thin interorbital septum.
+
+For purposes of description the skull may be divided into
+
+ (1) The cranial portion.
+ (2) The facial portion.
+ (3) The mandible.
+ (4) The hyoid.
+
+
+(1) THE CRANIAL PORTION.
+
+This is a rounded box expanded dorsally and posteriorly, but tapering
+antero-ventrally. In the young skull the divisional lines between the
+several bones can be easily seen, but in the adult they are quite
+obliterated.
+
+(_a_) The _dorsal surface_ is rounded, expanded in front and behind,
+but encroached upon in the middle by the cavities of the orbits. There
+is a prominent divisional line in front, separating it from the facial
+part of the skull. It is formed mainly by the _frontal_ (fig. 59, A,
+6) and _parietal_ bones, but the frontals diverge a little anteriorly
+and enclose between them the ends of the _nasal processes_ (fig. 59,
+A, 4) of the _premaxillae_. Just in front of the orbit the outer
+margins of the frontals are either notched or pierced by a pair of
+foramina.
+
+(_b_) At the _posterior end_ of the cranium the most prominent feature
+is the large, almost circular =foramen magnum=, through which the
+spinal cord and brain communicate; this in young birds is seen to be
+bounded by four distinct bones, dorsally by the =supra-occipital=,
+ventrally by the =basi-occipital=, and laterally by the
+=exoccipitals=.
+
+The =basi-occipital= forms the main part of a prominent convex knob,
+the =occipital condyle=, with which the atlas articulates. The
+occipital condyle is slightly notched above, and the ventral surface
+of the cranium is deeply pitted just in front of it; the exoccipitals
+also contribute slightly to its formation. Slightly in front of and
+ventral to the foramen magnum is a small foramen through which the
+hypoglossal nerve leaves the cranial cavity.
+
+The =supra-occipital= is separated from the parietal by a suture line
+along which run a pair of prominent ridges, the =lambdoidal crests=
+(fig. 60, B, 30). There are often a pair of prominent vacuities in the
+supra-occipital dorsal to the foramen magnum. The =epi-otics= and
+=opisthotics= become completely fused with the bones of the occipital
+segment at a very early stage.
+
+(_c_) The _ventral surface_ of the cranium is wide behind, where it is
+formed by a broad transverse membrane bone, the _basitemporal_ (fig.
+60, A, 23), the sides of which are fused with the auditory capsules.
+Slightly in front of and an eighth of an inch external to the
+hypoglossal foramen the cranial wall is pierced by a pair of foramina
+through which the tenth or pneumogastric nerves leave (fig. 60, A, X).
+At the sides of the basitemporal are a pair of depressions, the
+=tympanic recesses=, in each of which are three holes. Straight lines
+joining these holes would form an isosceles triangle with its apex
+directed forwards. Of the two holes at the base of the triangle, the
+one nearer the middle line and leading into the cranial cavity, is for
+the exit of the ninth or glossopharyngeal nerve (fig. 60, A, IX), it
+lies just in front of the pneumogastric foramen. The more external
+leads into the tympanic cavity, while the more anterior at the apex of
+the triangle is the =posterior opening of the carotid canal= (fig. 60,
+A, 25), which traverses the base of the cranium, and during life
+lodges the carotid artery.
+
+The anterior end of the basitemporal is pierced near the middle line
+by a pair of holes, the =anterior openings of the Eustachian canals=;
+while just in front of these and a little further removed from the
+middle line are the anterior openings of the =carotid canals=.
+Bristles passed in through the posterior openings of the carotid
+canals will emerge here (fig. 60, A, 27). In front of the basitemporal
+the base of the cranium is formed by the =rostrum= (fig. 60, A, 31),
+or thickened basal portion of the interorbital septum; this bears two
+prominent surfaces with which the pterygoids articulate. In some kinds
+of duck these surfaces are borne by well-marked basi-pterygoid
+processes.
+
+(_d_) _The side of the cranium._ At the base of the posterior end is
+seen the deep =tympanic cavity=. The dorsal part of this is divided by
+a vertical partition into two halves; of these the more anterior is
+the larger, and forms a deep funnel-shaped cavity, the =posterior
+opening of the Eustachian canal= (fig. 60, B, 26). A bristle passed
+into this opening emerges through the anterior opening of the
+Eustachian canal. The more posterior of the two is the =fenestral
+recess= (fig. 60, B, 28), and is in its turn divided by a slender
+horizontal bar into a dorsal hole, the =fenestra ovalis=, and a
+ventral hole, the =fenestra rotunda=. During life the fenestra ovalis
+lodges the proximal end of the =columellar= chain. Lying at the outer
+side and slightly dorsal to the tympanic cavity is a deep depression,
+the =lateral tympanic recess=, and immediately in front of this is the
+articular surface for the quadrate. The tympanic cavity is bounded
+below by the basitemporal, posteriorly by the exoccipital, and above
+by the _squamosal_, a membrane bone, which roofs over a good deal of
+the side of the cranium, and bears ventrally a prominent surface with
+which the quadrate articulates. Just in front of this is a large round
+hole, the =trigeminal foramen= (fig. 60, B, V), behind which the
+squamosal is drawn out into a short process.
+
+In front of the squamosal there is a prominent forwardly-projecting
+=postfrontal process= (fig. 60, 8), which ossifies from a different
+centre from that forming the squamosal, but in the adult is completely
+fused with it.
+
+The =orbit= forms a large more or less hemispherical cavity which
+lodges the eyeball. It is separated from its fellow of the opposite
+side by an imperfect partition, the =interorbital septum=. In the
+young skull it is seen to be bounded above by the frontal, with which
+the _lachrymal_ (fig. 60, 7) is fused anteriorly, forming a large
+backwardly-projecting process; while behind it is bounded by
+the =alisphenoid=. The interorbital septum is formed by the
+ossification and coalescence of the =mesethmoid= in front, with the
+=orbitosphenoid= behind, and the =rostrum= below. The boundary of the
+orbit below is very imperfect, the zygomatic arch being incomplete.
+
+The interorbital septum is pierced by the very prominent =optic
+foramen= (fig. 60, B, 2), just behind which are the two much smaller
+foramina for the exit of the oculomotor and pathetic (fig. 60, B, IV)
+nerves, the more anterior being that for the oculomotor.
+
+Above and slightly in front of the optic foramen is a median opening,
+the =olfactory foramen.= This leads into the cranial cavity behind,
+and in front is continued forwards as a groove between the
+interorbital septum and the frontal.
+
+
+(2) THE FACIAL PART OF THE SKULL.
+
+This includes the olfactory capsule and associated bones, and the
+upper jaw.
+
+The bones associated with the olfactory capsules are the _nasals_ and
+_vomer_. The _nasals_ (figs. 59 and 60, 5) lie on the dorsal surface
+immediately in front of the cranium, and are separated from one
+another by the nasal processes of the premaxillae. Each is completely
+fused in the adult with the corresponding maxillae and premaxillae, the
+three bones together forming the boundary of the =anterior nares.= The
+_vomer_ (fig. 60, 19) is unpaired and forms a small median vertical
+plate lying ventral to the anterior continuation of the interorbital
+septum.
+
+The bones of the upper jaw consist on each side of two slender arcades
+which in front converge and are attached to the large beak, while
+behind they diverge but are united by the =quadrate=.
+
+The =inner arcade= is formed by the pterygoid and palatine. The
+_pterygoid_ (fig. 60, 21) is a short flattened bone, which articulates
+behind with the quadrate, and on its inner side with a large flattened
+surface borne by the rostrum, in front it meets the palatine, or
+sometimes ends freely with a long antero-dorsally directed point.
+
+The _palatine_ (fig. 60, 20) is a slender irregular bone flattened
+dorso-ventrally at its anterior end where it articulates with the
+beak, and laterally behind. It gives off at its posterior end a
+process, which is sometimes united with the vomer, sometimes projects
+forwards, and meets its fellow dorsal to the vomer. In the large space
+between it and the vomer is the opening of the =posterior nares=.
+
+The _premaxillae_ (figs. 59 and 60, 2) are very large, and form nearly
+a third of the big shovel-shaped beak. They constitute the inner, and
+part of the front boundary of the anterior nares, and send back a pair
+of _nasal processes_ which partially separate the nasals from one
+another.
+
+The =outer arcade= forms the slender =suborbital bar=, and consists
+mainly of two rod-like bones, which in the adult are completely fused
+together. The posterior of these is the _quadratojugal_ (figs. 59 and
+60, 11) which articulates with the quadrate, the anterior is the small
+and slender _jugal_ or _malar_ (figs. 59 and 60, 10). The extreme
+anterior part of the bar is formed by the _maxillae_. The main part of
+the maxillae however lies anterior to the suborbital bar, and extends
+forwards along the side of the premaxillae forming all the lateral part
+of the beak (figs. 59 and 60, 1); it also sends inwards a plate, the
+=maxillo-palatine= (fig. 60, A, 29), which completely fuses with its
+fellow in the middle line, and forms the posterior boundary of the
+anterior palatine foramen. The term =desmognathous= describes the
+condition of the skull in which the maxillo-palatines fuse with one
+another in the middle line in this way.
+
+The =quadrate= (fig. 60, 12), which unites the two arcades behind, is
+a stout irregular four-cornered bone forming the =suspensorium=. It
+articulates by its dorso-posterior corner with the squamosal, and by
+its antero-internal corner with the pterygoid. The middle of its
+ventral surface forms a hemispherical knob with which the mandible
+articulates, while its dorso-anterior border is drawn out into a long
+point which extends towards the interorbital septum.
+
+
+(3) THE MANDIBLE.
+
+The =mandible= or lower jaw consists of two =rami= which are flattened
+and fused together in the middle line in front, while behind they
+diverge from one another and articulate with the quadrates.
+
+Each ramus is composed of five bones fused together, one being a
+cartilage bone, and the other four membrane bones. The =articular= is
+the only cartilage bone of the mandible, it bears the double condyle
+(figs. 59 and 60, 13) or concave articular surface for the quadrate,
+and is drawn out behind into a large hooked =posterior articular
+process=. The articular is also drawn out into a prominent process on
+each side of the articular surface for the quadrate, and is marked by
+a deep pit opening posteriorly. The articular is continuous in front
+with =Meckel's cartilage= which forms the original cartilaginous bar
+of the lower jaw, and is ensheathed by the membrane bones. Of these
+the _supra-angular_ forms the upper part of the mandible in front of
+the articular, its dorsal surface is drawn out into a small =coronoid
+process=, its outer surface also bearing a prominent process. The
+_angular_ is a small bone which underlies the articular and
+supra-angular on the inner side of the jaw. The _dentary_ (fig. 59,
+15) forms the anterior half of each ramus, and is the largest bone of
+the mandible; it is fused with its fellow at the symphysis in front,
+and extends back below the supra-angular. The _splenial_ is a small
+bone lying along the middle half of the inner side of each ramus of
+the mandible.
+
+
+(4) THE HYOID.
+
+With the hyoid apparatus is included the =columella=. This forms a
+minute rod of bone, one end of which is expanded and fits into the
+fenestra ovalis, while the other end, terminated by a triradiate piece
+of cartilage, is attached to the tympanic membrane. The structure is
+as a whole homologous with the auditory ossicles of mammals and the
+hyomandibular of fish.
+
+The =hyoid= consists of a median unpaired portion, formed of two
+pieces of bone, the =basi-hyal= (fig. 59, C, 16) in front, and the
+=uro-hyal= (fig. 59, C, 17) behind, the two being placed end to end
+and terminated anteriorly by an unpaired cartilaginous plate, the =os
+entoglossum.= At the posterior end there come off a pair of long
+=posterior cornua=, each of which consists of two pieces, a longer
+=basibranchial= (fig. 59, C, 18), and a shorter =cerato-branchial=.
+For the homology of these parts see p. 336.
+
+
+THE RIBS AND STERNUM.
+
+The last two cervical vertebrae bear long movable ribs which
+articulate by distinct capitular and tubercular processes, but do not
+meet the sternum. The thoracic ribs are eight in number, and each is
+divisible into a =vertebral= and a =sternal= portion. The first five
+thoracic ribs are flattened curved bars of bone, which articulate by a
+prominent =capitulum= with the centrum of the corresponding vertebra,
+and by a =tuberculum= with the transverse process. Projecting
+backwards from each is a large hooked =uncinate process.= The last
+three ribs which are without uncinate processes, become progressively
+more slender, and in the eighth the tubercular processes are lost.
+
+The sternal portions of the ribs are imperfectly ossified pieces,
+short and comparatively thick in the case of the anterior ribs, longer
+and more slender in the case of the posterior ribs.
+
+
+THE STERNUM[1].
+
+The =sternum= or breast bone is exceedingly large in the Duck, as in
+all birds, and projects back far beyond the thorax over much of the
+anterior part of the abdomen. It is an irregularly oblong plate of
+bone, abruptly truncated behind, somewhat concave dorsally, and drawn
+out ventrally into a prominent keel, the =carina=, which projects for
+some distance forwards beyond the body of the sternum, and tapers off
+gradually behind. The point where the carina joins the body of the
+sternum is at the anterior end drawn out into a small process, the
+=rostrum=[102]. Just dorsolateral to this are a pair of deep grooves,
+the =coracoid grooves=, with which the coracoids articulate.
+
+The sides of the sternum are drawn out in front into a pair of short
+blunt =costal processes;= and just behind these are a series of seven
+surfaces with which the ends of the sternal ribs articulate.
+Immediately behind these surfaces the sides are produced into a pair
+of long backwardly-projecting =xiphoid processes= which nearly meet
+processes from the posterior end of the sternum.
+
+
+2. THE APPENDICULAR SKELETON.
+
+This consists of the skeleton of the anterior and posterior limbs and
+of their respective girdles.
+
+
+A. THE PECTORAL GIRDLE[103].
+
+The pectoral girdle in almost all birds is strongly constructed and
+firmly united to the sternum. It consists of three bones, a dorsal
+element, the =scapula=, a posterior ventral element, the =coracoid=,
+and an anterior ventral element, the _clavicle_.
+
+The =scapula= forms a long curved flattened bone expanded at its
+anterior end, where it meets the coracoid, and lying across the ribs
+at its tapering posterior end. It helps to form the imperfect =glenoid
+cavity=, with which the humerus articulates. The =coracoid=, a shorter
+but stouter bone than the scapula, has its upper end or =head=
+thickened and bears on its posterior border an irregular surface, with
+part of which the scapula articulates, while the rest forms part of
+the glenoid cavity. The inner border of the coracoid adjoining the
+articular facet for the scapula is produced into a strong process
+which helps to complete the =foramen triosseum=, a space lying between
+the adjoining ends of the scapula and coracoid, through which the
+tendon of the second pectoral muscle passes. The lower part of the
+coracoid, which is much flattened and expanded, and abruptly truncated
+posteriorly, articulates with the coracoid groove of the sternum. The
+_clavicle_ is a thickened curved membrane bone, which is fused with
+its fellow in the middle line below, the two forming the _furcula_ or
+merrythought. Its dorsal end is drawn out into a process which
+articulates with the coracoid.
+
+
+THE ANTERIOR LIMB OR WING.
+
+This consists of three parts, a proximal part, the upper arm or
+=brachium=, a middle part, the fore-arm or =antibrachium=, and a
+distal part, the =manus=. When extended for flight the parts lie
+almost in the same straight line, but when at rest they are folded on
+one another in the form of a Z, the brachium and manus pointing
+backwards, and the antibrachium forwards. When extended for flight the
+surfaces and borders of the wing correspond in position with those of
+the primitive vertebrate limb[104], the pre-axial border being
+directed forwards and the postaxial backwards, while the dorsal and
+ventral surfaces look respectively upwards and downwards. But when the
+wing is at rest, the humerus as it extends backwards becomes slightly
+rotated, so that its dorsal surface looks more inwards than upwards,
+while the dorsal surface of the antibrachium looks partially outwards
+and upwards, and that of the manus mainly outwards.
+
+The =brachium= or =upper arm= contains only a single bone, the
+=humerus= (fig. 57, 1). This is a large nearly straight bone expanded
+at both ends. The proximal end is specially expanded, forming two
+=tuberosities=, and a large convex =head= articulating with the
+glenoid cavity. The =pre-axial tuberosity= is the smaller of the two,
+but is continued by a prominent =deltoid ridge=, which extends for a
+very short distance down the shaft. The =postaxial tuberosity= is the
+larger, and below it there is a very deep pit, the =pneumatic
+foramen=, which leads into an air cavity in the shaft of the bone. The
+shaft is long and straight, and at the distal end of the bone is the
+=trochlea= with two convex surfaces, one pre-axial with which the
+radius articulates, the other postaxial for the ulna.
+
+The =fore-arm= or =antibrachium= consists of two bones, the =radius=
+and =ulna=. These are of nearly equal length, and are separated from
+one another by a considerable space except at their terminations.
+
+The =radius= (fig. 57, 2), the pre-axial and smaller bone, is straight
+and fairly stout; its proximal end articulates with the humerus by a
+slightly cupped surface, while its distal end, which articulates with
+the carpus, is convex and somewhat expanded.
+
+The =ulna= (fig. 57, 3) is longer, stouter, and slightly curved. Its
+proximal end is expanded, forming two surfaces which articulate with
+the trochlea of the humerus; behind them it is drawn out into a short
+blunt =olecranon process=. Its distal end is less expanded, and
+articulates with the carpus and also with the radius.
+
+The =Manus=. This includes the carpus or wrist, and the hand.
+
+The =Carpus=. While in the embryo the carpus consists of five
+distinct elements arranged in a proximal row of two and a distal row
+of three, in the adult only the proximal bones can be clearly
+distinguished, the distal ones having become completely ankylosed with
+the metacarpals to form the =carpo-metacarpus=.
+
+The two distinct carpal bones are the radial carpal and the ulnar
+carpal. The radial carpal (fig. 57, 4) is a small somewhat cubical
+bone, wedged in between the manus and the radius and ulna. The ulnar
+carpal (fig. 57, 5) is a somewhat larger, more irregular bone, lying
+adjacent to the end of the ulna. It is deeply notched to receive the
+carpo-metacarpus.
+
+=The hand.= In the adult bird the hand is in a much modified
+condition; only the first three digits are represented, and the
+metacarpals are all fused with one another and with the distal
+carpalia to form the =carpo-metacarpus=.
+
+The most prominent part of the carpo-metacarpus is formed by the
+=second metacarpal= (fig. 57, 7), a stout, straight bone expanded at
+both ends. The =third metacarpal= (fig. 57, 8) is a more slender
+curved bone fused at both ends with the second metacarpal. The =first
+metacarpal= forms simply a small projection on the radial side of the
+proximal end of the second metacarpal.
+
+The =phalanges=. The first digit or =pollex= includes two phalanges,
+the distal one being very small and bearing a claw.
+
+The second digit includes three phalanges, the proximal one being
+somewhat flattened. The third digit has a single small phalanx.
+
+
+THE PELVIC GIRDLE.
+
+The bones constituting the pelvic girdle are not only as in other
+higher vertebrates ankylosed together forming the innominate bones,
+but are also ankylosed with a series of some seventeen sacral and
+pseudosacral vertebrae. The =acetabulum= (fig. 61, 5) with which the
+head of the femur articulates is incompletely ossified.
+
+The =ilium= (figs. 58 and 61, 1) is the largest bone of the pelvis. It
+forms a long flattened plate extending for a considerable distance
+both in front of and behind the acetabulum, and is fused along its
+whole length with the transverse processes and neural spines of the
+sacral and pseudosacral vertebrae. It forms more than half the
+acetabulum, above and behind which it is produced to form a process,
+the =antitrochanter= (fig 61, 8), with which the great trochanter of
+the femur articulates.
+
+[Illustration FIG. 61. LATERAL VIEW OF THE PELVIS AND SACRUM OF A DUCK
+(_Anas boschas_) × 2/3.
+
+ 1. ilium.
+ 2. ischium.
+ 3. pubis.
+ 4. pectineal process.
+ 5. acetabulum.
+ 6. ilio-sciatic foramen.
+ 7. fused vertebrae.
+ 8. antitrochanter.]
+
+The =ischium= (figs. 58 and 61, 2) is a flattened bone which forms
+about one-third of the acetabulum, and lies ventral to the posterior
+part of the ilium. Its anterior portion is separated from the ilium by
+the large oval =ilio-sciatic foramen= (fig. 61, 6), while behind this
+the two bones are completely fused.
+
+The =pubis= (figs. 58 and 61, 3) is a very long slender bar of bone
+which forms only a very small part of the acetabulum and runs back
+parallel to the ventral surface of the ischium with which it is
+loosely connected at its posterior end. For the greater part of their
+length the two bones are separated by the long narrow =obturator
+foramen=. Behind the ischium the pubis is produced into a long curved
+downwardly-projecting process, and in front of the acetabulum it bears
+a short blunt =pectineal= or =pre-pubic process= (fig. 61, 4) probably
+homologous with the pre-pubis of Orthopod Dinosaurs. The remainder of
+the pubis is homologous with the post-pubis of Orthopod Dinosaurs.
+
+
+THE POSTERIOR LIMB.
+
+The leg of the bird is somewhat differently constructed from that of
+other vertebrates owing to the fact that there is no free tarsus, the
+proximal tarsals having fused with the tibia, and the distal with the
+metatarsals.
+
+The =thigh= consists of a single bone, the femur. The =femur= is a
+comparatively short bone with a straight shaft and expanded ends. The
+proximal end bears on its inner side a rounded =head=, which
+articulates with the acetabulum. On its outer side is an irregular
+outgrowth, the =great trochanter=, while between the two is the
+surface which meets the antitrochanter of the ilium. The posterior end
+also is expanded and marked by a wide groove which lodges the
+=patella=. On each side of the groove is a strong =condylar ridge= for
+articulation with the tibia. The external condyle is deeply grooved
+behind for articulation with the fibula.
+
+The =crus= or =shin= consists of two separate bones, (1) the
+=tibio-tarsus=, formed by the fusion of the tibia with the proximal
+row of tarsals, and (2) the =fibula=.
+
+The =tibio-tarsus= is a thick straight bone nearly twice as long as
+the femur. Both ends of the bone are considerably expanded. The
+proximal end bears two slight depressions which articulate with the
+condyles of the femur, and a third depression which partly lodges the
+patella. The proximal end of the anterior or extensor surface is drawn
+out into a very prominent =cnemial crest= which bends over towards the
+postaxial side of the bone; a slight ridge is continued from it all
+the way down the shaft. The proximal part of the shaft of the
+tibio-tarsus bears a roughened ridge with which the fibula is closely
+connected. The distal end is expanded and rotated outwardly and forms
+a prominent pulley-like surface which articulates with the
+tarso-metatarsus.
+
+The =fibula= is reduced to the proximal portion only, which is
+expanded and articulates with a depression behind the external condyle
+of the femur. The fibula further extends about a third of the way down
+the shaft of the tibio-tarsus. The =patella= or =knee-cap= is a
+sesamoid bone due to an ossification in the tendon of the extensor
+muscles of the leg.
+
+The =ankle joint= lies between the proximal and distal tarsals which
+as previously mentioned fuse respectively with the tibia and
+metatarsus.
+
+The =Pes=. The pes includes four digits, and consists of the
+tarso-metatarsus and the phalanges. The proximal tarsals which are
+fused with the tibia also really belong to the pes.
+
+The =tarso-metatarsus= is a strong straight bone nearly as long as the
+femur, and is formed by the fusion of the distal tarsals with the
+second, third and fourth metatarsals. The proximal end of the bone is
+expanded and bears two facets for articulation with the tibio-tarsus,
+and near them on the posterior surface is a large roughened
+projection. The lines of junction between the several metatarsals are
+marked along the shaft by slight ridges. At the distal end of the bone
+the three metatarsals diverge from one another and each bears a
+prominent convex pulley-like surface. The =first metatarsal= is
+reduced to the distal end, which tapers to a point proximally, and is
+attached by ligaments near the distal end of the tarso-metatarsus.
+
+The =digits=. Four digits are present, each consisting of a metatarsal
+(already described) and a certain number of phalanges, the terminal
+one being in each case clawed. The first digit or =hallux= has two
+phalanges, the second three, the third four, and the fourth five.
+
+
+FOOTNOTES:
+
+[101] See R.S. Wray, _P.Z.S._, 1887, p. 343.
+
+[102] Often called the manubrium, but not homologous with the
+manubrium of the mammalian sternum.
+
+[103] Cp. fig. 63.
+
+[104] See p. 28.
+
+
+
+
+CHAPTER XIX.
+
+GENERAL ACCOUNT OF THE SKELETON IN BIRDS.
+
+
+EXOSKELETON.
+
+The epidermal exoskeleton of birds is very greatly developed, feathers
+constituting its most important part.
+
+Three kinds of feathers are found, viz. (_a_) _pennae_ including
+quills and coverts, (_b_) down feathers or _plumulae_, and (_c_)
+_filoplumes_ which are rudimentary feathers. The structure of the
+different kinds of feathers is described on pp. 303-306.
+
+Sometimes a fourth class of feathers, the _semiplumae_, is recognised.
+They have the stems of pennae, and the downy barbs and barbules of
+plumulae.
+
+In most birds the pennae are not uniformly distributed over the whole
+surface of the body, but are confined to certain tracts, the
+=pterylae=; while the intervening spaces or =apteria= are either bare
+or covered only with down feathers. In some birds, however, such as
+the Ratitae and the Penguins, pennae are evenly distributed over the
+whole body.
+
+In many birds the calamus or quill bears two vexilla or vanes, the
+second of which, called the =aftershaft= or =hyporachis=, is generally
+much the smaller, and is attached to the under surface of the main
+vexillum. In the Moas, Emeu and Cassowary the two vexilla in the adult
+bird are nearly equal in size; though in the nestling Emeu one is much
+longer than the other. The aftershaft is very small in most Passeres
+and gallinaceous birds, but is comparatively large in Parrots, Gulls,
+Herons and most birds of prey. It is absent or extremely small in the
+Ostrich, _Apteryx_, _Rhea_, Pigeons, Owls, Anseres, and others.
+
+The quill feathers include two groups, the =remiges= or wing quills,
+and the =rectrices= or tail quills. In most birds the primary remiges,
+or those which are attached to the bones of the manus, are ten or
+eleven in number, and are set in grooves in the bones, being firmly
+attached to them. In the Ostrich however the primaries are little
+specialised in character and are as many as sixteen in number. They
+are also less definitely attached to the bones; as their ends do not
+lie in grooves in the bones, but project beyond them.
+
+The secondary quills or those attached to the ulna vary much in number
+according to the length of the bone. The large dark quills in the
+wings of Cassowaries are the secondaries.
+
+The wing of Penguins is very little differentiated. It is covered at
+the margin by overlapping scales which gradually merge into scale-like
+feathers at the proximal end. The wing of the Penguin has nothing
+comparable to the remiges of other birds.
+
+In some birds, such as Herons (_Ardea_), there occur in places
+plumulae of a peculiar kind, which grow persistently and whose summits
+break off into fine powder as fast as they are formed. These feathers
+are known as _powder-down_ feathers. They occur also in some Parrots
+and are then scattered indiscriminately all over the body.
+
+Other exoskeletal structures besides feathers are commonly well
+developed. Thus the extremities of the jaws are sheathed in horny
+=beaks= whose form varies enormously according to the special mode of
+life.
+
+In ducks and geese the beak with the exception of the anterior end is
+soft, and its edges are raised into lamellae, while in the Mergansers
+these lamellae become pointed processes supported by bony outgrowths.
+These lamellae act as strainers. In Parrots and Hawks, on the other
+hand, nearly the whole of the beak is hard.
+
+The toes and tarso-metatarsus are usually featherless and are covered
+either with granular structures or with well-formed scales. The toes
+are nearly always provided with =claws=, and these vary in correlation
+with the character of the beak. Claws[105] also sometimes occur on the
+manus. Thus _Archaeopteryx_ and some Ostriches and Rheas have claws on
+all three digits. Most Ostriches and Rheas, and many Anseres and birds
+of prey, have them on the first two digits, while the Secretary bird
+(_Gypogeranus_) and many fowls, ducks, and birds of prey, especially
+kestrels, have a claw only on the pollex. In the Cassowary, Emeu,
+Apteryx and some Ostriches and Rheas only the second digit is clawed.
+
+Claws should not be confounded with =spurs=, which are conical horny
+structures developed on bony outgrowths of the radial side of the
+carpus, metacarpus, or metatarsus. They occur in a number of birds,
+but are most commonly developed in gallinaceous birds, by which they
+are used for fighting. A single spur occurs on the metacarpus in
+_Megapodius_, in _Palamedea_, in _Parra jacana_ and in _Hoplopterus
+spinosus_, the Spur-winged plover. The Derbian Screamer, _Chauna
+derbiana_, has two metacarpal spurs, borne on the first and second
+metacarpals. The Spur-winged goose, _Plectropterus gambensis_, has a
+carpal spur borne on the radial carpal. Metatarsal spurs are quite
+common.
+
+The male Solitaire (_Pezophaps_) has large bony excrescences on the
+wrist which may, like spurs, have been sheathed in horn and used for
+fighting.
+
+=Teeth= do not occur in any living birds, but conical teeth imbedded
+in separate sockets are present in _Archaeopteryx_ and _Ichthyornis_,
+while in _Hesperornis_ similar teeth occur implanted in continuous
+grooves in the mandibles and maxillae, the premaxillae being toothless.
+
+Except that teeth are partly dermal in origin, a dermal exoskeleton is
+quite unrepresented in birds.
+
+
+ENDOSKELETON.
+
+Perhaps the most striking feature of the endoskeleton of birds is its
+pneumaticity. In the embryo all the bones contain marrow, but as
+growth proceeds this becomes replaced by air to a variable extent in
+different forms. In all birds some part of the skeleton is pneumatic.
+Many small birds and _Apteryx_ and Penguins among larger ones have air
+only in the skull; in Pigeons air is present in all the bones except
+the caudal vertebrae, the leg bones, and those of the antibrachium and
+manus; in Hornbills every bone contains air.
+
+[Illustration FIG. 62. THIRD CERVICAL VERTEBRA OF AN OSTRICH
+(_Struthio camelus_). × 1. A anterior, B posterior, C dorsal view (A
+and B after MIVART).
+
+ 1. neural spine.
+ 2. neural canal.
+ 3. prezygapophysis.
+ 4. postzygapophysis.
+ 5. posterior articular surface of centrum.
+ 6. anterior articular surface of centrum.
+ 7. vertebrarterial canal.
+ 8. hypapophysis.]
+
+
+VERTEBRAL COLUMN.
+
+The vertebral column of birds is readily divisible into a very mobile
+cervical region, and an extremely rigid post-cervical region. In most
+birds the vertebral centra are without terminal epiphyses, but these
+structures are found in Parrots. The cervical vertebrae are generally
+large and vary in number from eight or nine to twenty-three in Swans.
+Except in some extinct forms, such as _Ichthyornis_ and _Apatornis_,
+in which they are biconcave, the centra are characterised by having
+saddle-shaped articulating surfaces, which in front are concave from
+side to side and slightly convex from above downwards, while
+posteriorly they are convex from side to side and concave from above
+downwards. The atlas is small and ring-like, and its centrum is fused
+with the axis forming the odontoid process. Cervical ribs are often
+well developed, and in some of the Ratitae they remain for a long time
+distinct from the vertebrae.
+
+The thoracic vertebrae are distinguished from the cervical by the fact
+that their true ribs are united to the sternum by means of sternal
+ribs. This distinction, however, though convenient, is somewhat
+arbitrary, as it has been shown that in the fowl and gannet, two pairs
+of ribs which in the adult are free from the sternum, are connected
+with it in the embryo. When, as in the Swans, the thoracic vertebrae
+are not all fused together, they generally have saddle-shaped
+articulating surfaces, but sometimes, as in the Penguins, Auks and
+Plovers, the centra are convex in front and concave behind. The trunk
+vertebrae generally have well-marked neural spines, while in the
+Divers the anterior ones have peculiar bifurcating hypapophyses.
+
+The trunk vertebrae are not readily divisible into thoracic and
+lumbar. There are two true sacral vertebrae, but as development
+proceeds a number of other vertebrae become fused with the true
+sacrals, the whole forming a large compound sacrum. These pseudosacral
+vertebrae generally include the lumbar, and some of the thoracic and
+caudal vertebrae. Sixteen to twenty vertebrae or even more may be
+included in the compound sacrum, and sometimes the whole of the trunk
+vertebrae are fused together. In _Archaeopteryx_ however but five
+vertebrae take part in the formation of the sacrum.
+
+In _Archaeopteryx_ there are twenty long caudal vertebrae, of which
+the last sixteen carry a pair of feathers apiece, but in all other
+birds the tail is short and in the great majority of cases the
+posterior vertebrae are fused together, forming the pygostyle. In the
+Ratitae and Tinamidae a pygostyle is rarely or imperfectly developed.
+In _Hesperornis_ there are twelve caudal vertebrae, six or seven of
+which are united by their centra only, forming an imperfect pygostyle.
+
+The free caudal vertebrae are generally amphicoelous.
+
+
+THE SKULL.
+
+The skull of all birds from _Archaeopteryx_ onwards is essentially
+similar, differing from the skull of reptiles mainly in the extent to
+which the cranium is arched, and its greater size in proportion to the
+jaws.
+
+Most of the bones of the cranium are pneumatic, and all show a marked
+tendency to fuse together, and have their outlines obliterated by the
+disappearance of the sutures. The several bones remain longest
+distinguishable in the Ratitae and to a less extent in the Penguins.
+The orbits are very large and lie almost entirely in front of the
+cranium; they are separated by an interorbital septum which is
+sometimes, as in _Chauna_ and _Scythrops_, very complete, sometimes,
+as in Hornbills and the Common Heron, very slightly developed. As a
+general rule the sclerotic is cartilaginous.
+
+The anterior nares are almost always situated far back at the base of
+the beak near the orbits, but in _Apteryx_ they are placed right at
+its extremity. In _Phororhacos_ they are placed very high up on the
+enormous beak and are not separated by any bony partition.
+
+The skull of Parrots has some peculiarities. In some Parrots the
+lachrymal sends back a process which meets the postorbital process of
+the frontal and completes the orbit. In most birds the upper beak is
+immovably fixed, but in some it is attached to the cranium, only by
+the nasals and by flexible processes of the premaxillae, so that by
+this means a kind of elastic joint is established and the beak is able
+to be moved on the cranium. In the Parrots and _Opisthocomus_ there is
+a regular highly movable joint.
+
+In Cassowaries the fronto-nasal region of the skull is produced into
+an enormous bony crest, and in Hornbills a somewhat similar structure
+occurs. Although true teeth do not occur in any known bird except
+_Archaeopteryx_, _Hesperornis_, and _Ichthyornis_, another extinct
+bird, _Odontopteryx_, has the margins of both jaws provided with
+forwardly-directed tooth-like serrations, formed of part of the actual
+jawbone: a living hawk, _Harpagus_, too, has a deeply notched bill, to
+which correspond serrations in the premaxillae.
+
+A basi-pterygoid process of the basisphenoid abuts against the
+pterygoid in Ratitae and in Tinamous, plovers, fowls, pigeons, ducks
+and geese among Carinatae, recalling the arrangement met with in many
+reptiles. The squamosal is sometimes, as in the fowl, united with the
+postorbital process of the frontal. In the Carinatae the quadrate
+articulates with the cranium by a double convex surface, in the
+Ratitae by a single one. The premaxillae are always comparatively large
+bones, the maxillae on the contrary are small, but give rise to
+important inwardly-projecting maxillo-palatine processes.
+
+The relations of the palatines, pterygoids, maxillae, and vomers vary
+considerably, and on them Huxley has based a classification of
+birds[106]. In the Ratitae and the Tinamous (Tinamidae), among
+Carinatae the vomers unite and form a large broad bone, separating the
+palatines and the pterygoids from the rostrum. Huxley uses the term
+=Dromaeognathous= to describe this condition. In all other Carinatae
+the vomers are narrow behind, and the palatines and pterygoids
+converge posteriorly and articulate largely with the rostrum. Three
+modifications of this condition are distinguished by Huxley, and
+termed =Schizognathous=, =Ægithognathous=, and =Desmognathous=.
+
+In the =Schizognathae= the vomers coalesce and form a narrow elongated
+bone, pointed in front, separating the maxillo-palatine processes of
+the premaxillae. Waders, fowls, penguins, gulls, some falcons and
+eagles, American vultures, some herons and many owls have the
+Schizognathous arrangement. In pigeons and sandgrouse there is no
+vomer, but the other bones have the Schizognathous arrangement.
+
+In the =Ægithognathae= the arrangement is the same as in the
+Schizognathae, except that the vomers are truncated in front.
+Passeres, swifts, woodpeckers, humming birds, rollers, hoopoes have
+this arrangement.
+
+In the =Desmognathae= (fig. 60, A) the maxillo-palatine processes
+approach one another in the middle line, and either unite with the
+vomers, or unite with one another, hiding the vomers. Thus a more or
+less complete bony roof is formed across the palate. The vomers in
+Desmognathae are small or sometimes absent. Ducks, storks, most
+herons, most birds of prey and owls, pelicans, cormorants, parrots,
+and flamingoes are Desmognathous.
+
+The mandible, as in other Sauropsids, consists of a cartilage bone,
+the articular, and a series of membrane bones, the dentary, splenial,
+coronoid, angular, and supra-angular, developed round the unossified
+Meckel's cartilage. The dentaries of the two rami are nearly always
+fused together, but in _Ichthyornis_ and _Archaeopteryx_ the two rami
+are but loosely united. There is often a fontanelle between the
+dentary and the posterior bones, while the angle is sometimes, as in
+the fowl, drawn out into a long curved process.
+
+The hyoid apparatus (fig. 59, C) consists of a median portion, and a
+pair of cornua. The median portion is composed of three pieces placed
+end to end, and called respectively the os entoglossum, the basi-hyal,
+and the uro-hyal. The os entoglossum is shown by development to be
+formed by the union of paired structures and is probably homologous
+with the hyoid arch of fishes. The basi-hyal and the long cornua, each
+of which is composed of two or three pieces placed end to end, are
+homologous with the first branchial arch of fishes, while the uro-hyal
+is probably homologous with the second branchial arch of fishes. In
+Woodpeckers the cornua are enormously long, and curve over the skull,
+extending as far forwards as the anterior nares.
+
+
+RIBS AND STERNUM.
+
+Well-developed ribs are attached to the posterior cervical vertebrae
+as well as to the thoracic vertebrae. The ribs generally have uncinate
+processes and separate capitula and tubercula, but uncinate processes
+are absent in _Chauna Palamedea_ and apparently in _Archaeopteryx_.
+
+The sternum (fig. 63) is greatly developed in all birds. In the
+embryo[107] it is seen to be derived from the union of right and left
+plates of cartilage, formed by the fusion of the ventral ends of the
+ribs. In the Ratitae and a few Carinatae, such as _Stringops_, it is
+flat, but in the great majority of birds it is keeled, though the
+development of the keel varies greatly. It is large in the flightless
+Penguins, which use their wings for swimming. Traces of an
+interclavicle may occur in the embryo.
+
+
+PECTORAL GIRDLE.
+
+[Illustration FIG. 63. SHOULDER-GIRDLE AND STERNUM OF
+
+ A. BLACK VULTURE (_Vultur cinereus_) × 1/3.
+ B. PEACOCK (_Pavo cristatus_) × 3/8.
+ C. PELICAN (_Pelicanus conspicillatus_) × 1/3. (All Camb. Mus.)
+
+ 1. carina of the sternum.
+ 2. coracoid.
+ 3. scapula.
+ 4. clavicle.
+ 5. costal process.
+ 6. surfaces for articulation with the sternal ribs.
+ 7. xiphoid processes.
+ 8. fontanelle.]
+
+The pectoral girdle is also strongly developed in all Carinatae, but
+is much reduced in Ratitae. In some Moas the sternum has no facet for
+the articulation of the coracoid, and the pectoral girdle appears to
+have been entirely absent; it is extremely small also in _Apteryx_.
+Clavicles are generally well developed in the Carinatae, and small
+ones are found also in _Hesperornis_, and in Emeus and Cassowaries. In
+the other living Ratitae and in _Stringops_ they are absent. In some
+Parrots, Owls and Toucans they do not meet one another ventrally.
+Clavicles are especially stout in some of the birds of prey. They do
+not generally touch the sternum, but sometimes, as in the Pelican
+(fig. 63, C), Adjutant and Frigate bird, they are fused with it.
+
+In all Ratitae the scapula and coracoid lie almost in the same
+straight line with one another, in the Carinatae they are nearly at
+right angles to one another.
+
+
+ANTERIOR LIMB.
+
+In the wing of nearly all birds the ulna is thicker than the radius,
+but in _Archaeopteryx_ the two bones are equal in size. In the wing of
+_Archaeopteryx_ there are three long digits with distinct metacarpals.
+In all other birds the digits are modified, the metacarpals being
+commonly fused and the phalanges reduced in number. In _Palamedea_ and
+some other birds the metacarpus bears a bony outgrowth, which when
+sheathed in horn forms a spur.
+
+In most of the Ratitae and in the extinct Dodo (_Didus_) and Solitaire
+(_Pezophaps_) the wing is very small, but the usual parts are
+recognisable. In _Hesperornis_ apparently only the humerus is present;
+in some Moas, in which the wing is imperfectly known, the presence of
+the humerus is indicated by traces of a glenoid cavity. In most Moas
+the wing is apparently completely absent. As compared with those in
+other Ratitae, the wings of the Ostrich and Rhea are well developed.
+In the Ostrich (fig. 64, B) and Rhea, as in nearly all Carinatae, the
+manus has three digits, but in _Apteryx_ there is only a single digit,
+the second. The Penguins (fig. 64, A) too among Carinatae have only
+two digits, but in their case it is the pollex which is missing. In
+the Ostrich the third digit has two phalanges, in all other living
+birds it has only one phalanx.
+
+
+PELVIC GIRDLE.
+
+[Illustration FIG. 64. BONES OF THE RIGHT WING OF
+
+ A. A PENGUIN × 1/3. (Camb. Mus.)
+ B. OSTRICH (_Struthio camelus_) × 1/7. (Partly after PARKER.)
+ C. GANNET (_Sula alba_) × 1/3. (Camb. Mus.)
+
+In C the distal phalanges of the pollex and second digit have been
+omitted.
+
+ 1. humerus.
+ 2. radius.
+ 3. ulna.
+ 4. second metacarpal.
+ 5. third metacarpal.
+ 6. pollex.
+ 7. second digit.
+ 8. cuneiform.
+ 9. sesamoid bone.]
+
+Birds have a very large pelvis and its characters are constant
+throughout almost the whole group. The ilium is very large, and is
+united along its whole length with the sacral and pseudosacral
+vertebrae. The ischium is broad and extends back parallel to the ilium
+with which in most birds it fuses posteriorly, further forward the
+ilio-sciatic foramen separates the two bones. In _Tinamus_,
+_Hesperornis_, _Apteryx_ (fig. 65, B, 2), and _Struthio_, the ischia
+are separate from the ilia along their whole length except at the
+acetabulum; in _Phororhacos_, on the other hand, the two bones are
+fused along almost their whole length. The bone usually called the
+pubis in birds corresponds to the post-pubis of Dinosaurs and forms a
+long slender rod (fig. 65, 3) lying parallel to the ischium. In many
+birds the ischia and pubes are united at their distal ends. This is
+the case in the Ostrich (fig. 65, D), in which the ilia and ischia are
+widely separated. In many birds the pubis is drawn out in front into
+the pectineal process, this is specially large in _Apteryx_ (fig. 65,
+B, 5), and in the embryos of many birds. It is probably homologous
+with the pre-pubis of Dinosaurs but in some birds is formed in part by
+the ilium. The acetabulum in birds is always perforate.
+
+In _Rhea_ (fig. 65, C, 2) and probably in _Archaeopteryx_ a symphysis
+ischii occurs, and in the ostrich alone among birds there is a
+symphysis pubis. In _Archaeopteryx_ all three bones of the pelvis are
+distinct, but they are imperfectly known. In Ichthyornis they are also
+distinct, in all other known birds they are fused together to a
+greater or less extent.
+
+[Illustration FIG. 65. PELVIC GIRDLE AND SACRUM OF
+
+ A. CASSOWARY (_Casuarius galeatus_) × 1/8.
+ B. OWEN'S APTERYX (_A. oweni_) × 1/2.
+ C. BROAD BILLED RHEA (_R. macrorhyncha_) × 1/6.
+ D. OSTRICH (_Struthio camelus_) × 1/10. (All Camb. Mus.)
+
+ 1. ilium.
+ 2. ischium.
+ 3. pubis.
+ 4. acetabulum.
+ 5. pectineal process.]
+
+
+POSTERIOR LIMB.
+
+The tibia is always well developed and has a very strong cnemial
+crest. The proximal tarsals are fused with its distal end, the whole
+forming a compound bone, the tibio-tarsus. There is frequently an
+oblique bar of bone crossing the anterior face of the tibio-tarsus at
+the distal end, just above the articular surface of the
+tarso-metatarsus, this is absent in Ostriches and _Æpyornis_. The
+fibula though in the embryo and in _Archaeopteryx_ equal in length to
+the tibia, is in the adult of other birds always imperfect, its
+proximal end is often fused with the tibia, and its distal end is
+commonly atrophied. In the Penguins however the distal end is
+complete. The distal tarsals fuse with the second, third and fourth
+metatarsals, forming a compound bone, the tarso-metatarsus. The first
+metatarsal is nearly always free but occasionally as in _Phaëthon_ it
+is fused with the others. No adult bird has more than four digits in
+the pes. In the Penguins the metatarsals are separate, and in many
+birds larger or smaller gaps exist between the fused metatarsals. In
+most birds the third metatarsal is curved so as not to lie in the same
+plane as the others, but in the Penguins they all three lie in the
+same plane. The metatarsals are clearly separated in _Archaeopteryx_.
+In Gallinaceous birds the tarso-metatarsus bears a bony outgrowth
+which is sheathed in horn and forms a spur.
+
+In most birds the first four toes are present while the fifth is
+always absent. The first toe commonly has two phalanges, the second
+three, the third four, and the fourth five. In Swifts the third and
+fourth toes have only three phalanges. Many birds, such as all Ratitae
+except _Apteryx_, have only three toes, the hallux being absent; in
+the Ostrich the second toe is also gone with the exception of a small
+metatarsal, so that the foot retains only the third and fourth digits,
+the third being much the larger of the two and bearing a claw, while
+the fourth is clawless.
+
+In the Swifts, Cormorants, and Penguins, all four toes are directed
+forwards. In most birds the hallux is directed backwards, and the
+other toes forwards. In the Owls the fourth toe can be directed
+backwards as well as the hallux, while in Parrots, Cuckoos,
+Woodpeckers, and Toucans the fourth toe is permanently reversed. In
+Trogons the second toe is reversed in addition to the hallux, but not
+the fourth.
+
+
+FOOTNOTES:
+
+[105] W.K. Parker, _Phil. Trans._ vol. 179, p. 385, 1888; and _Ibis_,
+1888, p. 124.
+
+[106] See T.H. Huxley, "On the Classification of Birds," _P.Z.S._
+1867.
+
+[107] B. Lindsay, _P.Z.S._ 1885, p. 684.
+
+
+
+
+CHAPTER XX.
+
+CLASS MAMMALIA.
+
+
+The skeleton of the members of this class, the highest of the
+vertebrata, has the following characteristics:--
+
+Some part of the integument at some period of life is always provided
+with hairs; these are epidermal structures arising from short papillae
+of the Malpighian layer of the epidermis, which at once grow inwards
+and become imbedded in pits of the dermis. Sometimes scales or spines
+occur, and epidermal exoskeletal structures in the form of hoofs,
+nails, claws and horns are also characteristic. As regards the
+endoskeleton, the vertebral centra have terminal epiphyses except in
+the Ornithodelphia and some Sirenia. In the skull the cranial region
+is greatly developed as compared with that in lower vertebrates, and
+whereas in many reptiles the true cranium is largely concealed by a
+false roof, in mammals the only relic of this secondary roof is found
+in the zygomatic arch, and postorbital bar. In the adult all the bones
+except the mandible, hyoid, and auditory ossicles are firmly united
+together. The basisphenoid is well ossified, and there is no
+parasphenoid. The pro-otic ossifies, and unites with the epi-otic and
+opisthotic before they coalesce with any other bones.
+
+The skull articulates with the vertebral column by means of two convex
+occipital condyles formed mainly by the exoccipitals, and the mandible
+articulates with the squamosal without the intervention of the
+quadrate. The latter is much reduced, and is converted into the
+tympanic ring, while the hyomandibular of fish is represented by the
+auditory ossicles[108].
+
+The teeth are always attached to the maxillae, premaxillae and
+mandibles, never to any of the other bones. They are nearly always
+implanted in distinct sockets, and are hardly ever ankylosed to the
+bone. The teeth of mammals are generally markedly heterodont, four
+forms, incisors, canines, premolars, and molars, being commonly
+distinguishable. Some mammals are _monophyodont_, having only a single
+set of teeth, but the great majority are _diphyodont_, having two
+sets, a deciduous or milk dentition, and a permanent dentition.
+
+The _incisors_, the front teeth, are simple, one-rooted, adapted for
+cutting, and are nearly always borne by the premaxillae. Next come the
+_canines_, one on each side in each jaw. They are generally large
+teeth adapted for tearing or holding, and get their name from the fact
+that they are largely developed in the dog. The remaining teeth form
+the grinding series, the more posterior of them being the _molars_,
+which are not preceded by milk teeth[109]. Between the molars and the
+canines are the _premolars_, which do as a rule have milk or deciduous
+predecessors, though very frequently the first of them is without a
+milk predecessor.
+
+In describing the dentition of any mammal, for the sake of brevity a
+formula is generally made use of. Thus, the typical mammalian
+dentition is expressed by the formula
+
+ _i_ 3/3 _c_ 1/1 _pm_ 4/4 _m_ 3/3 = 11/11,
+
+giving twenty-two teeth on each side, or forty-four altogether[110].
+The incisors are represented by _i_, the canines by _c_, the premolars
+by _p_ or _pm_, and the molars by _m_. The numbers above the lines
+represent the teeth in the upper jaw, those below the lines the teeth
+in the lower jaw. The milk dentition is expressed by a similar formula
+with _d_ (deciduous) prefixed to the letter expressing the nature of
+the tooth.
+
+The following terms are of frequent use as characterising certain
+forms of the grinding surfaces of teeth, and it will be well to define
+them at once.
+
+_Bunodont_ is a term applied to teeth with broad crowns raised into
+rounded tubercles, e.g. the grinding teeth of Pigs and Hippopotami;
+
+_Bilophodont_ to teeth marked by a simple pair of transverse ridges,
+with or without a third ridge running along the outer border of the
+tooth at right angles to the other two, e.g. the grinding teeth of
+_Lophiodon_, Kangaroo, Manatee, Tapir, _Dinotherium_;
+
+_Selenodont_ to teeth marked by crescentic ridges running from the
+anterior towards the posterior end of the tooth, e.g. the grinding
+teeth of the Ox and Sheep.
+
+Teeth whose crowns are low so that their whole structure is visible
+from the grinding surface are called _brachydont_, while those with
+higher crowns, in which the bases of the infoldings of enamel are
+invisible from the grinding surface are said to be _hypsodont_.
+Bunodont teeth are brachydont, the teeth of the Horse and Ox are
+hypsodont.
+
+Passing now to the appendicular skeleton--the shoulder girdle differs
+markedly from that of Sauropsids in the fact that the coracoid, except
+in the Ornithodelphia, is greatly reduced, generally forming only a
+small process on the scapula. In the pelvis the pubes meet in a
+ventral symphysis, except in some Insectivora and Chiroptera. In many
+mammals a fourth pelvic element, the _acetabular bone_, is
+distinguishable. The ankle joint is _cruro-tarsal_, or situated
+between the proximal tarsal bones and the tibia and fibula. Carpalia 4
+and 5 are united forming the _unciform_; and the ulnar sesamoid bone
+or _pisiform_ is generally well developed. In the proximal row of
+tarsal elements there are only two bones, the calcaneum and
+astragalus. Of these the calcaneum is the fibulare, and the
+astragalus is generally regarded as the tibiale and intermedium
+fused[111].
+
+
+_Subclass I._ ORNITHODELPHIA OR PROTOTHERIA.
+
+This subclass contains only a single order, the Monotremata, and the
+following characteristics are equally applicable to the subclass and
+to the order. The vertebral centra have no epiphyses, and the odontoid
+process remains for a long time free from the centrum of the second
+vertebra. With the exception of the atlas of _Echidna_ the cervical
+vertebrae are without zygapophyses. The cranial walls are smooth and
+rounded, and the sutures between the several bones early become
+completely obliterated as in birds. The mandible is a very slight
+structure, with no ascending ramus, and with the coronoid process (see
+p. 398) and angle rudimentary. The auditory ossicles show a low state
+of development. The tubercula of the ribs articulate with the sides of
+the centra of the thoracic vertebrae, not with the transverse
+processes. Some of the cervical ribs remain for a long time separate
+from the vertebrae. Well ossified sternal ribs occur. No true teeth
+are present in the adult. The young _Ornithorhynchus_ has functional
+molar teeth, but in the adult their place is taken by horny plates. In
+the Echidnidae neither teeth nor horny plates occur.
+
+The coracoid (fig. 66, 3) is complete and well developed, and
+articulates with the sternum. A precoracoid (epicoracoid) occurs in
+front of the coracoid, and there is a large interclavicle (fig. 66,
+6). The ridge on the scapula, corresponding to the spine of other
+mammals, is situated on the anterior border instead of in the middle
+of the outer surface. Epipubic bones are present. In the Echidnidae,
+but not in _Ornithorhynchus_[112], the central portion of the
+acetabulum is unossified as in birds. The humerus has a prominent
+deltoid crest; its ends are much expanded, and the distal end is
+pierced by an ent-epicondylar foramen. The fibula has a broad proximal
+process resembling an olecranon. The limbs and their girdles bear a
+striking resemblance to those of some Theromorphous reptiles.
+
+[Illustration FIG. 66. VENTRAL VIEW OF THE SHOULDER-GIRDLE AND STERNUM
+OF A DUCKBILL (_Ornithorhynchus paradoxus_) × 3/4 (after PARKER).
+
+ 1 and 2. scapula.
+ 3. coracoid.
+ 4. precoracoid (epicoracoid).
+ 5. glenoid cavity.
+ 6. interclavicle.
+ 7. clavicle.
+ 8. presternum.
+ 9. third segment of mesosternum.
+ 10. sternal rib.
+ 11. intermediate rib.
+ 12. vertebral rib.]
+
+The order Monotremata includes only two living families, the
+Echidnidae and Ornithorhynchidae.
+
+
+MESOZOIC MAMMALIA[113].
+
+It will be well here to briefly refer to certain mammals of small
+size, the remains of which have been found in deposits of Mesozoic
+age. In the great majority of cases they are known only by the lower
+jaw, or sometimes only by isolated teeth. A large number of them are
+commonly grouped together as the Multituberculata, and are sometimes,
+partly owing to the resemblance of their teeth to those of
+_Ornithorhynchus_, placed with the Prototheria, sometimes between the
+Prototheria and the Metatheria. They are characterised by having a
+single pair of large incisors in the lower jaw, and one large with one
+or two smaller incisors in each premaxillae. The lower canines are very
+small or altogether wanting. The incisors are separated by a diastema
+from the grinding teeth, which are sometimes (_Tritylodon_)
+characterised by the possession of longitudinal rows of little
+tubercles separated by grooves, sometimes by having the premolars
+provided with high cutting edges, whose surfaces are obliquely
+grooved. Some of the Mesozoic mammals found associated with the
+Multituberculata, have however a dentition of an altogether different
+type, with at least three lower incisors, well developed canines and
+premolars, and numerous molars with peculiar three-cusped or
+tritubercular grinding surfaces. These mammals, one of the best known
+of which is _Phascolotherium_, are commonly separated from the
+Multituberculata, and are divided by Osborn into two groups, one
+allied to the Marsupials, and one to the Insectivores. The group
+showing Marsupial affinities is further subdivided into carnivorous,
+omnivorous, and herbivorous subgroups. The members of both groups
+commonly have four premolars, and six to eight molars in each
+mandibular ramus.
+
+
+_Subclass II._ DIDELPHIA OR METATHERIA.
+
+This subclass, like the previous one, contains only a single order,
+viz. the Marsupialia[114]; but the forms referable to it are far more
+numerous than in the case of the Monotremata.
+
+The integument is always furry, and the teeth are always
+differentiated into incisors, canines, premolars and molars. Except in
+_Phascolomys_, the number of incisors in the upper and lower jaws is
+never equal, and the number in the upper usually exceeds that in the
+lower jaw. There is no such regular succession and displacement of
+teeth as in most mammals. Sometimes the anterior teeth are diphyodont,
+and as a general rule the tooth commonly regarded as the last premolar
+is preceded by a milk tooth. The majority of the permanent teeth of
+most Marsupials are regarded as belonging to the milk series for two
+reasons, (1) they are developed from the more superficial tissues of
+the jaws, (2) a second set, the permanent teeth, begin to develop as
+outgrowths from them, but afterwards become aborted[115].
+
+The odontoid process at an early stage becomes fused with the centrum
+of the second cervical vertebra, and the number of thoraco-lumbar
+vertebrae is always nineteen. The skull has several characteristic
+features. The tympanic bone remains permanently distinct, and the
+anterior boundary of the tympanic cavity is formed by the alisphenoid.
+The carotid canal perforates the basisphenoid, and the lachrymal canal
+opens either outside the orbit or at its margin. There are generally
+large vacuities in the palate. The angle of the mandible is (except in
+_Tarsipes_) more or less inflected; and as a rule the jugal furnishes
+part of the articular surface for the mandible. There is no
+precoracoid (epicoracoid) or interclavicle, and the coracoid is
+reduced to form a mere process of the scapula, not coming near the
+sternum.
+
+Epipubic, or so-called marsupial bones[116], nearly always occur, and
+a fourth pelvic element, the acetabular bone, is frequently developed.
+The fibula is always complete at its distal end, sometimes it is fused
+with the tibia, but often it is not only free but is capable of a
+rotatory movement on the tibia. This is the case in the families
+Phascolomyidae, Didelphyidae, and Phalangeridae.
+
+The Marsupialia can be subdivided into two main groups, according to
+the character of the teeth:--
+
+
+1. POLYPROTODONTIA.
+
+In this group the incisors are small, subequal and numerous, not less
+than 4/3. The canines are larger than the incisors, and the molars
+have sharp cusps. The members of this group are all more or less
+carnivorous or insectivorous. The group includes the families
+Didelphyidae, Dasyuridae, Peramelidae, and Notoryctidae[117].
+
+
+2. DIPROTODONTIA.
+
+In this group the incisors do not exceed 3/3, and are usually 3/1,
+occasionally 1/1. The first upper and lower incisors are large and
+cutting. The lower canines are always small or absent, and so in most
+cases are the upper canines. The molars have bluntly tuberculated, or
+transversely ridged crowns. The group includes the families
+Phascolomyidae, Phalangeridae, Macropodidae, and Epanorthidae.
+
+
+_Subclass III._ MONODELPHIA OR EUTHERIA.
+
+This great group includes all the Mammalia except the orders
+Monotremata and Marsupialia. Coming to their general characteristics--
+as in the Didelphia the odontoid process and cervical ribs early
+become fused with the centra which bear them, while the coracoid is
+reduced so as to form a mere process on the scapula, and there is no
+precoracoid (epicoracoid), such as is found in Ornithodelphia.
+Clavicles may be present or absent; when fully developed they
+articulate with the sternum, usually directly, but occasionally, as in
+some Rodents and Insectivores, through the remains of the sternal end
+of the precoracoid. There is never any interclavicle in the adult,
+though sometimes traces of it occur during development. In the pelvis
+the acetabula are imperforate; and well-developed epipubic bones are
+never found in the adult, though traces of them occur in some
+Carnivores and foetal Ungulates.
+
+
+_Order 1._ EDENTATA[118].
+
+Teeth are not, as the name of the order seems to imply, always
+wanting; and sometimes they are very numerous. They are, however,
+always imperfect, and, with very few exceptions, are homodont and
+monophyodont. They have persistent pulps, and so grow indefinitely and
+are never rooted. In all living forms they are without enamel,
+consisting merely of dentine and cement, and are never found in the
+front part of the mouth in the situation occupied by the incisors of
+other mammals. These characters derived from the teeth are the only
+ones common to the various members of the order, which includes the
+living sloths, ant-eaters, armadillos, pangolins and aard-varks,
+together with various extinct forms, chiefly found in beds of late
+tertiary age in both North and South America, the best known being the
+Megatheridae and Glyptodonts.
+
+
+_Order 2._ SIRENIA[119].
+
+The skeleton of these animals has a general fish-like form, in
+correlation with their purely aquatic habits. The fore limbs have the
+form of paddles, but the number of phalanges is not increased beyond
+the normal. There are no external traces of hind limbs.
+
+The whole skeleton and especially the skull and ribs is remarkably
+massive and heavy. The dentition varies; in the two living genera
+_Manatus_ and _Halicore_, incisor and molar teeth are present, in one
+extinct genus, _Rhytina_, teeth are entirely absent, while in another,
+_Halitherium_, the dentition is more decidedly heterodont than in
+living forms. In the two living genera the dentition is monophyodont,
+but in _Halitherium_ the anterior grinding teeth are preceded by milk
+teeth. The tongue and anterior part of the palate and lower jaw are
+covered with roughened horny plates. The skull is noticeable for the
+size and backward position of the anterior nares, also for the absence
+or small size of the nasal bones. There is no union of certain of the
+vertebrae to form a sacrum, and in living forms the centra are not
+terminated by well-formed epiphyses[120].
+
+The cervical vertebrae are much compressed, but they are never
+ankylosed together. In _Manatus_ there are only six cervical
+vertebrae. The caudal vertebrae have well-developed chevron bones. The
+humerus is distinctly articulated to the radius and ulna, and these
+two bones are about equally developed, and are often fused together.
+There are no clavicles, and the pelvis is vestigial, consisting of a
+pair of somewhat cylindrical bones suspended at some distance from the
+vertebral column. In living forms there is no trace of a posterior
+limb, but in _Halitherium_ there is a vestigial femur connected with
+each half of the pelvis.
+
+
+_Order 3._ CETACEA[121].
+
+In these mammals the general form is more fish-like than is the case
+even in the Sirenia. The skin is generally almost completely naked,
+but hairs are sometimes present in the neighbourhood of the mouth,
+especially in the foetus. In some Odontoceti vestiges of dermal
+ossicles have been described, and in _Zeuglodon_ the back was probably
+protected by dermal plates. The anterior limbs have the form of
+flattened paddles, showing no trace of nails, the posterior limb bones
+are quite vestigial or absent, and there is never any external sign of
+the limb. Teeth are always present at some period of the life history,
+but in the whalebone whales they are only present during foetal life,
+their place in the adult animal being taken by horny plates of baleen.
+In all living forms the teeth are simple and uniform structures
+without enamel; they have single roots, and the alveoli in which they
+are imbedded are often incompletely separated from one another. As in
+some forms traces of a replacing dentition have been described, it has
+been concluded that the functional teeth of Cetacea belong to the milk
+dentition.
+
+The texture of the bones is spongy. The cervical vertebrae are very
+short, and though originally seven in number, are in many forms
+completely fused, forming one solid mass (fig. 67). The odontoid
+process of the axis is short and blunt, or may be completely wanting.
+The lumbar and caudal vertebrae are large and numerous, and as
+zygapophyses are absent, are very freely movable on one another;
+zygapophyses are also absent from the posterior thoracic vertebrae.
+The lumbar vertebrae are sometimes more numerous than the thoracic.
+The epiphyses are very distinct, and do not unite with the centra till
+the animal is quite adult. None of the vertebrae are united to form a
+sacrum, but the caudal vertebrae have large chevron bones.
+
+[Illustration FIG. 67. CERVICAL VERTEBRAE OF A CA'ING WHALE
+(_Globicephalus melas_) × 1/4. (Camb. Mus.)
+
+ 1. centrum of seventh cervical vertebra.
+ 2. neural arch of seventh cervical vertebra.
+ 3. transverse process of atlas.
+ 4. foramen for exit of first spinal nerve.
+ 5. transverse process of axis.
+ 6. fused neural spines of atlas and axis.]
+
+The skull is peculiarly modified; the bones forming the occipital
+segment show a specially strong development, and the cranial cavity is
+short, high, and almost spherical. The supra-occipital is very large
+and rises up to meet the frontals, thus with the interparietal
+completely separating the parietals from one another.
+
+The frontals are expanded, forming large bony plates, which roof over
+the orbits. The zygomatic process of the squamosal is extremely large
+and extends forwards to meet the supra-orbital process of the frontal;
+the zygomatic process of the jugal is on the contrary very slender.
+The face is drawn out into a long rostrum, formed of the maxillae and
+premaxillae surrounding the vomer and the mesethmoid cartilage. The
+maxillae are specially large, and extend backwards so as to partially
+overlap the frontals. The nasals are always small, and the anterior
+nares open upwards between the cranium and rostrum. The periotics are
+loosely connected with the other bones of the skull and the tympanics
+are commonly large and dense. The mandible has hardly any coronoid
+process, and the condyles are at its posterior end.
+
+There are no clavicles, but the scapula and humerus are well
+developed. The humerus moves freely in the glenoid cavity, but all the
+other articulations of the anterior limb are imperfect; the various
+bones have flattened ends, and are connected with one another by
+fibrous tissue, which allows of hardly any movement. Frequently the
+carpus is imperfectly ossified.
+
+The number of digits in the manus is generally five, sometimes four,
+and when there are four digits it is the third and not the first that
+is suppressed. The number of phalanges in the second and third digits
+almost always exceeds that which is normal in mammals, and the
+phalanges are also remarkable for having epiphyses at both ends. The
+pelvis is represented by two small bones which lie suspended
+horizontally at some distance below the vertebral column; in some
+cases vestiges of the skeleton of the hind limb are attached to them.
+
+The Cetacea are divided into three suborders.
+
+
+_Suborder_ (1). ARCHAEOCETI.
+
+The members of this group are extinct; they differ from all living
+Cetacea in having the dentition heterodont and in the fact that the
+back was probably protected by dermal plates. The skull is elongated
+and depressed, and the brain cavity is very small. The temporal fossae
+are large, and there is a strong sagittal crest. The nasals and
+premaxillae are a good deal larger than they are in living Cetacea, and
+the anterior nares are usually far forward. The cervical vertebrae are
+not fused with one another, and the lumbar vertebrae are unusually
+elongated.
+
+The limbs are very imperfectly known, but while the humerus is much
+longer than in modern Cetaceans, it is nevertheless flattened
+distally, indicating that the limb was paddle-like, and that there was
+scarcely any free movement between the fore-arm and upper arm.
+
+The best known genus is _Zeuglodon_, which is found in beds of Eocene
+age in various parts of Europe, and in Alabama.
+
+
+_Suborder_ (2). MYSTACOCETI OR BALAENOIDEA.
+
+These are the Whalebone Whales or True Whales.
+
+Calcified teeth representing the milk dentition occur in the foetus,
+but the teeth are never functional, and always disappear before the
+close of foetal life. There is a definite though small olfactory
+fossa. The palate is provided with plates of baleen or whalebone. The
+skull is symmetrical, and is extremely large in proportion to the
+body. The nasals are moderately well developed, and the maxillae do not
+overlap the orbital processes of the frontals. The lachrymals are
+small and distinct from the jugals. The tympanics are ankylosed to the
+periotics, and the rami of the mandible do not meet in a true
+symphysis. The ribs articulate only with the transverse processes, and
+the capitula are absent or imperfectly developed. Only one pair of
+ribs meets the sternum, which is composed of a single piece.
+
+The group includes among others the Right whale (_Balaena_),
+Humpbacked whale (_Megaptera_), and Rorqual (_Balaenoptera_).
+
+
+_Suborder_ (3). ODONTOCETI.
+
+Teeth always exist after birth and baleen is never present. The teeth
+are generally numerous, but are sometimes few and deciduous; the
+dentition is homodont (except in _Squalodon_). The dorsal surface of
+the skull is somewhat asymmetrical, there is no trace of an olfactory
+fossa, the nasals are quite rudimentary, and the hind ends of the
+maxillae cover part of the frontals; in all these respects the skull
+differs from that of the Mystacoceti. The lachrymal may either be
+united to the jugal or may be large and distinct. The tympanic is not
+ankylosed to the periotic. The rami of the mandible are nearly
+straight and become united in a long symphysis. Some of the ribs have
+well developed capitula articulating with the vertebral centra. The
+sternum is almost always composed of several pieces as in other
+mammals, and several pairs of ribs are connected with it. There are
+always five digits to the manus, though the first and fifth are
+usually very little developed.
+
+The suborder includes the Sperm Whale (_Physeter_), Narwhal
+(_Monodon_), Dolphin (_Delphinus_), Porpoise (_Phocoena_), and many
+other living forms as well as the extinct _Squalodon_ which differs
+from the other members of the suborder in its heterodont dentition.
+
+_Order 4._ UNGULATA.
+
+This order includes a great and somewhat heterogeneous group of
+animals, a large proportion of which are extinct. They all (except
+certain extinct forms) agree in having the ends of the digits either
+encased in hoofs or provided with broad flat nails. The teeth are
+markedly heterodont and diphyodont, and the molars have broad crowns
+with tuberculated or ridged surfaces. Clavicles are never present in
+the adult except in a few generalised extinct forms such as
+_Typotherium_, and it is only recently that vestigial clavicles have
+been discovered in the embryo[122]. The scaphoid and lunar are always
+distinct.
+
+The order Ungulata may be subdivided into two main groups, Ungulata
+vera and Subungulata.
+
+
+_Section I._ UNGULATA VERA[123].
+
+The cervical vertebrae except the atlas are generally opisthocoelous.
+The feet are never _plantigrade_[124]. In all the living and the great
+majority of the extinct forms the digits do not exceed four, the first
+being suppressed. In the carpus the os magnum articulates freely with
+the scaphoid, and is separated from the cuneiform by the lunar and
+unciform. In the tarsus the cuboid articulates with the astragalus as
+well as with the calcaneum, and the proximal surface of the astragalus
+is marked by a pulley-like groove. All the bones of the carpus and
+tarsus strongly interlock. These characters with regard to the carpus
+and tarsus do not hold in _Macrauchenia_ and its allies. The humerus
+never has an ent-epicondylar foramen.
+
+The group is divided into two very distinct suborders:--
+
+
+_Suborder_ (1). ARTIODACTYLA.
+
+The Artiodactyla have a number of well marked characters, one of the
+most obvious being the fact that many of the most characteristic forms
+have large paired outgrowths on the frontal bones. These may be (1)
+solid deciduous bony _antlers_, or (2) more or less hollow bony
+outgrowths which are sheathed with permanently growing horn.
+
+The premolar and molar teeth are usually dissimilar, the premolars
+being one-lobed and the molars two-lobed; the last lower molar of both
+the milk and permanent dentitions is almost always three-lobed.
+
+The grinding surfaces of the molar teeth have a tendency to assume one
+of two forms. In the Pigs and their allies the crowns are bunodont[1],
+while in the more highly specialised Ruminants the crowns are
+selenodont[125]. The nasals are not expanded posteriorly, and there is
+no alisphenoid canal[126]. The thoraco-lumbar vertebrae are always
+nineteen. The symphysis of the ischia and pubes is very elongated, and
+the femur has no third trochanter. The limbs never have more than four
+digits, and are symmetrical about a line drawn between the third and
+fourth digits; the digits, on the other hand, are never symmetrical in
+themselves. The astragalus has pulley-like surfaces both proximally
+and distally, and articulates with the navicular and cuboid by two
+nearly equal facets. The calcaneum articulates with the lower end of
+the fibula when that bone is fully developed.
+
+In the Artiodactyla are included the following living groups:--
+
+_a._ Suina. Pigs and Hippopotami.
+
+_b._ Tylopoda. Camels and Llamas.
+
+_c._ Tragulina. Chevrotains.
+
+_d._ Ruminantia or Pecora. Deer, giraffes, oxen, sheep and antelopes.
+
+
+_Suborder_ (2). PERISSODACTYLA[127].
+
+In this group there are never any bony outgrowths from the frontals.
+The grinding teeth form a continuous series, the posterior premolars
+resembling the molars in complexity, and the last lower molar
+generally has no third lobe. The cervical vertebrae with the
+exception of the atlas almost always have markedly opisthocoelous
+centra, but in _Macrauchenia_ they are flat. The nasals are expanded
+posteriorly, and an alisphenoid canal is present. The thoraco-lumbar
+vertebrae are never less than twenty-two in number and are usually
+twenty-three. The femur has a third trochanter (except in
+_Chalicotherium_). The third digit of the manus and pes is symmetrical
+in itself, and larger than the others, and in some cases the other
+digits are quite vestigial. The number of the digits of the pes is
+always odd. The astragalus is abruptly truncated distally, and the
+facet by which it articulates with the cuboid, is much smaller than
+that by which it articulates with the navicular. The calcaneum does
+not articulate with the fibula, except in _Macrauchenia_. The group
+includes many extinct forms, and the living families of the Tapirs,
+Horses and Asses, and Rhinoceroses.
+
+
+_Section II._ SUBUNGULATA.
+
+In this group is placed a heterogeneous collection of animals, the
+great majority of which are extinct. There is really no characteristic
+which is common to them all, and which serves to distinguish them as a
+group from the Ungulata vera. But the most distinctive character
+common to the greatest number of them is to be found in the carpus,
+whose bones in most cases retain their primitive relation to one
+another, the os magnum articulating with the lunar and sometimes just
+meeting the cuneiform, but in living forms at any rate not
+articulating with the scaphoid. The feet frequently have five
+functional digits, and may be plantigrade. The proximal surface of the
+astragalus is generally flattened instead of being pulley-like as in
+Ungulata vera.
+
+
+_Suborder_ (1). TOXODONTIA.
+
+This suborder includes some very aberrant extinct South American
+ungulates, which have characters recalling the Proboscidea, both
+groups of Ungulata vera, and the Rodentia. The limbs are
+subplantigrade or digitigrade, and the digits are three, rarely five,
+in number, the third being most developed. The carpus resembles that
+of the Ungulata vera, in that the bones interlock and the magnum
+articulates with the scaphoid. In the tarsus, however, the bones do
+not interlock. The astragalus has a pulley-like proximal surface
+(except in _Astrapotherium_, in which it is flat), and articulates
+only with the navicular, not meeting the cuboid. The calcaneum has a
+large facet for articulation with the fibula, as in Artiodactyla.
+There is no alisphenoid canal, and the orbit is confluent with the
+temporal fossa. Some of the forms (e.g. _Nesodon_) referred to this
+group have the typical mammalian series of forty-four teeth, but in
+others the canines are undeveloped. In _Toxodon_ all the cheek teeth
+have persistent pulps, while in _Nesodon_ and _Astrapotherium_ they
+are rooted. A clavicle is sometimes present (_Typotherium_), and the
+femur sometimes has a third trochanter (_Typotherium_ and
+_Astrapotherium_), sometimes is without one (_Toxodon_).
+
+The remains of these curious Ungulates have been found in beds of late
+Tertiary age in South America.
+
+
+_Suborder_ (2). CONDYLARTHRA[128].
+
+This group includes some comparatively small extinct ungulates, which
+are best known from the Lower Eocene of Wyoming, though their remains
+have also been found in deposits of similar age in France and
+Switzerland. Their characters are little specialised, and they show
+relationship on the one hand to the Ungulata vera and on the other to
+the Hyracoidea. They also have characters allying them to the
+Carnivora. They generally have the typical mammalian series of
+forty-four teeth, the molars being brachydont and generally bunodont.
+The premolars are more simple than the molars. The limbs are
+plantigrade, and have five digits with rather pointed ungual
+phalanges. The os magnum, as in living Subungulates, articulates with
+the lunar, not reaching the scaphoid. The astragalus has an elongated
+neck, a pulley-like proximal and a convex distal articular surface,
+and does not articulate with the cuboid. The humerus has an
+ent-epicondylar foramen, and the femur has a third trochanter. The
+best known genus is _Phenacodus_; it is perhaps the most primitive
+ungulate whose skeleton is thoroughly well known, and is of special
+interest from the fact that it is regarded as the lowest stage in the
+evolutionary series of the horse. Its remains are found in the Lower
+Eocene of Wyoming.
+
+
+_Suborder_ (3). HYRACOIDEA[129].
+
+This group of animals is very isolated, having no very close allies,
+either living or extinct. The digits are provided with flat nails,
+except the second digit of the pes, which is clawed. Canine teeth are
+absent, and the dental formula is usually given as _i_ 1/2, _c_ 0/0,
+_pm_ 4/4, _m_ 3/3. The upper incisors are long and curved, and have
+persistent pulps as in Rodents; their terminations are, however,
+pointed, not chisel-shaped, as in Rodents. The lower incisors have
+pectinated edges. The grinding teeth have a pattern much like that in
+_Rhinoceros_. In the skull (fig. 83) the postorbital processes of the
+frontal and jugal almost or quite meet. The jugal forms part of the
+glenoid cavity for articulation with the mandible, and also extends
+forwards so as to meet the lachrymal. There is an alisphenoid canal.
+There are as many as twenty-one or twenty-two thoracic vertebrae, and
+the number of thoraco-lumbar vertebrae reaches twenty-eight or thirty.
+There are no clavicles, and the scapula has no acromion; the coracoid
+process is, however, well developed. The ulna is complete. In the
+manus the second, third and fourth digits are approximately equal in
+size, the fifth is smaller, and the first is vestigial. The femur has
+a slight ridge representing the third trochanter. The fibula is
+complete, but is generally fused with the tibia proximally. There is a
+complicated articulation between the tibia and astragalus, which has a
+pulley-like proximal surface. In the pes the three middle digits are
+well developed, but there is no trace of a hallux, and the fifth digit
+is represented only by a vestigial metatarsal.
+
+The only representatives of the suborder are some small animals
+belonging to the genus _Procavia_ (_Hyrax_), which is found in Africa
+and Syria; some of the species are by many authors placed in a
+distinct genus _Dendrohyrax_.
+
+
+_Suborder_ (4). AMBLYPODA[130].
+
+This suborder includes a number of primitive extinct Ungulates, many
+of which are of great size. Their most distinguishing characteristics
+are afforded by the extremities. In the carpus the bones interlock a
+little more than is the case in most Subungulata, and the corner of
+the os magnum reaches the scaphoid, while the lunar articulates
+partially with both magnum and unciform, instead of only with the
+magnum. In the tarsus the cuboid articulates with both the calcaneum
+and the astragalus, which is remarkably flat. The manus and pes are
+short, nearly or quite plantigrade, and have the full number of
+digits. The cranial cavity is singularly small. Canine teeth are
+present in both jaws, and the grinding teeth have short crowns, marked
+by =V=-shaped ridges. The pelvis is large, the ilia are placed
+vertically, and the ischia do not take part in the ventral symphysis.
+
+The best known animals belonging to this suborder are the
+Uintatheriidae (Dinocerata)[131], found in the Upper Eocene of
+Wyoming. They are as large as elephants, and are characterised by the
+long narrow skull drawn out into three pairs of rounded protuberances,
+by the strong occipital crest, and by the very large upper canines.
+
+
+_Suborder_ (5). PROBOSCIDEA.
+
+This suborder includes the largest of land mammals, the Elephants, and
+certain of their extinct allies. The limbs are strong, and are
+vertically placed; the proximal segment is the longest, and the manus
+and pes are pentedactylate and subplantigrade. The digits are all
+enclosed in a common integument, and each is provided with a broad
+hoof. The vertebral centra are much flattened and compressed,
+especially in the cervical region. The number of thoracic vertebrae is
+very great, reaching twenty. The skull (figs. 96 and 97) is extremely
+large, this being due to the great development of air cells, which
+takes place in nearly all the bones of the adult skull. In the young
+skull there are hardly any air cells, and the growth of the cranial
+cavity does not by any means keep pace with the growth of the skull in
+general. The supra-occipital is very large, and forms a considerable
+part of the roof of the skull. The nasals and jugals are short, and
+the premaxillae very large. The rami of the mandible meet in a long
+symphysis, and the ascending portion is very high. Canine teeth are
+absent, and the incisors have the form of ever-growing tusks composed
+mainly of dentine; in living forms they are present in the upper jaw
+only. The grinding teeth are large, and in living forms have a very
+complex structure and mode of succession. In some of the extinct
+forms, such as _Mastodon_ and especially _Dinotherium_, the teeth are
+much more simple. In every case the teeth have the same general
+structure, consisting of a series of ridges of dentine, coated with
+enamel. In the more specialised forms the valleys between the ridges
+are filled up with cement. The acromion of the scapula has a recurved
+process, similar to that often found in rodents. Clavicles are absent.
+The radius and ulna are not ankylosed, but are incapable of any
+rotatory movement. All the bones of the extremities are very short and
+thick; the scaphoid articulates regularly with the trapezoid and the
+lunar with the magnum. The ilia are vertically placed, and are very
+much expanded; the ischia and pubes are small, and form a short
+symphysis. The femur has no third trochanter, and the tibia and fibula
+are distinct. The fibula articulates with the calcaneum, and the
+astragalus is very flat.
+
+ * * * * *
+
+Here brief reference may be made to the TILLODONTIA[132], a group of
+extinct mammals found in the Eocene beds of both Europe and North
+America. They seem to connect together the Ungulata, Rodentia, and
+Carnivora.
+
+The skull resembles that of bears, but the grinding teeth are of
+Ungulate type, while the second incisors resemble those of rodents,
+and have persistent pulps. The femur has a third trochanter, and the
+feet resemble those of bears in being plantigrade and having pointed
+ungual phalanges, differing, however, in having the scaphoid and lunar
+distinct.
+
+
+_Order 5._ RODENTIA.
+
+The Rodents form a very large and well-defined group of mammals easily
+distinguishable by their peculiar dentition. Canines are absent, and
+the incisors are very large and curved, growing from persistent pulps.
+They are rectangular in section and are much more thickly coated with
+enamel on their anterior face than elsewhere; consequently, as they
+wear down they acquire and retain a chisel-shaped (scalpriform) edge.
+There is never more than one pair of incisors in the mandible, and
+except in the Hares and Rabbits, there is similarly only a single pair
+in the upper jaw. These animals are, too, the only rodents which have
+well developed deciduous incisors. There is always a long diastema
+separating the incisors from the grinding teeth. The grinding teeth,
+which are arranged in a continuous series, vary in number from two to
+six in the upper jaw, and from two to five in the lower jaw. The
+number of premolars is always below the normal, often they are
+altogether wanting, but generally they are 1/1. Sometimes the grinding
+teeth form roots, sometimes they grow persistently.
+
+The premaxillae are always large, and the orbits always communicate
+freely with the temporal fossae. The condyle of the mandible is
+elongated from before backwards, and owing to the absence of a
+postglenoid process to the squamosal, a backward and forward motion of
+the jaw can take place. The zygomatic arch is complete, but the jugal
+is short and only forms the middle of it. The palate is small, being
+sometimes, as in the hares, narrowed from before backwards, sometimes
+as in the mole-rats (Bathyerginae) narrowed transversely.
+
+The thoraco-lumbar vertebrae are usually nineteen in number. Clavicles
+are generally present, and the acromion of the scapula is commonly
+very long. The feet are as a rule plantigrade, and provided with five
+clawed digits.
+
+There are two main groups of Rodentia; the Duplicidentata, or Hares
+and Rabbits, which have two pairs of upper incisors, whose enamel
+extends round to the posterior surface; and the Simplicidentata, in
+which there is only a single pair of upper incisors, whose enamel is
+confined to the anterior surface. This group includes all the Rodents
+except the Hares and Rabbits.
+
+
+_Order 6._ CARNIVORA.
+
+The living Carnivora form a natural and well-marked group, but as is
+the case with so many other groups of animals, when their extinct
+allies are included, it becomes impossible to readily define them.
+
+The manus and pes never have less than four well-developed digits, and
+these are nearly always provided with more or less pointed nails,
+generally with definite claws. The hallux and pollex are never
+opposable. The dentition is diphyodont and markedly heterodont. The
+teeth are always rooted, except in the case of the canines of the
+Walrus. The incisors are generally 3/3, and are comparatively small,
+while the canines are large, pointed, and slightly recurved. The cheek
+teeth are variable, and are generally more or less compressed and
+pointed; sometimes their crowns are flattened and tuberculated, but
+they are never divided into lobes by deep infoldings of enamel. The
+squamosal is drawn out into a postglenoid process, and the mandible
+has a large coronoid process. The condyle of the mandible is
+transversely elongated, and the glenoid fossa is very deep; in
+consequence of this arrangement the mandible can perform an up and
+down movement only, any rotatory or back and fore movement being
+impossible. The jugal is large, and the zygomatic arch generally
+strong, while the orbit and temporal fossa are in most cases
+completely confluent. The scapula has a large spine. The clavicle is
+never complete and is often absent, this forming an important
+distinction between the skeleton of a Carnivore and of any Insectivore
+except _Potamogale_. The humerus often has an ent-epicondylar foramen,
+and the radius and ulna, tibia and fibula are always separate. The
+manus is often capable of the movements of pronation and supination,
+and the scaphoid, lunar and centrale are in living forms always united
+together.
+
+The order Carnivora includes three suborders.
+
+
+_Suborder_ (1). CREODONTA[133].
+
+This suborder contains a number of extinct Carnivora, which present
+very generalised characters.
+
+The cranial cavity is very small; and the fourth upper premolar and
+first lower molar are not differentiated as carnassial teeth[134], as
+they are in modern Carnivora. The Creodonta also differ from modern
+Carnivora in the fact that the scaphoid and lunar are usually
+separate, and that the femur has a third trochanter. The feet are
+plantigrade.
+
+They resemble the Condylarthra, another very generalised group, in
+having an ent-epicondylar foramen.
+
+They occurred throughout the Tertiary period in both Europe and North
+America, and have also been found in India. One of the best known
+genera is _Hyaenodon_.
+
+
+_Suborder_ (2). CARNIVORA VERA or FISSIPEDIA.
+
+The skeleton is mainly adapted for a terrestrial mode of life, and the
+hind limbs have the normal mammalian position. In almost every case
+the number of incisors is 3/3. Each jaw always has one specially
+modified _carnassial_ or sectorial tooth which bites like a scissors
+blade against a corresponding tooth in the other jaw. In front of it
+the teeth are always more or less pointed, while behind it they are
+more or less broadened and tuberculated. In the manus the first digit,
+and in the pes the first and fifth digits are never longer than the
+rest, and the digits of both limbs are almost invariably clawed. Some
+forms are plantigrade, some digitigrade, some subplantigrade. The
+group includes all the ordinary terrestrial Carnivora, and is divided
+into three sections:--
+
+Æluroidea[135], including the cats, civets, hyaenas, and allied forms.
+
+Cynoidea, including the dog tribe.
+
+Arctoidea, including the bears, raccoons, weasels, and allied forms.
+
+
+_Suborder_ (3). PINNIPEDIA[136].
+
+In this suborder the limbs are greatly modified and adapted for a more
+or less purely aquatic life, the proximal and middle segments of the
+limbs are shortened, while the distal segment, especially in the leg,
+is much elongated and expanded. There are always five well-developed
+digits to each limb, and in the pes the first and fifth digits are
+generally larger than the others. The digits generally bear straight
+nails instead of claws, but even nails are sometimes absent. There is
+no carnassial tooth, and the teeth in other ways differ considerably
+from those of Carnivora vera. The incisors are always fewer than 3/3;
+while the cheek teeth generally consist of four premolars and one
+molar, all of very uniform character, being compressed with conical
+crowns, and never more than two roots.
+
+The suborder includes three families--Otariidae (Eared Seals),
+Trichechidae (Walrus), and Phocidae (Seals).
+
+
+_Order 7._ INSECTIVORA[137].
+
+This order contains a large number of small generally terrestrial
+mammals. The limbs are plantigrade or subplantigrade, and are
+generally pentedactylate. All the digits are armed with claws, and the
+pollex and hallux are not opposable. The teeth are diphyodont,
+heterodont, and rooted. The cheek teeth have tuberculated crowns, and
+there are never less than two pairs of incisors in the mandible; often
+the incisors, canines, and premolars are not clearly differentiated
+from one another, and special carnassial teeth are never found. The
+cranial cavity is small, and the facial part of the skull is generally
+much developed; often the zygomatic arch is incomplete. Clavicles are
+well developed (except in _Potamogale_), and the humerus generally has
+an ent-epicondylar foramen. The femur frequently has a ridge
+representing the third trochanter. There are two suborders:
+
+
+_Suborder_ (1). DERMOPTERA.
+
+This suborder includes only a very aberrant arboreal genus
+_Galeopithecus_, remarkable for its greatly elongated limb bones, and
+peculiar dentition. The incisors of the lower jaw are deeply
+pectinated or divided by several vertical fissures, the canines and
+outer upper incisors have two roots. Ossified inter centra occur in
+the thoraco-lumbar region of the vertebral column.
+
+
+_Suborder_ (2). INSECTIVORA VERA.
+
+This suborder includes all the ordinary Insectivora, such as moles,
+shrews and hedgehogs. The upper and lower incisors are conical, not
+pectinated.
+
+
+_Order 8._ CHIROPTERA[138].
+
+This order is perhaps the best marked and most easily defined of all
+the orders of mammals. The anterior limbs form true wings and the
+whole skeleton is modified in relation to flight.
+
+The anterior limbs are vastly larger than the posterior; for all the
+bones except the carpals are much elongated, and this applies
+specially to the phalanges of all the digits except the pollex.
+
+The pollex is clawed and so is sometimes the second digit; the other
+digits of the manus are without nails or claws. The teeth are
+divisible into the four usual types and the series never exceeds _i_
+2/3 _c_ 1/1 _pm_ 3/3 _m_ 3/3 × 2, total 38. The milk teeth are quite
+unlike the permanent teeth. The orbit is not divided by bone from the
+temporal fossa. The vertebral column is short, and in old animals the
+trunk vertebrae have a tendency to become partially fused together.
+The cervical vertebrae are remarkably wide, and the development of
+spinous processes is everywhere slight. The presternum has a prominent
+keel for the attachment of the pectoral muscles. The clavicles are
+very long and strong, and the scapula has a long spine and coracoid
+process. The ulna is vestigial, consisting only of a proximal end
+ankylosed to the radius. All the carpals of the proximal row--the
+scaphoid, lunar and cuneiform--are united, forming a single bone. The
+pelvis is very weak and narrow, and only in the Rhinolophidae do the
+pubes meet in a symphysis. The anterior caudal vertebrae are
+frequently united to the ischia. The fibula is generally vestigial,
+and the knee joint is directed backwards instead of forwards. The pes
+has five slender clawed digits, and the calcaneum is often drawn out
+into a spur which helps to support the membrane connecting the hind
+limbs with the tail.
+
+There are two suborders of Chiroptera:
+
+1. The MEGACHIROPTERA or Flying foxes, which almost always have smooth
+crowns to the molar teeth, and the second digit of the manus clawed.
+
+2. The MICROCHIROPTERA including all the ordinary bats which have
+cusped molar teeth, and the second digit of the manus clawless.
+
+
+_Order 9._ PRIMATES.
+
+The dentition is diphyodont and heterodont, the incisors generally
+number 2/2, and the molars, except in the Hapalidae (Marmosets), are
+3/3. The cheek teeth are adapted for grinding, and the molars are more
+complex than the premolars. A process from the jugal meets the
+postorbital process of the frontal completing the postorbital bar.
+
+The clavicle is well developed, and the radius and ulna are never
+united. The scaphoid and lunar of the carpus, and commonly also the
+centrale, remain distinct from one another. As a rule both manus and
+pes have five digits, but the pollex may be vestigial. The pollex is
+opposable to the other digits, and so is the hallux except in Man; the
+digits are almost always provided with flat nails. The humerus has no
+ent-epicondylar foramen and the femur has no third trochanter.
+
+The order Primates is divisible into two suborders:
+
+
+_Suborder_ (1). LEMUROIDEA.
+
+The skull has the orbit communicating freely with the temporal fossa
+beneath the postorbital bar (except in _Tarsius_). The lachrymal
+foramen is external to the margin of the orbit. Both pollex and hallux
+are well developed. In the pes the second digit is terminated by a
+long pointed claw, and so is also the third in _Tarsius_. The lumbar
+region of the vertebral column is long, sometimes including as many as
+nine vertebrae. Besides the Lemurs the group includes the aberrant
+_Tarsius_ and _Chiromys_.
+
+
+_Suborder_ (2). ANTHROPOIDEA.
+
+The skull has the orbit almost completely shut off from the temporal
+fossa, and the lachrymal foramen is situated within the orbit. The
+pollex is sometimes vestigial or absent. The second digit of the pes
+has a flattened nail except in the Hapalidae, in which all the digits
+of the pes except the hallux are clawed.
+
+The Anthropoidea are divided into five families:
+
+1. Hapalidae or Marmosets.
+
+2. Cebidae or American Monkeys.
+
+3. Cercopithecidae or Old World Monkeys.
+
+4. Simiidae or Anthropoid Apes.
+
+5. Hominidae or Men.
+
+
+FOOTNOTES:
+
+[108] This is Gadow's view; according to Huxley the quadrate forms the
+malleus; according to Baur it forms the zygomatic process of the
+squamosal, and according to Broom the interarticular mandibular
+cartilage.
+
+[109] According to Leche, _Morphol. Jahrb._ XIX. p. 502, the molar
+teeth belong morphologically to the first series, i.e. they are milk
+teeth without vertical successors.
+
+[110] The researches of Bateson, _P.Z.S._ 1892, p. 102, have shown
+that cases of individual variation in the number of teeth are common.
+
+[111] Baur, however, suggests (_Anat. Anz._ vol. IV. 1889), that a
+tibial sesamoid found in _Procavia_, many rodents, edentates and
+_Ornithorhynchus_ is a vestigial tibiale, and that the astragalus is
+the intermedium.
+
+[112] This perforation of the acetabulum in _Echidna_ is a secondary
+character occurring late in development, and consequently is not of
+phylogenetic importance.
+
+[113] See R. Owen, "Monograph of the Fossil Mammalia of the Mesozoic
+Formation," _Pal. Soc. Mon._ 1871.
+
+H.F. Osborn, "Structure and Affinities of Mesozoic Mammals," _J. of
+Philad. Acad._ 1888, vol. IX.
+
+O.C. Marsh, "Jurassic Mammals," _Amer. J. Sci._ 1878 _et seq._
+
+[114] See Oldfield Thomas, _Brit. Mus. Cat. of Marsupialia and
+Monotremata_ (1888).
+
+[115] W. Kükenthal, _Anat. Anz._ VI. p. 364, 1891. C. Röse, _Anat.
+Anz._ VII. p. 639.
+
+[116] These bones however have no connection with the marsupium, being
+nearly equally developed in both male and female. They are simply
+sesamoid bones forming ossifications in the inner tendon of the
+external oblique muscle, and are developed as supports for the
+abdominal wall. Very similar structures have been independently
+developed in various Amphibians, Reptiles and monodelphian Mammals.
+See W. Leche, _Biol. Fören._ III. p. 120.
+
+[117] See H. Gadow, _P.Z.S._ 1892, p. 361.
+
+[118] See W.H. Flower, "On the Mutual Affinities of the Animals
+composing the order Edentata," _P.Z.S._ 1882, p. 358. For the fossil
+Edentates of N. America see E. Cope, _Amer. Natural._ 1889; for those
+of S. America see various papers by F. Ameghino, H. Burmeister and R.
+Owen. Also T.H. Huxley, "On the Osteology of Glyptodon," _Phil.
+Trans._ 1865.
+
+[119] See J.F. Brandt, _Symbolae Sirenologicae_, St Petersburg, 1846,
+1861, 1868.
+
+[120] Epiphyses are fully developed in _Halitherium_, and traces occur
+in _Manatus_.
+
+[121] See P.J. van Beneden and P. Gervais, _Ostéographie des Cétacés_,
+1869-80.
+
+[122] H. Wincza, _Morphol. Jahrb._ XVI., p. 647.
+
+[123] See M. Pavlow, "Études sur l'histoire paléontologique des
+Ongulés." _Bull. Soc. Moscou_, 1887--1890.
+
+[124] In a _plantigrade_ animal the whole of the foot is placed on the
+ground in walking. A _digitigrade_ animal places only its toes on the
+ground. An intermediate condition is distinguished by the term
+_subplantigrade_.
+
+[125] See p. 345.
+
+[126] See p. 401.
+
+[127] See E.D. Cope, "The Perissodactyla," _Amer. Natural._, 1887.
+
+[128] See E.D. Cope, "The Condylarthra," _Amer. Natural._, 1884, and
+"Synopsis of the Vertebrates of the Puerco series," _Tr. Amer. Phil.
+Soc._, 1888. O.C. Marsh, "A new order of extinct Eocene Mammals
+(Mesodactyla)," _Amer. J. Sci._, 1892.
+
+[129] See O. Thomas, "On the species of Hyracoidea," _P.Z.S._, 1892,
+p. 50.
+
+[130] See E.D. Cope, "The Amblypoda," _Amer. Natural._, 1884 and 1885.
+
+[131] See O.C. Marsh, "The Dinocerata," _U.S. Geol. Survey_, 1884,
+vol. X.
+
+[132] See O.C. Marsh, _Amer. J. Sci._, 1875 and 1876.
+
+[133] E.D. Cope, "The Creodonta," _Amer. Natural._, 1884. W.B. Scott,
+"Revision of the N. American Creodonta," _P. Ac. Philad._, 1892.
+
+[134] See next paragraph.
+
+[135] St G. Mivart, _The Cat_, London, 1881.
+
+[136] St G. Mivart, _P.Z.S._, 1885.
+
+[137] St G. Mivart, "On the Osteology of Insectivora," _J. Anat.
+Physiol. norm. path._, 1867 and 1868, and _P.Z.S._, 1871. G.E. Dobson,
+_Monograph of the Insectivora_, London, 1882--90.
+
+[138] See G.E. Dobson, _Brit. Mus. Catalogue of Chiroptera_, 1878. See
+also other papers by the same author and by Oldfield Thomas.
+
+
+
+
+CHAPTER XXI.
+
+THE SKELETON OF THE DOG[139] (_Canis familiaris_).
+
+
+I. EXOSKELETON.
+
+The exoskeleton of the dog includes three sets of structures: 1.
+hairs, 2. claws, 3. teeth. =Hairs= and =claws= are epidermal
+exoskeletal structures, while =teeth= are partly of dermal, and partly
+of epidermal origin.
+
+1. =Hairs= are delicate epidermal structures which grow imbedded in
+little pits or follicles in the dermis. Specially large hairs forming
+the =vibrissae= or =whiskers= grow attached to the upper lip.
+
+2. =Claws= are horny epidermal sheaths, one of which fits on to the
+pointed distal phalanx of each digit. They are sharply curved
+structures, and being in the dog non-retractile, their points are
+commonly much blunted by friction with the ground. The claws of the
+pollex, and of the hallux when it is present, however do not meet the
+ground, and therefore remain comparatively sharp.
+
+3. =Teeth=[140]. Although as regards their mode of origin, teeth are
+purely exoskeletal or tegumentary structures, they become so
+intimately connected with the skull that they appear to belong to the
+endoskeleton.
+
+Each tooth, as has been already described, consists of three distinct
+tissues, dentine and cement of dermal origin, and enamel of epidermal
+origin.
+
+[Illustration FIG. 68. DENTITION OF A DOG (_Canis familiaris_) × 1/2.
+(Camb. Mus.)
+
+ _i_ 2. second incisor.
+ _c._ canine.
+ _pm_ 1, _pm_ 4. first and fourth premolars.
+ _m_ 1. first molar.]
+
+The teeth of the dog (fig. 68) form a regular series arranged along
+the margins of both upper and lower jaws, and imbedded in pits or
+=alveoli= of the maxillae, premaxillae, and mandibles. They are all
+fixed in the bone by tapering roots, and none of them grow from
+persistent pulps.
+
+They are divisible into four distinct groups, the =incisors=,
+=canines=, =premolars= and =molars=. There are three incisors, one
+canine and four premolars on each side of each jaw. But while there
+are three molars on each side of the lower jaw, the last is wanting in
+the upper jaw. The dentition of the dog may then be represented by the
+formula
+
+ _i_ 3/3 _c_ 1/1 _pm_ 4/4 _m_ 2/3 × 2 = 42.
+
+In each jaw there is one large specially modified tooth called the
+=carnassial=, the teeth in front of this are more or less pointed and
+compressed, while those behind it are more or less flattened and
+tuberculated.
+
+=Teeth of the upper jaw.=
+
+The first and second =incisors= are small teeth with long conical
+roots and somewhat chisel-shaped crowns. Surrounding the base of the
+crown there is a rather prominent ridge, terminated laterally by a
+pair of small cusps. This ridge, the =cingulum=, serves to protect the
+edge of the gums from injury by the hard parts of food. The third
+incisor is a good deal like the others but larger, and has the
+cingulum well developed though not terminated by lateral cusps. All
+the incisors are borne by the premaxillae, the remaining teeth by the
+maxillae.
+
+The =canine= is a large pointed tooth, slightly recurved and with a
+long tapering root.
+
+The =premolars= are four in number, and in all the cingulum is fairly
+well seen. The first is a very small tooth with a single tapering
+root, the second and third are larger and have two roots, while the
+fourth, the =carnassial=, is much the largest and has three roots.
+Each of the second, third and fourth premolars has a stout blade, the
+middle portion of which is drawn out into a prominent cone; the
+posterior part of the fourth premolar forms a compressed ridge, and
+at the antero-internal edge of the tooth there is a small inner
+tubercle.
+
+The two =molar= teeth are of very unequal size. The first, which has
+two anterior roots and one posterior, is wider than it is long, its
+outer portion being produced into two prominent cusps, while its inner
+portion is depressed. The second molar is a small tooth resembling the
+first in its general appearance, but with much smaller outer cusps.
+
+=Teeth of the lower jaw.=
+
+The three =incisors= of the lower jaw have much the same character as
+the first two of the upper jaw; while the =canine= is identical in
+character with that of the upper jaw.
+
+The four =premolars= gradually increase in size from the first to the
+last, but none are very large. The first premolar is a single-rooted
+tooth resembling that of the upper jaw; the second, third and fourth
+are two-rooted, like the second and third of the upper jaw, which they
+closely resemble in other respects.
+
+The first =molar= forms the =carnassial= (fig. 84, V), and with the
+exception of the canine, is much the largest tooth of the lower jaw;
+it is a two-rooted tooth, with a long compressed bilobed blade, and a
+posterior tuberculated talon or heel. The second molar is much
+smaller, though likewise two-rooted, while the third molar is very
+small and has only a single root. All the teeth except the molars are
+preceded in the young animal by temporary =milk teeth=. These milk
+teeth, though smaller, are very similar to the permanent teeth by
+which they are ultimately replaced.
+
+
+II. ENDOSKELETON.
+
+1. THE AXIAL SKELETON.
+
+This includes the vertebral column, the skull, and the ribs and
+sternum.
+
+
+A. THE VERTEBRAL COLUMN.
+
+This consists of a series of about forty vertebrae arranged in
+succession so that their centra form a continuous rod, and their
+neural arches a continuous tube, surrounding a cavity, the =neural
+canal=.
+
+The vertebrae may be readily divided into five groups:--
+
+1. The =cervical= or neck vertebrae.
+
+2. The =thoracic= or chest vertebrae which bear ribs.
+
+3. The =lumbar= vertebrae which are large and ribless.
+
+4. The =sacral= vertebrae which are fused with one another and united
+with the pelvis.
+
+5. The =caudal= or tail vertebrae which are small.
+
+Except in the sacral region the vertebrae are movably articulated to
+one another, while their centra are separated from one another by
+cartilaginous =intervertebral discs=.
+
+
+GENERAL CHARACTERS OF A VERTEBRA.
+
+Take as a type the =fourth lumbar vertebra=. It may be compared to a
+short tube whose inner surface is smooth and regular, and whose outer
+surface is thickened and drawn out in a variety of ways. The basal
+part of the vertebra is the =centrum= or body which forms the
+thickened floor of the neural canal. Its two ends are slightly convex
+and are formed by the =epiphyses=, two thin plates of bone which are
+at first altogether distinct from the main part of the centrum, but
+fuse with it as the animal grows older; its sides are drawn out into a
+pair of strong =transverse processes=, which project forwards,
+outwards, and slightly downwards. The =neural arch= forms the sides
+and roof of the neural canal, and at each end just above the centrum
+bears a pair of =intervertebral notches= for the passage of the spinal
+nerves, the posterior notches being considerably deeper than the
+anterior. The neural arch is drawn out into a series of processes.
+Arising from the centre of the dorsal surface is a prominent median
+=neural spine= or =spinous process=, which projects upwards and
+slightly forwards; its anterior edge is vertical, while its posterior
+edge slopes gradually. At the two ends of the neural arch arise the
+two pairs of =zygapophyses= or articulating surfaces, which
+interlock with those of the adjacent vertebrae. The anterior or
+=prezygapophyses= look inwards, and are large and concave; they are
+borne upon a pair of large blunt outgrowths of the neural arch, the
+=metapophyses=. The posterior or =postzygapophyses= are slightly
+convex and look outwards and downwards; they are borne upon backwardly
+projecting outgrowths of the neural arch. Lastly there are a pair of
+minute projections arising from the posterior end of the neural arch,
+below the postzygapophyses. These are the =anapophyses=. In young
+individuals the development of all the processes of the various
+vertebrae is less marked, and the epiphyses are obviously distinct.
+
+[Illustration FIG. 69. A, ATLAS AND B, AXIS VERTEBRA OF A DOG (_Canis
+familiaris_) (after VON ZITTEL).
+
+ 1. transverse process of atlas.
+ 2. vertebrarterial canal.
+ 3. foramen for exit of spinal nerve.
+ 4. neural spine.
+ 5. odontoid process.
+ 6. anterior articulating surface of centrum.
+ 7. centrum.
+ 8. transverse process of axis.
+ 9. postzygapophysis.]
+
+
+THE CERVICAL VERTEBRAE.
+
+These are seven in number, as in almost all mammals. They are
+characterised by the fact that they have small ribs fused with them,
+forming transverse processes perforated by canals through which the
+vertebral arteries run.
+
+The first, or =atlas= vertebra (fig. 69, A), differs much from all the
+others; it is drawn out into a pair of wide wing-like transverse
+processes (fig. 69, A, 1), and forms a ring surrounding a large
+cavity. This cavity is during life divided into two parts by a
+transverse ligament; the upper cavity is the true neural canal, while
+the lower lodges the =odontoid process= of the second vertebra, which
+is the detached centrum of the atlas. The neural arch is broad and
+regular; it has no spinous process, and is perforated in front by a
+pair of foramina for the passage of the first spinal nerves. The
+mid-ventral portion of the atlas is rather thick, and bears a minute
+backwardly-projecting hypapophysis. The bases of the broad transverse
+processes are perforated by the =vertebrarterial canals= (fig. 69, A,
+2). The atlas bears at each end a pair of large articulating surfaces;
+those at the anterior end articulate with the condyles of the skull,
+and are very deeply concave; those at the posterior end for
+articulation with the axis, are nearly as large, but are flattened.
+The atlas ossifies from three centres, one forming the mid-ventral
+portion, the others the two halves of the remainder.
+
+The second, or =axis= vertebra (fig. 69, B), also differs much from
+the other cervicals. The long and broad centrum has a very flat dorsal
+surface, and is produced in front into the conical =odontoid process=
+(fig. 69, B, 5), and bears a pair of very large convex outwardly
+directed surfaces for articulation with the atlas. At its posterior
+end it is drawn out into a pair of small backwardly-directed spines,
+the transverse processes; these are perforated at their bases by the
+vertebrarterial canals. The neural arch is deeply notched in front and
+behind for the passage of the spinal nerves, and is drawn out above
+into a very long compressed neural spine (fig. 69, B, 4), which
+projects a long way forwards, and behind becomes bifid and thickened,
+bearing a pair of flat downwardly directed postzygapophyses. In the
+young animal the odontoid process is readily seen to ossify from a
+centre anterior to that forming the anterior epiphysis of the axis.
+
+The remaining five cervical vertebrae, the third to the seventh
+inclusive, have rather flattened wide centra, obliquely truncated at
+either end. The neural spine progressively increases in size as the
+vertebrae are followed back. The transverse processes vary
+considerably; those of the third are divided into a thicker
+backwardly-, and a more slender forwardly-projecting portion; those of
+the fourth and fifth mainly extend downwards, and that of the sixth is
+divided into a horizontal portion and a downwardly-projecting
+=inferior lamella=. All the cervical vertebrae except the seventh have
+the bases of the transverse processes perforated by the
+vertebrarterial canals. The prezygapophyses in each case look upwards
+and slightly inwards, while the postzygapophyses look downwards and
+slightly outwards.
+
+
+THE THORACIC VERTEBRAE.
+
+The =thoracic vertebrae= are twelve or thirteen in number, and all
+bear movably articulated ribs. As a group they are characterised by
+their comparative shortness, and in the case of the first eight or
+nine by the great length of the backwardly-sloping neural spine. The
+posterior thoracic vertebrae approach in character the succeeding
+lumbar vertebrae.
+
+As type of the anterior thoracic vertebrae, take any one between the
+second and sixth inclusive. The centrum is short, and has its
+terminations vertically truncated. At the top of the centrum, at both
+anterior and posterior ends on each side, is a demi-facet (fig. 70, A,
+4), which, together with that on the adjacent vertebra, forms an
+articulating surface for the capitulum of the rib. The neural arch is
+small and deeply notched behind for the passage of the spinal nerve.
+It is drawn out above into a very long neural spine (fig. 70, A, 1),
+whose base extends back over the succeeding vertebra and bears the
+downwardly-directed postzygapophyses (fig. 70, A, 6). The summit of
+the neural arch is deeply notched in front, and on each side of the
+notch are the prezygapophyses, which look almost vertically upwards.
+The transverse processes are short and blunt, and are flattened below
+(fig. 70, A, 3) for the articulation of the tubercula of the ribs.
+
+[Illustration FIG. 70. A, SECOND THORACIC, AND B, SECOND LUMBAR
+VERTEBRA OF A DOG (_Canis familiaris_) SEEN FROM THE RIGHT SIDE (after
+VON ZITTEL).
+
+ 1. neural spine.
+ 2. centrum.
+ 3. transverse process bearing in A the facet for articulation with
+ the tuberculum of the rib.
+ 4. facet for articulation with the capitulum of the rib.
+ 5. metapophysis.
+ 6. postzygapophysis.]
+
+The posterior three or four thoracic vertebrae differ much from the
+others. The centra are longer, the neural spines short and not
+directed backwards, the articular facets for the heads of the ribs are
+confined to the anterior end of the centrum of each vertebra, not
+overlapping on to the preceding vertebra. The transverse processes are
+small and irregular, and metapophyses and anapophyses are developed.
+The prezygapophyses also look more inwards, and the postzygapophyses
+more outwards than in the more typical thoracic vertebrae.
+
+
+THE LUMBAR VERTEBRAE.
+
+The =lumbar vertebrae= are seven in number, and their general
+characteristics have been already described. As a group they are
+characterised by their large size, and the great development of the
+transverse processes, metapophyses and neural spines.
+
+
+THE SACRAL VERTEBRAE.
+
+Three vertebrae are commonly found fused together, forming the
+=sacrum=; the divisions between the three being indicated by the
+foramina for the exit of the spinal nerves.
+
+Of these three vertebrae, the first is much the largest, and is firmly
+united to the ilium on each side by a structure formed by the
+transverse processes and expanded ribs. In the adult this structure
+forms one continuous mass, but in the young animal a ventral portion
+formed by the rib is clearly distinguishable from a dorsal portion
+formed by the transverse process. All three have low neural spines.
+The anterior sacral vertebra bears a large pair of prezygapophyses,
+while the posterior one bears a small pair of postzygapophyses.
+
+
+THE CAUDAL VERTEBRAE.
+
+The =caudal vertebrae= are about nineteen in number. The earlier ones
+have well-developed neural arches, transverse processes, and
+zygapophyses, but as the vertebrae are followed back they gradually
+lose all their processes, and the neural arch as well, becoming at
+about the thirteenth from the end reduced to simple cylindrical
+centra.
+
+
+B. THE SKULL.
+
+The =skull= consists of the following three parts: (_a_) the cranium,
+with which are included the skeletal supports of the various special
+sense organs, and the bones of the face and upper jaw; (_b_) the lower
+jaw or mandible, which is movably articulated to the cranium, and
+(_c_) the hyoid.
+
+
+(_a_) THE CRANIUM.
+
+The cranium is a compact bony box, forming the anterior expanded
+portion of the axial skeleton. It has a longitudinal axis, the
+=craniofacial= axis around which the various parts are arranged, and
+this axis is a direct continuation of that of the vertebral column.
+Similarly the cavity of the cranium is a direct continuation of the
+spinal canal. The posterior part of the craniofacial axis, which has
+relations only with the cranium, is called the =basicranial axis=.
+
+In the dog as in the other types previously described, the skull in
+its earliest stages is cartilaginous, containing no bone. In the
+adult, however, the cartilage is to a great extent replaced by bone,
+and in addition to this cartilage bone, membrane bone is largely
+developed, and intimately united with the cartilage bone to form one
+complete whole.
+
+In the description of the dog's skeleton, as in those of the previous
+types, the names of the membrane bones are printed in italics, while
+those of the cartilage bones are printed in thick type.
+
+Most of the numerous foramina perforating the skull walls will be
+described after the bones have been dealt with.
+
+For purposes of description the cranium may be further subdivided
+into:--
+
+1. The cranium proper or brain case.
+
+2. The sense capsules.
+
+3. The upper jaw.
+
+
+1. THE CRANIUM PROPER OR BRAIN CASE.
+
+Taking the membrane and cartilage bones together, they are seen to be
+more or less arranged in three segments, which however must not be
+regarded as homologous with the segments forming the vertebral column.
+
+[Illustration FIG. 71. DIAGRAM OF THE RELATIONS OF THE PRINCIPAL BONES
+IN THE MAMMALIAN SKULL (modified after FLOWER).
+
+Cartilage is dotted. Cartilage bones are marked by dots and dashes,
+membrane bones are left white.
+
+ 1. basi-occipital.
+ 2. exoccipital.
+ 3. supra-occipital.
+ 4. basisphenoid.
+ 5. alisphenoid.
+ 6. parietal.
+ 7. presphenoid.
+ 8. orbitosphenoid.
+ 9. frontal.
+ 10. periotic, immediately below which is the tympanic.
+ 11. lachrymal.
+ 12. ethmo-turbinal.
+ 13. maxillo-turbinal.
+ 14. nasal.
+ 15. mesethmoid.
+ 16. vomer.
+ 17. pterygoid.
+ 18. palatine.
+ 19. maxillae.
+ 20. premaxillae.
+ 21. squamosal.
+ 22. mandible.
+ 23. tympano-hyal.
+ 24. stylo-hyal.
+ 25. epi-hyal.
+ 26. basi-hyal. Between this and the epi-hyal is the cerato-hyal.
+ 27. thyro-hyal.
+ 28. jugal.
+
+Nerve exits are indicated by Roman numerals.]
+
+The =occipital segment= is the most posterior of the three, and
+consists of four cartilage bones, which in the adult are commonly
+completely fused together. They surround the great =foramen magnum=
+(fig. 75, 2) through which the brain and spinal cord communicate.
+Forming the lower margin of the foramen magnum is a large flat
+unpaired bone, the =basi-occipital= (fig. 75, 5). Above this on each
+side are the =exoccipitals=, whose sides are drawn out into a pair of
+downwardly-directed =paroccipital processes=, which are applied to the
+tympanic bullae[141]. The inner side of each exoccipital is converted
+into the large rounded =occipital condyle= (fig. 72, 13) by which the
+skull articulates with the atlas vertebra. The dorsal boundary of the
+foramen magnum is formed by a large unpaired flat bone, the
+=supra-occipital= (figs. 72 and 75, 1), which is continuous with a
+small bone, the _interparietal_, prolonged forwards between the
+parietal bones of the next segment.
+
+In old animals the interparietal forms the hind part of a prominent
+ridge running along the mid-dorsal surface of the skull and called the
+=sagittal crest=, while the junction line of the occipital and
+parietal segments forms a prominent =occipital crest=.
+
+The plane in which the bones of the occipital segment lie is called
+the occipital plane; the angle that it makes with the basicranial axis
+varies much in different mammals.
+
+The =parietal segment= consists of both cartilage and membrane bones.
+It is formed of five bones, which are in contact with those of the
+occipital segment on the dorsal and ventral surfaces, while laterally
+they are separated by the interposition of the auditory bones, and to
+some extent of the squamosal. The =basisphenoid= (fig. 75, 6), an
+unpaired bone forming the ventral member of this segment, is the
+direct continuation of the basi-occipital. It tapers anteriorly, but
+is rather deep vertically, its upper or dorsal surface bearing a
+depression, the =sella turcica=, which lodges the pituitary body of
+the brain. From the sides of the basisphenoid arise the =alisphenoids=
+(fig. 75, 11) a pair of bones of irregular shape generally described
+as wing-like; each gives off from its lower surface a =pterygoid
+plate=, which is united in front with the palatine, and below with the
+pterygoid. The alisphenoids are united above with a pair of large
+nearly square bones, the _parietals_ (fig. 73, 2), which meet one
+another in the mid-dorsal line. The line of junction is frequently
+drawn out into a strong ridge, which forms the anterior part of the
+=sagittal crest=.
+
+[Illustration FIG. 72. VERTICAL LONGITUDINAL SECTION TAKEN A LITTLE TO
+THE LEFT OF THE MIDDLE LINE THROUGH THE SKULL OF A DOG (_Canis
+familiaris_) × 3/5. (Camb. Mus.)
+
+ 1. supra-occipital.
+ 2. interparietal.
+ 3. parietal.
+ 4. frontal.
+ 5. cribriform plate.
+ 6. nasal.
+ 7. mesethmoid.
+ 8. maxillae.
+ 9. vomer.
+ 10. ethmo-turbinal.
+ 11. maxillo-turbinal.
+ 12. premaxillae.
+ 13. occipital condyle.
+ 14. basi-occipital.
+ 15. tympanic bulla.
+ 16. basisphenoid.
+ 17. pterygoid.
+ 18. palatine.
+ 19. alisphenoid.
+ 20. internal auditory meatus.
+ 21. tentorium.
+ 22. foramen lacerum posterius.
+ 23. floccular fossa.
+ 24. coronoid process.
+ 25. condyle.
+ 26. angle.
+ 27. mandibular symphysis.
+ 28. inferior dental foramen.
+ 29. stylo-hyal.
+ 30. epi-hyal.
+ 31. cerato-hyal.
+ 32. basi-hyal.
+ 33. thyro-hyal.
+ XII. condylar foramen.]
+
+The =frontal segment=, which surrounds the anterior part of the brain,
+is closely connected along almost its whole posterior border with the
+parietal segment.
+
+Its base is formed by the =presphenoid= (fig. 75, 12), a very deep
+unpaired bone, narrow and compressed ventrally, and with an irregular
+dorsal surface. The presphenoid is continuous with a second pair of
+wing-like bones, the =orbitosphenoids=. Each orbitosphenoid meets the
+alisphenoid behind, but the relations of the parts in this region are
+somewhat obscured by a number of large foramina piercing the bones,
+and also by an irregular vacuity, the =foramen lacerum anterius= or
+=sphenoidal fissure=, which lies between the orbitosphenoid and
+alisphenoid, separating the lateral parts of the parietal and frontal
+segments, in the same way as the space occupied by the auditory bones
+separates the lateral parts of the occipital and parietal segments.
+The orbitosphenoids pass obliquely forwards and upwards, and are
+united above with a second pair of large membrane bones, the
+_frontals_ (fig. 73, 3). The outer side of each frontal is drawn out
+into a rather prominent rounded =postorbital process= (fig. 73, 10),
+from which a ridge converges backwards to meet the sagittal crest. The
+anterior part of the frontal is produced to form the long nasal
+process, which is wedged in between the nasal and maxillae.
+
+[Illustration FIG. 73. DORSAL VIEW OF THE CRANIUM OF A DOG (_Canis
+familiaris_) × 2/3.
+
+ 1. supra-occipital.
+ 2. parietal.
+ 3. frontal.
+ 4. nasal.
+ 5. maxillae (facial portion).
+ 6. premaxillae.
+ 7. squamosal.
+ 8. jugal.
+ 10. postorbital process of frontal.
+ 11. infra-orbital foramen.
+ 12. anterior palatine foramen.
+ 13. lachrymal foramen.
+ _i_ 1. first incisor.
+ _c._ canine.
+ _pm_ 4. fourth premolar.]
+
+The cranial cavity is continuous in front with the =nasal= or
+=olfactory cavities=, but the passage is partially closed by a screen
+of bone, the =cribriform plate= (fig. 72, 5), which is placed
+obliquely across the anterior end of the cranial cavity, and is
+perforated by a number of holes through which the olfactory nerves
+pass. The plane of the cribriform plate is called the =ethmoidal
+plane=, and as was the case also with the occipital plane, the angle
+that it makes with the basicranial axis varies much in different
+mammals, and is of importance. The =olfactory fossa= in which lie the
+olfactory lobes of the brain, is partially separated from the
+=cerebral fossa=, or cavity occupied by the cerebral hemispheres, by
+ridges on the orbitosphenoids and frontals. The presphenoid is
+connected in front with a vertical plate formed partly of bone, partly
+of unossified cartilage; this plate, the =mesethmoid= (fig. 72, 7),
+separates the two olfactory cavities which lodge the olfactory organs.
+Its anterior end always remains unossified, and forms the septal
+cartilage of the nose.
+
+The brain case may then, to use the words of Sir W.H. Flower, be
+described as a tube dilated in the middle and composed of three bony
+rings or segments, with an aperture at each end, and a fissure or
+space at the sides between each of them.
+
+
+2. THE SENSE CAPSULES.
+
+Each of the three special sense organs, of hearing, of sight, and of
+smell, is in the embryo provided with a cartilaginous or membranous
+protecting capsule; and two of these, the auditory and olfactory
+capsules, become afterwards more or less ossified, and intimately
+related to the cranium proper.
+
+
+(1) =Bones in relation to the Auditory capsules.=
+
+These bones lie on each side wedged into the vacuity between the
+lateral parts of the occipital and parietal segments; they are three
+in number, the =periotic=, the _tympanic_ and the _squamosal_.
+
+The =periotic= is the most important of them, as it replaces the
+cartilaginous auditory capsule of the embryo, and encloses the
+essential organ of hearing. It commences to ossify from three centres
+corresponding to the pro-otic, epi-otic and opisthotic of lower
+skulls, such as those of the Turtle and Crocodile.
+
+These ossifications however very early combine to form a single bone,
+the =periotic=, which nevertheless consists of two portions, the
+=petrous= and the =mastoid=, differing considerably from one another.
+
+[Illustration FIG. 74. DIAGRAM OF THE MAMMALIAN TYMPANIC CAVITY AND
+ASSOCIATED PARTS (modified from LLOYD MORGAN).
+
+ 1. external auditory meatus.
+ 2. tympanic membrane.
+ 3. malleus.
+ 4. incus.
+ 5. lenticular.
+ 6. stapes.
+ 7. fenestra ovalis.
+ 8. fenestra rotunda.
+ 9. Eustachian tube.
+ 10. cavity occupied by the cochlea.
+ 11. cavity occupied by the membranous labyrinth.]
+
+The =petrous portion= lies dorsally and anteriorly, and is much the
+more important of the two, as it encloses the essential part of the
+auditory organ. It forms an irregular mass of hard dense bone,
+projecting into the cranial cavity, and does not appear on the
+external surface at all. The =mastoid portion= lies ventrally and
+posteriorly, is smaller, and formed of less dense bone than is the
+petrous portion, from which it differs also in the fact that it
+appears on the surface of the skull, just external to the exoccipital.
+The petrous portion bears a ridge, which together with a ridge on the
+supra-occipital, and the =tentorium= (fig. 72, 21), a transverse fold
+of the dura mater[142], separates the large cerebral fossa from the
+=cerebellar fossa=, which is much smaller than the cerebral fossa and
+lies behind and partly beneath it. The plane of the tentorium is
+called the =tentorial plane=, and the angles that it makes with the
+basicranial axis and with the occipital and ethmoidal planes vary much
+in different mammals.
+
+The periotic has its inner surface marked by important depressions,
+while both inner and outer surfaces are pierced by foramina. At about
+the middle of its inner surface are seen two deep pits, one lying
+immediately above the other. Of these the more ventral is a foramen,
+the =internal auditory meatus= (fig. 72, 20), through which the VIIth
+(facial) and VIIIth (auditory) nerves leave the cranial cavity, the
+facial nerve passing through the bone and afterwards leaving the skull
+by the =stylomastoid foramen= (fig. 75, VII), while the auditory
+passes to the inner ear. The more dorsal of the two pits is not a
+foramen but the =floccular fossa= (fig. 72, 23) which lodges the
+floccular lobe of the cerebellum. In some skulls another wide and
+shallow but fairly prominent depression is seen dorsal to and slightly
+behind the floccular fossa, this also lodges part of the cerebellum.
+Behind the internal auditory meatus, between the periotic and
+exoccipital is seen the internal opening of the =foramen lacerum
+posterius= (fig. 72, 22). The shape of this opening varies. The
+ventro-anterior border of the periotic is marked by a deep notch, the
+sides of which sometimes unite, converting it into a foramen.
+
+On the outer side of the periotic, and clearly seen only after the
+removal of the tympanic, are two holes, the =fenestra ovalis= and the
+=fenestra rotunda=.
+
+The _tympanic_ (figs. 72, 15 and 75, 4) is a greatly expanded
+boat-shaped bone, which forms the auditory bulla and lies immediately
+ventral to the periotic; it is separated from the periotic by the
+=tympanic cavity= into which the fenestra rotunda and the fenestra
+ovalis open.
+
+There are several other openings into the tympanic cavity.
+
+(_a_) On the external surface is a large oval opening, the =external
+auditory meatus= bounded by a thickened rim.
+
+(_b_) Into the outer and anterior part of the cavity the outer end of
+the =Eustachian tube= opens; while the inner end passes through a
+foramen (fig. 75, 22) just external to the foramen lacerum medium, on
+its way to open into the pharynx.
+
+(_c_) The internal carotid artery also enters the tympanic cavity by a
+canal which commences in the foramen lacerum posterius, and passes
+forwards to open on the inner side of the bulla. The artery then
+passes forwards, and barely appearing on the ventral surface of the
+cranium, enters the brain cavity through the foramen lacerum medium
+(fig. 75, 9).
+
+Immediately behind the tympanic, between it and the mastoid process of
+the periotic and the paroccipital process of the exoccipital is the
+=stylomastoid foramen= (fig. 75, VIII).
+
+Within the tympanic cavity are four small bones, the =auditory
+ossicles= (cp. fig. 74), called respectively the =malleus=, =incus=,
+=lenticular= and =stapes=; these together form a chain extending from
+the fenestra ovalis to the tympanic membrane.
+
+The =malleus= has a somewhat rounded head (fig. 100, B, 1) which
+articulates with the incus, while the other end of the bone is drawn
+out into a long process, the =manubrium=, which lies in relation to
+the tympanic membrane. The head is also more or less connected by a
+thin plate of bone, the =lamella=, to another outgrowth, the
+=processus longus=. The =incus= (fig. 100, B, 3) is somewhat
+anvil-shaped, and is drawn out into a process which is connected with
+the =lenticular=, a nodule of bone interposed between the incus and
+the stapes, with which it early becomes united. The =stapes= (fig.
+100, B, 2) is stirrup-shaped, consisting of a basal portion from which
+arise two =crura=, which meet and enclose a space, the =canal=.
+
+The _squamosal_ (fig. 73, 7) is a large bone occupying much of the
+side wall of the cranial cavity, and articulating above with the
+parietal, and behind with the supra-occipital, while in front it
+overlaps the frontal and alisphenoid. But though it occupies so large
+a space on the outer wall, it forms very little of the internal wall
+of the skull, but is really like a bony plate attached to the outer
+surface of the cranial wall. The squamosal is drawn out into a strong
+forwardly-directed =zygomatic process= which meets the jugal or malar.
+The ventral side of the zygomatic process is hollowed out, forming the
+=glenoid fossa= (fig. 75, 8), a smooth laterally elongated surface
+with which the lower jaw articulates, while the hinder edge of the
+glenoid fossa is drawn out into a rounded =postglenoid process= (fig.
+75, 23). The articulation is such as to allow but little lateral play
+of the lower jaw.
+
+
+(2) =Bones in relation to the Optic capsules.=
+
+The only bone developed in relation to the optic capsule on each side
+is the _lachrymal_. This is a small membrane bone lying between the
+frontal and palatine behind, and the maxillae and jugal in front. It is
+perforated by a prominent =lachrymal foramen= (fig. 73, 13) which
+opens within the orbit.
+
+
+(3) =Bones in relation to the Olfactory capsules.=
+
+In connection with the =olfactory capsules=, five pairs of bones are
+developed, two pairs being membrane bones, and three pairs cartilage
+bones.
+
+Of membrane bones, the _nasals_ (fig. 73, 4) are a pair of long narrow
+bones, lying closely side by side, and forming the main part of the
+roof of the olfactory chamber. Their posterior ends overlap the
+frontals, and the outer margin of each is in contact with the nasal
+process of the frontal, and with the maxillae and premaxillae.
+
+Lying immediately ventral to the nasals, and on each side of the
+perpendicular mesethmoid, are the =ethmoid= or =turbinal= bones,
+which have a curious character, being formed of a number of delicate
+plates intimately folded on one another. The posterior pair of these
+bones, the =ethmo-turbinals= (fig. 72, 10), are the larger, and form a
+mass of intricately folded lamellae attached behind to the cribriform
+plate, and passing laterally into two thin plates of bone, which abut
+on the maxillae. The uppermost lamella of each ethmo-turbinal is larger
+than the others and more distinct. It is sometimes distinguished as
+the =naso-turbinal=, and forms an imperfect lower boundary to a canal,
+which is bounded above by the nasals. In front of and somewhat below
+the ethmo-turbinals, lie another pair of bones of similar character,
+the =maxillo-turbinals= (fig. 72, 11).
+
+The last bone to be mentioned in connection with the olfactory
+capsules is a membrane bone, the _vomer_ (fig. 72, 9). This is a
+slender vertically-placed bone, whose anterior part lies between the
+maxillo-turbinals, while behind it extends beyond the mesethmoid, so
+as to underlie the anterior part of the presphenoid. The anterior part
+of the vomer forms a kind of trough, while further back in the region
+of the ethmo-turbinals it sends out a pair of strong lateral plates,
+each of which, passing below the ethmo-turbinal, joins the side wall
+of the nasal cavity, and forms a partition dividing the nasal cavity
+into a lower =narial passage= and an upper =olfactory chamber=.
+
+
+THE JAWS.
+
+In the embryo both upper and lower jaws are formed of cartilaginous
+bars, but in the adult not only has the cartilage entirely
+disappeared, but even cartilage bone is absent, the jaws being formed
+of membrane bone.
+
+
+3. THE UPPER JAW.
+
+[Illustration FIG. 75. VENTRAL VIEW OF THE CRANIUM OF A DOG (_Canis
+familiaris_) × 3/5. (Camb. Mus.)
+
+ 1. supra-occipital.
+ 2. foramen magnum.
+ 3. occipital condyle.
+ 4. tympanic bulla.
+ 5. basi-occipital.
+ 6. basisphenoid.
+ 7. external auditory meatus.
+ 8. glenoid fossa.
+ 9. foramen lacerum medium and anterior opening of carotid canal.
+ 10. postglenoid foramen.
+ 11. alisphenoid.
+ 12. presphenoid.
+ 13. vomer.
+ 14. jugal.
+ 15. pterygoid.
+ 16. palatal process of palatine.
+ 17. maxillae (palatal portion).
+ 18. posterior palatine foramina.
+ 19. anterior palatine foramen.
+ 20. premaxillae.
+ 21. alisphenoid canal.
+ 22. Eustachian foramen.
+ 23. postglenoid process of squamosal.
+ II. optic foramen.
+ III, IV, V{1}, VI. foramen lacerum anterius.
+ V{2}. foramen rotundum.
+ V{3}. foramen ovale.
+ VII. stylomastoid foramen.
+ IX, X, XI. foramen lacerum posterius.
+ XII. condylar foramen.
+ _i_ 2. second incisor.
+ _c._ canine.
+ _pm_ 1, _pm_ 4. first and fourth premolars.
+ _m_ 1. first molar.]
+
+The bones of the upper jaw are closely connected with those of the
+cranium proper and olfactory capsules. The most posterior of them is
+the _pterygoid_ (fig. 75, 15), a thin vertically placed plate of bone,
+which articulates above with the basisphenoid, the presphenoid, and
+the strong pterygoid process of the alisphenoid. The ventral end of
+the pterygoid is drawn out into a small backwardly-projecting =hamular
+process=. In front the pterygoid articulates with the _palatine_, a
+much larger bone, consisting of (1) a vertical portion, which passes
+up to meet the orbitosphenoid and frontal, and sends inwards a plate
+which meets the presphenoid and vomer, forming much of the roof of the
+posterior part of the narial passage; and (2) a strong horizontal
+portion, the =palatal process= (fig. 75, 16), which passes inwards and
+meets its fellow in the middle line, forming the posterior part of the
+bridge of bone supporting the hard palate. The palatal process is
+continuous in front, with a large bone, the _maxillae_, which, like
+the palatine, consists of vertical and horizontal portions. The
+vertical, or =facial portion= (fig. 73, 5), is the largest, and
+constitutes the main part of the side of the face in front of the
+orbit, forming also the chief part of the outer wall of the nasal
+cavity. It is continuous in front with the premaxillae, above with the
+nasal and frontal, and behind with the lachrymal, jugal, and palatine.
+The horizontal, or =palatal portion= (fig. 75, 17), forms the anterior
+part of the bony plate supporting the hard palate, and meets its
+fellow in a long straight symphysis. The junction line between the
+palatal and facial portions is called the =alveolar border=, and
+along it are attached the canine, premolar, and molar teeth.
+
+The anterior part of the upper jaw on each side is formed by a small
+bone, the _premaxillae_, which bears the incisor teeth. It, like the
+maxillae, has a palatal portion (fig. 75, 20), which meets its fellow
+in the middle line, and an ascending portion, which passes backwards
+as the =nasal process=, tapering regularly and lying between the nasal
+and the maxillae. The two premaxillae form the outer and lower borders
+of the anterior nares. The last bone to be mentioned in connection
+with the upper jaw and face is the _jugal_ or _malar_ (figs. 73, 8,
+and 75, 14), a strong bone which forms the anterior half of the
+zygomatic arch. It is firmly united in front to the maxillae, and
+behind meets the zygomatic process of the squamosal, being drawn out
+dorsally into a short =postorbital process= at the point of meeting.
+This process lies immediately below the postorbital process of the
+frontal, and if the two met, as they do in some mammals, they would
+partially shut off the orbit from a larger posterior cavity, the
+=temporal fossa=. In the living animal a ligament unites the two
+postorbital processes.
+
+
+(_b_) THE LOWER JAW OR MANDIBLE.
+
+This consists of two elongated symmetrical halves, the =rami=, which
+are united to one another at the median symphysis in front, while
+behind they diverge considerably, and each articulates with the
+glenoid surface of the corresponding squamosal. In young animals the
+rami are united at the symphysis by fibrous tissue, but in old animals
+they sometimes become fused together. The upper or alveolar border
+bears the teeth, and behind them is drawn out into a high laterally
+compressed =coronoid process= (fig. 72, 24), which is hollowed on its
+outer surface. Immediately behind the coronoid process is the
+transversely elongated =condyle= (fig. 72, 25), which fits into the
+glenoid cavity in such a way as to allow free up and down movement of
+the jaw, with but little rolling motion. The posterior end of the jaw
+below the condyle forms a short rounded process, the =angle= (fig.
+72, 26). Two prominent foramina are to be seen in the lower jaw. These
+are firstly the =inferior dental foramen= (fig. 72, 28), which lies on
+the inner surface below the coronoid process; through it an artery and
+a branch of the fifth nerve enter to supply the teeth, and secondly
+the =mental foramen=, which lies on the outer side near the anterior
+end, and through which a branch of the same nerve emerges.
+
+
+(_c_) THE HYOID.
+
+The =Hyoid= of the dog consists of a transverse median piece, the
+=basi-hyal=[1] (fig. 72, 32), from which arise two pairs of =cornua=.
+The =anterior cornu= is much the longer of the two, and consists
+principally of three short separate ossifications, placed end to end
+and called respectively the =cerato-hyal=[143], =epi-hyal=, and
+=stylo-hyal=. All of them are short rods of bone, contracted in the
+middle, and expanded at the ends, where they are tipped with
+cartilage. The cerato-hyal (fig. 72, 31) lies next to the basi hyal.
+The stylo-hyal is terminated by a much smaller bone, the
+=tympano-hyal=, which lies in a canal between the tympanic and
+periotic, and is ankylosed to the periotic just to the anterior and
+inner side of the stylomastoid foramen.
+
+The =posterior cornu= of the hyoid is much smaller than the anterior;
+it consists of a short bone, the =thyro-hyal= (fig. 72, 33), which
+connects the basi-hyal with the thyroid cartilage of the larynx.
+
+FORAMINA OF THE SKULL.
+
+The foramina, or apertures perforating the walls of the skull, are
+very numerous, and may either be due to holes actually penetrating the
+bone, or may be small vacuities between the margins of two elsewhere
+contiguous bones.
+
+They may be divided into two groups, the first including
+
+I. The holes through which the =twelve cranial nerves= leave the
+cranial cavity.
+
+_a._ The most anterior of these nerves, the olfactory, leaves the
+skull by a number of small holes piercing the =cribriform plate= (fig.
+72, 5).
+
+_b._ The second, or optic, passes out by a large hole, the =optic
+foramen= (fig. 75, II) piercing the orbitosphenoid. The optic foramen
+is the most anterior of the three prominent holes seen within and
+immediately behind the orbit.
+
+_c._ The third, fourth, and sixth nerves, i.e. those supplying the eye
+muscles, and with them the first or ophthalmic branch of the large
+fifth or trigeminal nerve, pass out by a large hole, the =foramen
+lacerum anterius= (fig. 75, III, IV, V{1}, VI), which, as has been
+already mentioned, lies between the orbitosphenoid and alisphenoid.
+
+_d._ Immediately behind the foramen lacerum anterius, the alisphenoid
+is perforated by a prominent round hole, the =foramen rotundum= (fig.
+75, V{2}), through which the second branch of the trigeminal nerve
+passes out.
+
+_e._ A quarter of an inch further back there is another prominent
+hole, the =foramen ovale= (fig. 75, V{3}), through which the third
+branch of the trigeminal nerve leaves the cranium.
+
+_f._ The seventh or facial nerve, as already mentioned, leaves the
+cranial cavity and enters the auditory capsule, through an opening in
+the periotic called the =internal auditory meatus=, while it finally
+leaves the skull by the =stylomastoid foramen= (fig. 75, VII), which
+lies between the tympanic bulla, the paroccipital process, and the
+mastoid portion of the periotic.
+
+_g._ The eighth or auditory nerve on leaving the cranial cavity,
+passes with the facial straight into the auditory capsule through the
+=internal auditory meatus= (fig. 72, 20). It is then distributed to
+the organ of hearing.
+
+_h._ The ninth, tenth and eleventh nerves leave the skull through the
+=foramen lacerum posterius= (fig. 75, IX, X, XI), a large space lying
+between the auditory bones and the exoccipital.
+
+_i._ Finally, the twelfth nerve, the hypoglossal, passes out through
+the prominent =condylar foramen= (fig. 75, XII), which perforates the
+exoccipital just behind the foramen lacerum posterius.
+
+
+II. OTHER OPENINGS IN THE SKULL.
+
+_a._ The =anterior narial opening= lies at the anterior end of the
+skull, and is bounded by the premaxillae and nasals. In the natural
+condition it is divided into two by a vertical partition, formed by
+the =narial septum=, the anterior unossified part of the mesethmoid.
+
+_b._ Penetrating the middle of the maxillae at the side of the face is
+the rather large =infra-orbital foramen= (fig. 73, 11), through which
+part of the second branch of the trigeminal nerve passes out from the
+orbit to the side of the face.
+
+_c._ Several foramina are seen perforating the anterior part of the
+orbit. The most dorsal of these, perforating the lachrymal bone, is
+the =lachrymal foramen= (fig. 73, 13). Lying below and slightly
+external to this is a large foramen, through which part of the second
+branch of the trigeminal enters on its way to the infra-orbital
+foramen and so to the side of the face. Lastly, lying below these, and
+perforating the palatine, are two closely apposed foramina, the
+=internal orbital foramina=, through which part of the first or
+ophthalmic branch of the trigeminal nerve leaves the orbit, passing
+into the nasal cavity.
+
+_d._ The anterior part of the palate between the premaxillae and the
+maxillae is perforated by a pair of long closely apposed apertures, the
+=anterior palatine foramina= (fig. 75, 19). They transmit part of the
+trigeminal nerve.
+
+_e._ Towards the posterior part of the palate are two pairs of small
+=posterior palatine foramina= (fig. 75, 18). These perforate the
+palatine and transmit branches of the trigeminal nerve and certain
+blood-vessels.
+
+_f._ The =posterior narial opening= is bounded chiefly by the
+palatines.
+
+_g._ The =alisphenoid canal= (fig. 75, 21) is a short canal
+penetrating the base of the alisphenoid bone, and transmitting the
+external carotid artery. It lies between the foramen rotundum and the
+foramen ovale.
+
+_h._ Between the auditory bulla and the foramen ovale are seen two
+openings. The more external of these is the opening of the =Eustachian
+canal= (fig. 75, 22), which communicates with the tympanic cavity. The
+more internal is the =foramen lacerum medium= (fig. 75, 9), through
+which the internal carotid enters the cranial cavity.
+
+_i._ The =external auditory aperture= (fig. 75, 7) is a large opening
+with rough edges at the outer side of the tympanic bulla.
+
+_j._ Between it and the glenoid surface of the squamosal is the
+=postglenoid= foramen (fig. 75, 10) through which a vein passes out.
+
+_k._ Lastly, there is the great =foramen magnum= (fig. 75, 2), between
+the occipital condyles. Through it the brain and spinal cord
+communicate.
+
+
+C. THE RIBS AND STERNUM.
+
+[Illustration FIG. 76. STERNUM AND STERNAL RIBS OF A DOG (_Canis
+familiaris_) × 1/2.
+
+ 1. presternum.
+ 2. first sternebra of mesosternum.
+ 3. last sternebra of mesosternum.
+ 4. xiphisternum. The flattened cartilaginous plate terminating the
+ xiphisternum is not shown.
+ 5. first sternal rib.]
+
+These, together with the thoracic vertebrae, form the skeletal
+framework of the thorax. Each rib is a curved rod, which at its dorsal
+end is movably articulated to the vertebra, and at its ventral end is
+either connected with the sternum, or ends freely. In the dog there
+are thirteen pairs of ribs, nine pairs of which are directly connected
+with the sternum, while the remaining four end freely and are known as
+=floating ribs=. Each rib is obviously divided into two parts, a
+dorsal or =vertebral part=, and a ventral or =sternal part=. The
+vertebral portion, which forms about two-thirds of the whole rib, is a
+flattened, regularly curved rod, completely ossified. Its dorsal end
+is rounded, forming the =head= or =capitulum=, which articulates with
+a concave surface furnished partly by the corresponding vertebra and
+partly by the vertebra next in front. The last three or four however
+articulate with one vertebra only. A short way behind the capitulum on
+the dorsal side of the rib is a rounded outgrowth, the =tubercle= or
+=tuberculum=, by means of which the rib articulates with the
+transverse process. The portion of the rib between the head and the
+tubercle is known as the =neck=. The =sternal portion= of the rib
+(fig. 76) is a short bar of calcified or imperfectly ossified
+cartilage, about one-third of the length of the corresponding bony
+portion. The anterior sternal ribs are somewhat more cartilaginous
+than the posterior ones. The vertebral portions increase in length
+from the first which is very stout, and has the capitulum and
+tuberculum very distinct, to about the eighth or ninth; afterwards
+they gradually diminish in size. The first nine to eleven have the
+capitula and tubercula separate, afterwards they gradually merge
+together.
+
+
+THE STERNUM.
+
+This is an elongated cylindrical structure lying in the mid-ventral
+wall of the thorax, and is divided into eight segments or
+=sternebrae=. The anterior segment, the =presternum= (fig. 76, 1) or
+=manubrium sterni= is expanded in front; the next six segments, which,
+together form the =mesosternum= are elongated, somewhat contracted in
+the middle and expanded at the ends. The last segment or
+=xiphisternum= (fig. 76, 4) is long and narrow, and terminates in a
+flattened expanded plate of cartilage. The first pair of sternal ribs
+articulate with the sides of the presternum, and the remaining pairs
+between the successive sternebrae. Between the last sternebra and the
+xiphisternum two pairs articulate. Development shows that the sternum
+is formed by the union in the middle line of two lateral portions;
+this can be well seen in the presternum and xiphisternum of the puppy,
+but no traces of this median division remain in the adult dog.
+
+
+2. THE APPENDICULAR SKELETON.
+
+The appendicular skeleton consists of the bones of the anterior and
+posterior limbs, and of their respective supports, the pectoral and
+pelvic girdles.
+
+
+THE PECTORAL GIRDLE.
+
+The =pectoral girdle= lies external to the ribs, and has no bony
+attachment to the axial skeleton. In almost all Mammalia it is, as
+compared with that in Sauropsids, very incomplete; and in the dog it
+is even more reduced than in the majority of Mammalia. The dorsal
+portion or =scapula= is well developed, but the ventral portion is
+almost entirely absent.
+
+The =scapula= is somewhat triangular in shape, the apex being
+directed downwards and forwards, and being expanded to form the
+shallow =glenoid cavity= with which the head of the humerus
+articulates. The inner surface of the scapula is nearly flat, while
+the outer is drawn out into a very prominent ridge, the =spine=,
+which, arising gradually near the dorsal end, runs downwards, dividing
+the surface into two nearly equal parts, the =prescapular= and
+=postscapular fossae=, and ends in a short blunt process, the
+=acromion=. The anterior border of the scapula is somewhat curved, and
+is called the =coracoid border=; it is terminated ventrally by a
+slight blunt swelling, the =coracoid process=, which ossifies from a
+different centre from the rest of the scapula, and is probably the
+sole representative of the =coracoid=. The dorsal or =suprascapular
+border= of the scapula is rounded, while the posterior or =glenoid
+border= is nearly straight. The clavicle[144] or collar bone, which in
+a large proportion of mammals is well seen, in the dog is very
+imperfectly developed; it is short and broad, and is suspended in the
+muscles, not reaching either the scapula or sternum.
+
+
+THE ANTERIOR LIMB.
+
+The anterior limb of the dog is divisible into the usual three
+portions, the =brachium= or =upper arm=, the =antibrachium= or
+=fore-arm=, and the =manus= or =wrist= and =hand=.
+
+The =brachium= or =upper arm= includes only a single bone, the
+=humerus=.
+
+The =humerus= is a stout elongated bone, articulating by its large
+proximal =head= (fig. 77, 1) with the glenoid cavity of the scapula,
+and at its distal end by the =trochlea= with the bones of the
+fore-arm. The head passes on its inner side into an area roughened for
+the attachment of muscles and called the =lesser tuberosity= (fig. 77,
+2); while in front it is divided by the shallow =bicipital groove=
+from a large roughened area, the =greater tuberosity= (fig. 77, 3),
+which is continued as a slight roughened ridge, extending about
+one-third of the way down the outer side of the shaft. This ridge,
+which in many animals is much more strongly developed than it is in
+the dog, is called the =deltoid ridge=. The =trochlea= (fig. 77, 5) at
+the distal end of the bone is a pulley-like surface, elevated at the
+sides and grooved in the middle. It articulates with the radius and
+ulna of the fore-arm. On each side of it are slight roughened
+projections, the =internal= and =external condyles= (fig. 77, 7). In
+the cat and many other animals there's a foramen, the =ent-epicondylar
+foramen= above the internal condyle, but in the dog this is not
+developed. Passing up the shaft from the external condyle is a slight
+ridge, the =supinator= or =ectocondylar ridge=; this is better
+developed in many mammals. Immediately above the trochlea in front and
+behind are the deep =supra-trochlear fossae=, which communicate with
+one another through the =supra-trochlear foramen= (fig. 77, 8). The
+posterior of these, the =olecranon fossa=, is much the deeper, and
+receives the olecranon process of the ulna when the arm is extended.
+The head and tuberosities of the humerus ossify from one centre, the
+shaft from a second, and the trochlea and condyles from a third.
+
+The =fore-arm= or =antibrachium= contains two bones, the =radius= and
+=ulna=; they are immovably articulated with one another, but not
+fused. The pre-axial bone, the =radius= (fig. 77, B), which lies more
+or less in front of the ulna, is external to the ulna at its proximal
+end, and at its distal end is internal to that bone. It articulates
+with the external portion of the trochlea, while the ulna articulates
+with the internal portion. It is a straight bone with its distal end
+slightly larger than its proximal end. The proximal end articulates
+with the trochlea, the distal end with the bones of the carpus.
+
+[Illustration FIG. 77. BONES OF THE LEFT UPPER ARM AND FORE-ARM OF A
+DOG (_Canis familiaris_) × 1/2.
+
+A, humerus (seen from the posterior side); B, radius, C, ulna, both
+seen from the anterior side.
+
+ 1. head.
+ 2. lesser tuberosity.
+ 3. greater tuberosity.
+ 4. shaft of the humerus.
+ 5. trochlea.
+ 6. internal condyle.
+ 7. external condyle.
+ 8. supra-trochlear foramen.
+ 9. proximal end of the radius.
+ 10. shaft of the radius.
+ 11. olecranon.
+ 12. surface for articulation with the trochlea.
+ 13. surface for articulation with the radius.
+ 14. distal end of the ulna.]
+
+The postaxial bone, the =ulna= (fig. 77, C), has the proximal end much
+enlarged, forming the =olecranon= (fig. 77, 11), and tapers gradually
+to the distal end. Near its proximal end the ulna is marked by a deep
+=sigmoid notch=, which bears on its inner side a concave surface
+(fig. 77, 12) for articulation with the trochlea. The pointed proximal
+end of the sigmoid notch is called the =coronoid process=. Somewhat
+in front of and below the sigmoid notch is a smaller hollow (fig. 77,
+13), with which the radius articulates.
+
+In the young animal the ends of both radius and ulna are seen to
+ossify from centres different from those forming the shafts. The
+epiphyses forming both ends of the radius, and the distal end of the
+ulna are large, while that at the proximal end of the ulna is small,
+and forms only the end of the olecranon.
+
+The =Manus= is divided into
+
+_a._ The =carpus= or =wrist=, formed of a group of small bones.
+
+_b._ The =hand=, which includes firstly some elongated bones, the
+=metacarpals=, forming what corresponds to the palm of the hand, and
+secondly the phalanges, which form the =fingers=.
+
+The =Carpus= or =wrist=. The carpus of the dog consists of seven small
+bones, arranged in a proximal row of three, and a distal row of four.
+It differs much from the simpler type met with in the newt. The
+largest bone of the proximal row is the =scapho-lunar= (fig. 80, 1),
+formed by the fused =scaphoid= (radiale), =lunar= (intermedium), and
+=centrale=; it has a large convex proximal surface for articulation
+with the radius, and articulates distally with the trapezium,
+trapezoid, and magnum, and internally with the cuneiform. The
+=cuneiform= (ulnare) (fig. 80, 2) has a posterior rounded surface
+articulating with the ulna; it articulates in front with the unciform,
+and internally with the =pisiform= (fig. 80, 7), which is a
+comparatively large sesamoid bone on the ulnar side of the carpus.
+Frequently also there is a small sesamoid bone on the radial side of
+the carpus. The =trapezium= (carpale 1), =trapezoid= (carpale 2), and
+=magnum= (carpale 3) (fig. 80, 5) are all small bones, and support
+respectively the first, second, and third metacarpals. The =unciform=
+(carpalia 4 and 5) (fig. 80, 6) is larger, and supports the fourth and
+fifth metacarpals.
+
+The hand has five =digits=, each consisting of an elongated
+=metacarpal=, followed by =phalanges=, the last of which, the =ungual
+phalanx=, is pointed and curved, and bears the claw. Each of the
+metacarpals is seen in the young animal to have its distal end formed
+by a prominent epiphysis, and each of the phalanges, except those
+bearing the claws, has a similar epiphysis at its proximal end.
+
+The =pollex= (fig. 80, A, I ) is far shorter than the other digits,
+and normally does not touch the ground in walking. It has only two
+phalanges, while each of the other digits has three. A pair of small
+sesamoid bones are developed on the ventral or flexor side of the
+metacarpo-phalangeal articulations of all the digits except the
+pollex. Frequently similar sesamoid bones occur also on the dorsal
+side of the phalangeal articulations.
+
+
+THE PELVIC GIRDLE.
+
+The =pelvic girdle= consists of two halves, which lie nearly parallel
+to the vertebral column.
+
+Each half is firmly united to its fellow in a ventral symphysis
+behind, and is in front expanded and united to the sacrum. Each half
+or =innominate bone= is seen in the young animal to consist of four
+distinct parts, the =ilium= or dorsal element, the =pubis= or anterior
+ventral element, the =ischium= or posterior ventral element, and a
+small fourth part, the =acetabular= or =cotyloid= bone, wedged in
+between the three others. These parts, though all distinct in the
+young animal, are in the adult so completely fused that their
+respective boundaries cannot be distinguished. At about the middle of
+the outer surface of the innominate bone is a very deep cavity, the
+=acetabulum= (fig. 78, A, 1) with which the head of the femur
+articulates; all the bones except the pubis take part in its
+formation.
+
+The =ilium= is a rather long bone, expanded in front and contracted
+behind; it forms about half the acetabulum. On its inner or =sacral
+surface= (fig. 78, 4) is a large roughened patch for articulation
+with the sacrum; its outer or =gluteal surface= is concave. The
+posterior part of the bone is flattened below, forming the narrow
+=iliac surface= (fig. 78, A, 5).
+
+[Illustration FIG. 78. RIGHT INNOMINATE BONE, A, OF A FULL-GROWN
+TERRIER, B, OF A COLLIE PUPPY. × 1.
+
+A is seen from the ventral side, B from the inner or sacral side.
+
+ 1. acetabulum.
+ 2. thyroid foramen.
+ 3. supra-iliac border of ilium.
+ 4. sacral surface.
+ 5. iliac surface.
+ 6. acetabular border.
+ 7. pubic border.
+ 8. ischial border.
+ 9. ischium.
+ 10. tuberosity of ischium.
+ 11. ischial symphysis.
+ 12. pubis.
+ 13. pubic symphysis.
+ 14. cotyloid or acetabular bone.]
+
+The =ischium= (fig. 78, 9) is a wide flattened bone forming the
+posterior part of the innominate bone. It meets the pubis ventrally,
+but is separated from it for the greater part of its length by the
+large =obturator= or =thyroid foramen= (fig. 78, 2). At its posterior
+end externally it bears a rather prominent roughened =ischial
+tuberosity= (fig. 78, A, 10). The ischium meets its fellow in a
+ventral symphysis, and forms about one-third of the acetabulum.
+
+[Illustration FIG. 79. FRONT VIEW OF THE LEFT LEG BONES OF A DOG
+(_Canis familiaris_) × 1/2.
+
+A femur, B tibia, C fibula, D patella.
+
+ 1. head of femur.
+ 2. neck.
+ 3. great trochanter.
+ 4. shaft.
+ 5. external condyle.
+ 6. internal condyle.
+ 7. fabella.
+ 8. cnemial crest.]
+
+The =pubis= (fig. 78, 12) is smaller than either the ischium or ilium;
+it does not take part in the formation of the acetabulum, and like the
+ischium, meets its fellow in a ventral symphysis. The =acetabular
+bone= (fig, 78, B, 14) is small and triangular, and is wedged in
+between the other three. It forms about one-sixth of the acetabulum.
+
+
+THE POSTERIOR LIMB.
+
+The =posterior limb=, like the anterior, is divisible into three
+parts; these are the =thigh=, the =crus= or =shin=, and the =pes=.
+
+The =thigh= contains only a single bone, the =femur=.
+
+The =femur= is a long straight bone with a nearly smooth shaft and
+expanded ends. The proximal end bears on its inner side the large
+rounded =head= (fig. 79, A, 1) which articulates with the acetabulum.
+External to the head and divided from it by a deep pit is a large
+rough outgrowth, the =great trochanter= (fig. 79, 3). The deep pit is
+the =trochanteric= or =digital fossa=. On the inner side below the
+head is a smaller roughened surface, the =lesser trochanter=. The
+lower or distal end of the bone bears two prominent rounded surfaces,
+the =condyles=, which articulate with the tibia. They are separated
+from one another by the deep =intercondylar notch=, which is continued
+above and in front as a shallow groove, lodging a large sesamoid bone,
+the =patella= or =knee-cap=. At the back of the knee-joint are a pair
+of smaller sesamoids, the =fabellae= (fig. 79, 7).
+
+In the young animal there are three epiphyses to the shaft of the
+femur, one forming the head, one the great trochanter, and one the
+distal end.
+
+The =crus= or =shin= contains two bones, the =tibia= and =fibula=. The
+=tibia= is a fairly thick straight bone, expanded at both ends,
+especially at the head or proximal end. The proximal end is triangular
+in cross section, and bears two facets for articulation with the
+condyles of the femur. The anterior surface of the proximal end of the
+tibia is marked by the strong =cnemial crest= (fig. 79, 8), which runs
+some way down the shaft. The distal end of the tibia articulates with
+the astragalus by an irregular, somewhat square surface.
+
+The shaft of the tibia ossifies from one centre, the distal end from a
+second, and the proximal end from two more.
+
+[Illustration FIG. 80. A, RIGHT MANUS, B, RIGHT PES OF A DOG (_Canis
+familiaris_) × 1/2 (after VON ZITTEL).
+
+ 1. bone representing the fused scaphoid, lunar and centrale.
+ 2. cuneiform.
+ 3. trapezium.
+ 4. trapezoid.
+ 5. magnum.
+ 6. unciform.
+ 7. pisiform.
+ 8. first metacarpal.
+ 9. fifth metacarpal.
+ 10. astragalus.
+ 11. calcaneum.
+ 12. navicular.
+ 13. middle cuneiform.
+ 14. external cuneiform.
+ 15. cuboid.
+ 16. first metatarsal.
+
+The digits are numbered with Roman numerals.]
+
+The =fibula= (fig. 79, C) is a distinct but very slender bone,
+somewhat expanded at both ends. It lies external to the tibia and
+articulates by its proximal end with the head of the tibia, and by its
+distal end with the calcaneum. Its shaft and proximal end ossify from
+one centre, and its distal end from a second.
+
+
+The =Pes=.
+
+The structure of the =pes= corresponds closely with that of the manus.
+It is divided into:--
+
+_a._ The =tarsus= or =ankle= formed of a group of small bones.
+
+_b._ The =foot=, which includes, firstly, some elongated bones, the
+=metatarsals=, forming what corresponds to the sole of the foot, and
+secondly the =phalanges=, which form the toes.
+
+The =Tarsus=. The tarsus of the dog consists of seven bones arranged
+in two rows, of two and four respectively, with a =centrale= between
+them. The two bones of the proximal row are the =astragalus= and
+=calcaneum=.
+
+The =astragalus= (fig. 80, 10) corresponds to the fused =tibiale= and
+=intermedium= of the typical tarsus. Its proximal end is much wider
+than its distal end, and forms a large rounded =condyle= articulating
+with the tibia, while its posterior end meets the navicular. It lies
+to the dorsal side of the foot.
+
+The =calcaneum= (fibulare) (fig. 80, 11), the thickest bone in the
+pes, lies somewhat behind, and to the outer side of the astragalus. It
+articulates with the astragalus and fibula, and is drawn out behind
+into a long rounded process, which forms the heel, and is in the young
+animal terminated by an epiphysis. Between the proximal and distal
+rows of tarsals is the =navicular= (centrale) (fig. 80, 12), a
+somewhat flattened and square bone articulating with the astragalus.
+
+The distal row of tarsals consists of four bones. The =internal
+cuneiform= (tarsale 1) is a smooth flattened bone lying to the inner
+side of the foot; it articulates with the first metatarsal and with
+the navicular. The =middle cuneiform= (tarsale 2) (fig. 80, 13) is a
+still smaller bone, lying external to the internal cuneiform. It
+articulates with the second metatarsal and with the navicular. The
+=external cuneiform= (tarsale 3) (fig. 80, 14) is a larger, somewhat
+square bone lying external to the middle cuneiform. It articulates
+with the third metatarsal and with the navicular. The =cuboid=
+(tarsalia 4 and 5) (fig. 80, 15) is a considerably larger bone lying
+to the outer side of the foot. It articulates with the fourth and
+fifth metatarsals and with the calcaneum.
+
+The pes has sometimes five digits, sometimes four, the hallux being
+absent. Even when present the =hallux= (fig. 80, =B, I=) is commonly
+much reduced, and may be quite vestigial, and represented only by a
+small nodular metatarsal.
+
+Each of the other digits consists of a long metatarsal, which in the
+young animal has a prominent epiphysis at its distal end, and of three
+phalanges. The proximal and middle phalanges have epiphyses at their
+proximal ends, while the distal phalanx is without epiphyses and is
+claw-shaped.
+
+
+FOOTNOTES:
+
+[139] W. Ellenberger and H. Baum, _Anatomie des Hundes_, Berlin, 1891.
+
+[140] T.H. Huxley, "Dental and cranial characters of the Canidae,"
+_P.Z.S._ 1880.
+
+[141] See p. 392.
+
+[142] The dura mater is a membrane which lines the cranial cavity and
+is formed of tough connective tissue.
+
+[143] These are not strictly homologous with the basi-hyal and
+cerato-hyal of the Dogfish.
+
+[144] See note to p. 25.
+
+
+
+
+CHAPTER XXII.
+
+GENERAL ACCOUNT OF THE SKELETON IN MAMMALIA.
+
+
+THE EXOSKELETON AND VERTEBRAL COLUMN.
+
+
+EPIDERMAL EXOSKELETON.
+
+=Hair=, which forms the characteristic Mammalian exoskeleton, varies
+much in different animals, and in different parts of the same animal.
+A large proportion of mammals have the surface fairly uniformly
+covered with hair of one kind only. In some forms however there are
+two kinds of hair, a longer and stiffer kind alone appearing on the
+surface, and a shorter and softer kind forming the under fur. In most
+mammals hairs of a special character occur in certain regions, such as
+above the eyes, on the margins of the eyelids, and on the lips and
+cheeks, here forming the vibrissae or whiskers.
+
+Sometimes as in _Hippopotamus_, _Orycteropus_ and the Sirenia, the
+hair, though scattered over the whole surface, is extremely scanty,
+while in the Cetacea it is limited to a few bristles in the
+neighbourhood of the mouth, or may even be absent altogether in the
+adult. In most mammals the hairs are shed and renewed at intervals,
+sometimes twice a year, before and after the winter. The vibrissae or
+large hairs which occur in many animals upon the upper lip, and the
+mane and tail of Equidae are probably persistent.
+
+In the hedgehogs, porcupines and _Echidna_ certain of the hairs are
+modified and greatly enlarged, forming stiff spines. Similar spines
+occur in the young of _Centetes_, and in _Acanthomys_ among the
+Muridae.
+
+Several other forms of epidermal exoskeleton are met with in mammals,
+including:--
+
+(_a_) =Scales=. These overlie the bony scutes of armadillos and occur
+covering the tail in several groups of mammals, such as beavers and
+rats. In the Manidae the body is covered by flat scales which overlap.
+
+(_b_) The =horns= of Bovine Ruminants. These, which must on no account
+be confused with antlers, are hollow cases of hardened epidermis
+fitting on to bony outgrowths of the frontals. In almost every case
+they are unbranched structures growing continuously throughout life,
+and are very rarely shed entire. In the Prongbuck _Antilocapra_
+however they are bifurcated and are periodically shed. Horns are
+nearly always limited to a single pair, but the four-horned antelope
+_Tetraceros_ has two pairs, the anterior pair being the smaller.
+
+(_c_) The =horns of Rhinoceroses=. These are conical structures
+composed of a solid mass of hardened epidermal cells growing from a
+cluster of long dermal papillae. From each papilla there grows a fibre
+which resembles a thick hair, and cementing the whole together are
+cells which grow from the interspaces between the papillae. These
+fibres differ from true hairs in not being developed in pits in the
+dermis. Rhinoceros horns may be either one or two in number, and are
+borne on the fronto-nasal region of the skull. They vary much in
+length, the longest recorded having the enormous length of fifty-seven
+inches.
+
+(_d_) _Nails_, _hoofs_ and =claws=. In almost all mammals except the
+Cetacea, these are found terminating the digits of both limbs. =Nails=
+are more or less flattened structures, =claws= are pointed and
+somewhat curved. In most mammals the nails tend to surround the ends
+of the digits much more than they do in man. Sometimes the nail of one
+digit differs from that of all the others; thus the second digit of
+the pes in the Hyracoidea and Lemuroidea is terminated by a long claw,
+the other digits having flat nails. In the Felidae the claws are
+retractile, the ungual phalanx with claw attached folding back when
+the animal is at rest into a sheath, above, or by the side of the
+middle phalanx. In the Sloths and Bats enormously developed claws
+occur, forming hooks by which the animals suspend themselves. In
+_Notoryctes_ the third and fourth digits of the manus bear claws of
+great size; similar claws occur in _Chrysochloris_, being correlated
+in each case with fossorial habits. The nail at its maximum
+development entirely surrounds the terminal phalanx of the digit to
+which it is attached, and is then called a =hoof=. Hoofs are specially
+characteristic of the Ungulata.
+
+(_e_) =Spurs= and =beaks= are structures which are hardly represented
+among mammals, while so characteristic of birds. They are however both
+found in the Monotremata. In both _Echidna_ and _Ornithorhynchus_ the
+male has a peculiar hollow horny spur borne on a sesamoid bone
+articulated to the tibia. The jaws in _Ornithorhynchus_ are cased in
+horny beaks similar to those of birds, and are provided with horny
+pads which act as teeth.
+
+(_f_) =Horny plates= of a ridged or roughened character occur upon the
+anterior portion of the palate, and of the mandibular symphysis in all
+three genera of recent Sirenia; also upon the toothless anterior
+portion of the palate in Ruminants.
+
+(_g_) The =baleen of whales= also belongs to the epidermal
+exoskeleton. It consists of a number of flattened horny plates
+arranged in a double series along the palate. The plates are somewhat
+triangular in form and have their bases attached to the palate at
+right angles to its long axis, while their apices hang downwards into
+the mouth cavity. The outer edge of each plate is hard and smooth,
+while the inner edge and apex fray out into long fibres which look
+like hair. At the inner edge of each principal plate are subsidiary
+smaller plates. The plates are formed of a number of fibres each
+developed round a dermal papilla in the same way as are the fibres
+forming the horns of _Rhinoceros_. Baleen and Rhinoceros horn likewise
+agree in that the fibres are bound together by less hardened
+epithelial cells, which readily wear away and allow the harder fibres
+to fray out. The greatest development of baleen occurs in the Northern
+Right whale, _Balaena mysticetus_, in which the plates number three
+hundred and eighty or more on each side, and reach a length of ten or
+twelve feet near the middle of the series.
+
+
+DERMAL EXOSKELETON.
+
+Mammals show two principal kinds of exoskeletal structures which are
+entirely or partially dermal in origin, viz. the bony scutes of
+armadillos, and teeth.
+
+The =bony scutes of armadillos= are quadrate or polygonal in shape and
+are in general aggregated together, forming several shields protecting
+various regions of the body. The head is generally protected by a
+_cephalic_ shield, the anterior part of the body by a _scapular_, and
+the posterior by a _pelvic_ shield. The tail is also generally encased
+in bony rings, and scutes are irregularly scattered over the surface
+of the limbs. The mid-body region is protected by a varying number of
+bands of scutes united by soft skin, so as to allow of movement.
+Corresponding to each dermal scute is an epidermal plate. In
+_Chlamydophorus_ the scutes are mainly confined to the posterior
+region where they form a strong vertically-placed shield which
+coalesces with the pelvis. The anterior part of the body is mainly
+covered by horny epidermal plates with very little ossification
+beneath. In the gigantic extinct Glyptodonts the body is covered with
+a solid carapace formed by the union of an immense number of plates,
+and there are no movable rings. The top of the head is defended by a
+similar plate, the tail is generally encased in an unjointed bony
+tube, and there is commonly a ventral plastron.
+
+In _Phocaena phocaenoides_ the occurrence of vestigial dermal ossicles
+has been described, and in _Zeuglodon_ the back was probably protected
+by dermal plates.
+
+TEETH[145].
+
+Teeth are well developed in the vast majority of mammalia, and are of
+the greatest morphological and systematic importance, many extinct
+forms being known only by their teeth. Mammalian teeth differ from
+those of lower animals in various well-marked respects. (1) They are
+attached only to the maxillae, premaxillae and mandible, never to the
+palatines, pterygoids or other bones. (2) They frequently have more
+than one root. (3) They are always, except in some Odontoceti, placed
+in distinct sockets. (4) They are hardly ever ankylosed to the bone.
+(5) They are in most cases markedly heterodont. (6) They are commonly
+developed in two sets, the milk dentition and permanent dentition.
+
+It sometimes happens that teeth after being formed are reabsorbed
+without ever cutting the gum. This is the case, for instance, with the
+upper incisors of Ruminants.
+
+The form of mammalian teeth varies much, some are simple conical
+structures comparable to those of most reptiles, and these may either
+have persistent pulps, as in the case of the upper canines of the
+Walrus and the tusks of Elephants, or may be rooted as in most canine
+teeth. Some teeth have chisel-shaped edges, and this may be their
+original form, as in the human incisors, or may, as in those of
+Rodents, be brought about by the more rapid wearing away of the
+posterior edge, the anterior edge being hardened by a layer of enamel.
+Then, again, the crown may, as in the majority of grinding teeth, be
+more or less flattened. The various terms used in describing some of
+the forms of the surface of grinding teeth are defined on page 345.
+
+[Illustration FIG. 81. SKULL OF A YOUNG INDIAN RHINOCEROS (_R.
+unicornis_), SHOWING THE CHANGE OF THE DENTITION × 1/7. (Brit. Mus.)
+
+ 1. nasal.
+ 2. frontal.
+ 3. parietal.
+ 4. zygomatic process of squamosal.
+ 5. jugal.
+ _mI{1}._ milk incisor.
+ _mc._ milk canine.
+ _mpm{1}._ milk premolar.
+ _I{1}._ first incisor.
+ _c._ canine.
+ _pm{2}_, _pm{3}_, _pm{4}_. 2nd, 3rd and 4th premolars.
+ _m{1}_, _m{2}_. first and second molars.]
+
+The teeth of the Aard Varks are compound, and differ completely from
+those of all other mammals (see p. 425).
+
+As a rule, the higher the general organisation of an animal the
+better are its milk teeth developed, and the more do they form a
+reproduction on a small scale of the permanent set. This fact is well
+seen in the Primates, Carnivora and Ungulata. The method of notation
+by which the dentition of any mammal can be briefly expressed as a
+formula has been already described. The regular mammalian arrangement
+of teeth for each side is expressed by the formula
+
+ _i_ 3/3 _c_ 1/1 _pm_ 4/4 _m_ 3/3 × 2; total, 44.
+
+MONOTREMATA. In _Echidna_ teeth are quite absent. In the young
+_Ornithorhynchus_[146] functional molar teeth of a multi-tubercular
+type resembling those of some Mesozoic mammalia are present, but in
+the adult they disappear, their office being discharged by horny
+plates.
+
+MARSUPIALIA[147] have a heterodont dentition, which has generally been
+regarded as almost monophyodont, the only tooth which has an obvious
+deciduous predecessor being the last premolar. The researches of
+Röse[148] and Kükenthal[149] tend to show that the teeth of Marsupials
+are developed in the same way as in other mammals, and are diphyodont.
+In the case of the premolars, teeth which are homologous with the
+permanent teeth of other mammals begin to develop as lateral
+outgrowths from the milk teeth, but afterwards become absorbed, so
+that the teeth which actually persist belong to the milk series. The
+last premolar, however, does as a rule develop and replace its milk
+predecessor; sometimes, however, as in _Didelphys_, it takes its place
+among the milk molars without replacing one of them.
+
+The types of dentition characteristic of the different groups of
+placental mammals may mostly be paralleled among the Marsupials. Thus
+among the polyprotodont forms the Didelphyidae or opossums, and some
+of the Dasyuridae, such as _Sarcophilus_ and _Thylacinus_, have a
+typical carnivorous dentition with small incisors, large canines, and
+molars with pointed compressed crowns. The dental formula of
+_Thylacinus_, is _i_ 4/3 _c_ 1/1 _pm_ 3/3 _m_ 4/4, total 46.
+
+In _Myrmecobius_ five or six molar teeth occur on each side, and the
+total number of teeth reaches fifty-two or fifty-six. The teeth bear
+rows of tubercles, and resemble those of the Multituberculate mesozoic
+Mammals[150], more than do those of any other living form. Calcified
+teeth have recently been described[151] in _Myrmecobius_ earlier than
+the functional or milk set. This would relegate the milk teeth of
+mammals in general to a second series, and the permanent teeth to a
+third. In _Notoryctes_ the dental formula[152] is given as _i_ 3/2 _c_
+1/1 _pm_ 2/3 _m_ 4/4, total 40. The canines are small, and the
+anterior molars have strongly developed cusps, and much resemble those
+of _Chrysochloris_ (Insectivora).
+
+Among the diprotodont types the Phascolomyidae, or Wombats, have a
+dentition recalling that of the Rodents. All the teeth grow from
+persistent pulps, and the incisors have enamel only on the anterior
+surface as in Rodents. The dental formula is _i_ 1/1 _c_ 0/0 _pm_ 1/1
+_m_ 4/4, total 24. There are indications of a vestigial second pair of
+incisors.
+
+The Macropodidae, or Kangaroos, have a herbivorous dentition with the
+formula _i_ 3/1 _c_ (0--1)/0 _pm_ 2/2 _m_ 4/4. The incisors are sharp
+and cutting, and are separated by a long diastema or gap from the
+molars, which have their crowns marked by ridges or cusps. There are
+indications of several vestigial incisors.
+
+_Coenolestes_, a remarkable form recently described from America,
+belongs to the diprotodont section, and is the only living member of
+the section known outside the Australian region[153]. An exceptional
+dentition is seen in the case of the extinct _Thylacoleo_, in which
+the functional teeth are reduced to two pairs; one pair of large
+cutting incisors and one of compressed sharp-edged premolars.
+
+EDENTATA. Some Edentata, viz. the ant-eaters (Myrmecophagidae) are, as
+far as is known, absolutely toothless at all stages of their
+existence; being the only mammals except _Echidna_ in which no tooth
+germs have been discovered; others, viz. the Manidae, though showing
+foetal tooth germs, are quite toothless in post-foetal life; others,
+viz. some of the armadillos, have the largest number of teeth met with
+in land mammals. The teeth are homodont except in the Aard Varks, and
+grow from persistent pulps. In the sloths (Bradypodidae) and the
+Megatheriidae, there are five pairs of teeth in the upper and four in
+the lower jaw. The teeth of sloths consist of a central axis of
+vasodentine, surrounded firstly by a thin coating of hard dentine, and
+secondly by a thick coating of cement.
+
+In no living Edentate have the teeth any enamel; it has, however, been
+described as occurring in certain early Megatheroid forms from S.
+America[154], and an enamel organ has also been discovered in an
+embryo _Dasypus_[155]. In the Armadillos (Dasypodidae) the number of
+teeth varies from 8/8 or 7/7 in _Tatusia_, to upwards of 25/25 in
+_Priodon_, which therefore may have upwards of a hundred teeth, the
+largest number met with in any land mammal. In _Tatusia_ all the teeth
+except the last are preceded by two-rooted milk teeth. The Aard Varks
+are diphyodont, and milk teeth are also known in a species of
+_Dasypus_, but with these exceptions Edentates are, as far as is
+known, monophyodont. In _Glyptodon_ the teeth are almost divided into
+three lobes by two deep grooves on each side.
+
+The Aard Varks (Orycteropodidae) are quite exceptional as regards
+their teeth, which are cylindrical in shape, and are made up of a
+number of elongated denticles fused together. Each denticle contains a
+pulp cavity from which a number of minute tubes radiate outwards.
+These teeth are diphyodont and somewhat heterodont, eight to ten pairs
+occur in the upper jaw and eight in the lower, but they are not all in
+place at one time. The last three teeth in each jaw are not preceded
+by milk teeth[156].
+
+SIRENIA. The teeth of Sirenia show several very distinct types, the
+least modified being that of the extinct Halitheriidae, which have
+large incisors in the upper jaw, and five or six pairs of tuberculated
+grinding teeth in each jaw, the anterior ones being preceded by milk
+teeth.
+
+In both the living genera the dentition is monophyodont. In _Manatus_
+the dentition is _i_ 2/2 _pm_ and _m_ 11/11. The incisors are
+vestigial, and disappear before maturity. The grinding teeth have
+square enamelled crowns marked by transverse tuberculated ridges. They
+are not all present in the jaw at the same time. In _Halicore_ the
+upper jaw bears a pair of straight tusklike incisors; in the male
+these have persistent pulps and project out of the mouth; in the
+female they soon cease to grow and are never cut. They are separated
+by a long diastema from the grinding teeth which have tuberculated
+crowns and are 5/5 or 6/6 in number, but are not all in place at once.
+Several other pairs of slender teeth occur in the young animal, but
+are absorbed or fall out before maturity. In _Rhytina_ teeth are
+altogether absent.
+
+
+CETACEA.
+
+_ARCHAEOCETI._ _Zeuglodon_ has the following dentition, _i_ 3/3 _c_
+1/1 _pm_ and _m_ 5/5, total 36. The incisors and canines are simple
+and conical; the cheek teeth are compressed and have serrated cutting
+edges like those in some seals.
+
+In the _MYSTACOCETI_, or whalebone whales, calcified tooth germs
+probably belonging to the milk dentition are present in the embryo,
+but they are never functional, and are altogether absent in the adult.
+The anterior of these germs are simple, the posterior ones are
+originally complex, but subsequently split up into simple teeth like
+those of the anterior part of the jaw. Hence according to Kükenthal,
+who described these structures, the Cetacean dentition was originally
+heterodont.
+
+In the living _ODONTOCETI_ the dentition is homodont and monophyodont.
+In some cases traces occur of a replacing dentition which never comes
+to maturity, and renders it probable that the functional teeth of the
+Odontoceti are really homologous with the milk teeth of other mammals.
+Some of the dolphins afford the apparently simplest type of mammalian
+dentition known. The teeth are all simple, conical, slightly recurved
+structures, with simple tapering roots and without enamel. The
+dentition is typically _piscivorous_, being adapted for seizing active
+slippery animals such as fish. The prey is then swallowed entire
+without mastication. Sometimes the teeth are excessively numerous,
+reaching two hundred or more (fifty to sixty on each side of each jaw)
+in _Pontoporia_. This multiplication of teeth is regarded by Kükenthal
+as due to the division into three parts of numbers of trilobed teeth
+similar to those of some seals.
+
+In the Sperm whale, _Physeter_, the lower jaw bears a series of twenty
+to twenty-five stout conical recurved teeth, while in the upper jaw
+the teeth are vestigial and remain imbedded in the gum. An extinct
+form, _Physodon_, from the Pliocene of Europe and Patagonia is allied
+to the Sperm whale, but has teeth in both jaws. In the Grampus _Orca_,
+the teeth number about 12/12, and are very large and strong. In some
+forms the teeth are very much reduced in number; thus in _Mesoplodon_
+the dentition consists simply of a pair of conical teeth borne in the
+mandible. In the Narwhal _Monodon_ the dentition is practically
+reduced to a single pair of teeth, which lie horizontally in the
+maxillae, and in the female normally remain permanently in the alveoli.
+In the male the right tooth remains rudimentary, while the left is
+developed into an enormous cylindrical tusk marked by a spiral groove.
+Occasionally both teeth develop into tusks, and there is reason for
+thinking that two-tusked individuals are generally or always female.
+In the extinct _Squalodon_ the dentition is decidedly heterodont, and
+the molars have two roots. The dental formula is
+
+ _i_ 3/3 _c_ 1/1 _pm_ 4/4 _m_ 7/7, total 60.
+
+It is probable that the homodont condition of modern Odontoceti is not
+primitive, but due to retrogressive evolution.
+
+
+UNGULATA.
+
+Just as in the Cetacea a piscivorous dentition is most typically
+developed, so the Ungulata are, as a group, the most characteristic
+representatives of a _herbivorous_ dentition in its various forms.
+
+
+UNGULATA VERA.
+
+_ARTIODACTYLA_. As regards the living forms, the Artiodactyla can be
+readily divided into two groups, namely those with bunodont and those
+with selenodont teeth. It has, however, been shown that selenodont
+teeth always pass through an embryonic bunodont stage[157]. The
+bunodont type is best seen in Pigs and Hippopotami and such extinct
+forms as _Hyotherium_. In _Hippopotamus_ the dental formula is _i_
+(2-3)/(1-3) _c_ 1/1 _pm_ 4/4 _m_ 3/3.
+
+The incisors and canines of _Hippopotamus_ are very large and grow
+continuously. The genus _Sus_, which affords a good instance of an
+_omnivorous_ type of dentition, has the regular unmodified Mammalian
+dental formula _i_ 3/3 _c_ 1/1 _pm_ 4/4 _m_ 3/3, total 44. The
+canines, specially in the male, are large and have persistent pulps,
+and the upper canines do not have the usual downward direction but
+pass outwards and upwards. In the Wart Hog, _Phacochaerus_, they are
+enormously large, but a still more extraordinary development of teeth
+is found in _Babirussa_. In the male _Babirussa_ the canines, which
+are without enamel, are long, curved and grow continuously. Those of
+the upper jaw never enter the mouth, but pierce the skin of the face
+and curve backwards over the forehead. The dental formula of
+_Babirussa_ is _i_ 2/3 _c_ 1/1 _pm_ 2/2 _m_ 3/3, total 34.
+
+The Wart Hog has a very anomalous dentition, for as age advances all
+the teeth except the canines and last molars show signs of
+disappearing; both pairs of persisting teeth are however very large.
+
+Various extinct Ungulata such as _Anoplotherium_ have teeth which are
+intermediate in character between the bunodont and selenodont types.
+_Anoplotherium_ has the regular mammalian series of forty-four teeth.
+The crowns of all the teeth are equal in height, and there is no
+diastema--an arrangement found in no living mammal but man.
+
+We come now to the selenodont Artiodactyla.
+
+The Tylopoda--camels (Camelidae) and Llamas (Aucheniidae) when young
+have the full number of incisors, but in the adult the two upper
+middle ones are lost. The molars are typically selenodont and
+hypsodont[158]. In the Camel the dental formula is _i_ 1/3 _c_ 1/1
+_pm_ 3/2 _m_ 3/3, total 34. The upper incisors, canines and first
+premolars of the Camel are very small teeth, and the first premolar
+is separated by a long diastema from the others.
+
+The Tragulina or Chevrotains have no upper incisors, while the canines
+are largely developed, especially in the male.
+
+The Ruminantia or Pecora are very uniform as regards their dentition.
+The upper incisors are always absent, for though their germs are
+developed they are reabsorbed without ever becoming visible, and as a
+rule the upper canines are absent too, while the lower canines are
+incisiform. The grinding teeth are typically selenodont, and in the
+lower jaw form a continuous series separated by a wide diastema from
+the canines. The dental formula is usually
+
+ _i_ 0/3 _c_ 0-1/1 _pm_ 3/3 _m_ 3/3.
+
+The canines are largely developed in the male Muskdeer (_Moschus_) and
+in _Hydropotes_.
+
+_PERISSODACTYLA_. The premolars and molars have a very similar
+structure and form a continuous series of large square teeth with
+complex crowns. The crowns are always constructed on some modification
+of the bilophodont[159] plan, as is easily seen in the case of the
+forms with brachydont teeth, but in animals like the Horse, in which
+the teeth are very hypsodont, this arrangement is hard to trace. All
+four premolars in the upper jaw are preceded by milk teeth, while in
+Artiodactyla the first has no milk predecessors.
+
+In the Tapiridae the grinding teeth are brachydont and the lower ones
+are typically bilophodont. The last two upper molars have the
+transverse ridges united by an outer longitudinal ridge. The dentition
+is _i_ 3/3 _c_ 1/1 _pm_ 4/3 _m_ 3/3, total 42.
+
+In some of the extinct Perissodactyles such as _Lophiodon_[160], the
+dentition is brachydont and bilophodont, the grinding teeth in general
+resembling the posterior upper molars of the Tapir. The same type of
+brachydont tooth is seen in _Palaeotherium_ but the transverse ridges
+are crescentic instead of straight, and are separated from one another
+by shallow valleys without cement. Some of the Palaeotheridae have the
+regular series of forty-four teeth.
+
+A complete series of forms is known showing how from the simple
+brachydont teeth of the Palaeotheridae, were derived the complicated
+hypsodont teeth of the Equidae. The increase in depth of the tooth was
+accompanied by increase in the depth and complexity of the enamel
+infoldings, and of the cement filling them.
+
+Both upper and lower grinding teeth of the Equidae are much
+complicated by enamel infoldings, but their derivation from the
+bilophodont type can still be recognised. The diastema in front of the
+premolars is longer in the living Equidae than in their extinct
+allies. In the adult horse the dental formula is _i_ 3/3 _c_ 1/1 _pm_
+3/3 _m_ 3/3, total 40, with often a vestigial first upper premolar
+(fig. 82, _pm_ 1). The last molar is not more complex than the others,
+and in the female the canine is quite vestigial. The incisors are
+large and adapted for cutting and have the enamel curiously folded in
+forming a deep pit. The milk dentition is _di_ 3/3 _dc_ 0/0 _dpm_ 3/3,
+total 24. The last milk premolar is not more complex than the premolar
+that succeeds it. The horse affords an excellent instance of a
+typically _herbivorous_ type of dentition, the cutting incisors,
+reduced canines and series of large square flat-crowned grinding teeth
+being most characteristic.
+
+In _Rhinoceros_ the grinding teeth are much like those of _Lophiodon_,
+having an outer longitudinal ridge from which two crescentic
+transverse ridges diverge. The upper premolars are as complex as the
+molars, and there are no canines; in some species incisors also are
+absent. The dental formula is
+
+ _i_ (0--2)/(0--1) _c_ 0/(0--1) _pm_ 4/4 _m_ 3/3.
+
+[Illustration FIG. 82. PALATAL ASPECT OF THE CRANIUM AND MANDIBLE OF A
+DONKEY (_Equus asinus_) × 1/5. (Camb. Mus.)
+
+ 1. supra-occipital.
+ 2. occipital condyle.
+ 3. basi-occipital.
+ 4. vacuity representing the confluent foramen lacerum posterius
+ and foramen lacerum medium.
+ 5. auditory bulla.
+ 6. glenoid surface.
+ 7. vomer.
+ _i_ 1, _i_ 3. first and third incisors.
+ _c._ canine.
+ _pm_ 1, _pm_ 2. first and second premolars.
+ _m_ 1. first molar.]
+
+Among the Titanotheriidae _Palaeosyops_[161] has very brachydont teeth
+whose crowns have been described as _buno-selenodont_, the inner pair
+of columns being bunodont, the outer, selenodont. Similar grinding
+teeth occur in _Chalicotherium_. Some of the Titanotheriidae have the
+regular mammalian series of forty-four teeth.
+
+
+SUBUNGULATA.
+
+_TOXODONTIA._ _Nesodon_ has the regular dental formula; its grinding
+teeth are rooted and the upper ones resemble those of Rhinoceros. The
+second upper and third lower incisors form ever-growing tusks. There
+is a marked difference between the deciduous and permanent dentition.
+_Astrapotherium_ likewise has large rooted cheek teeth of a
+rhinocerotic type, and each jaw bears a pair of permanently growing
+tusks, those of the lower jaw being the canines. The dental formula is
+
+ _i_ 1/3 _c_ 0/1 _pm_ 2/1 _m_ 3/3, total 28.
+
+In _Toxodon_ the upper incisors and molars are large and curved and
+all the teeth have persistent pulps. In _Typotherium_ there are no
+tusks, but the upper incisors are chisel-like, recalling those of
+Rodents.
+
+The _CONDYLARTHRA_ have brachydont, generally bunodont teeth, with the
+premolars simpler than the molars. They generally have the regular
+dental formula.
+
+_HYRACOIDEA._ The dental formula of _Procavia_ is usually given as _i_
+1/2 _c_ 0/0 _pm_ 4/4 _m_ 3/3, total 34; in young individuals however
+there occur a second pair of upper incisors which early fall out. The
+upper incisors resemble those of Rodents in being long and curved and
+growing from persistent pulps. They are however triangular in
+transverse section, not rectangular, having two antero-lateral faces
+covered with enamel and a posterior face without enamel. Their
+terminations are pointed, not chisel-shaped as in Rodents. The lower
+incisors (fig. 83, _i_ 1) are pectinate or partially divided by
+vertical fissures, and the grinding teeth are of the rhinocerotic
+type.
+
+[Illustration FIG. 83. SKULL OF _Procavia (Dendrohyrax) dorsalis_ ×
+2/3. (Camb. Mus.)
+
+ 1. nasal.
+ 2. parietal.
+ 3. external auditory meatus.
+ 4. paroccipital process of the exoccipital.
+ 5. jugal.
+ 6. lachrymal foramen.
+ _i_ 1. first incisor.]
+
+_AMBLYPODA._ Two of the best known forms belonging to this extinct
+group differ much as regards dentition. For while _Coryphodon_ has the
+regular dental formula, and the canines of both jaws of moderate size,
+in _Uintatherium_ the dentition is very specialised, there are no
+upper incisors, and the upper canines form a pair of enormous tusks.
+The grinding teeth form a continuous series marked by =V=-shaped ridges
+and the dental formula is _i_ 0/3 _c_ 1/1 _pm_ 3/3 _m_ 3/3 total 34.
+
+_PROBOSCIDEA._ The incisors are composed entirely of dentine and have
+the form of conical tusks projecting greatly from the mouth. In
+living forms they are confined to the upper jaw, in some species of
+the extinct _Mastodon_ however they occur in the lower jaw also. In
+_Dinotherium_ they are probably absent from the upper jaw, but form a
+pair of downwardly and backwardly-directed tusks growing from the
+elongated symphysis of the mandible.
+
+The grinding teeth in the various Proboscidea show a very remarkable
+series of modifications. In _Dinotherium_ they are bilophodont or else
+are marked by three straight transverse ridges. The dental formula is
+_i_ 0?/1 _c_ 0/0 _pm_ 2/2 _m_ 3/3, and the teeth have the normal
+method of succession. In _Mastodon_ as in _Dinotherium_ the grinding
+teeth are marked by transverse ridges, but the ridges are subdivided
+into conical or mammillary cusps, and similar cusps often occur
+between the ridges. These cusps are covered with very thick enamel and
+the spaces between them are not filled up with cement. There are six
+of these grinding teeth for each side of each jaw but only three are
+in place at once. The first three are milk teeth as they may be
+succeeded vertically by others.
+
+In the true Elephants the number and depth of the enamel folds is much
+increased, and the spaces between the folds are filled up with cement.
+A very complete series of extinct forms is known with teeth
+intermediate in character between those of _Mastodon_ and those of the
+Mammoth and living elephants. The dental formula of _Elephas_ is
+
+ _di_ 1/0 _i_ 1/0 _c_ 0/0 _dm_ 3--4/3--4 _m_ 3/3.
+
+Sir W.H. Flower describes[162] the mode of succession of teeth in
+Elephants as follows: "As regards the mode of succession that of
+modern Elephants is as before mentioned very peculiar. During the
+complete lifetime of the animal there are but six molar teeth on each
+side of each jaw with occasionally a rudimentary one in front,
+completing the typical number of seven. The last three represent the
+true molars of ordinary mammals, those in front appear to be milk
+molars which are never replaced by permanent successors, but the whole
+series gradually moves forwards in the jaw, and the teeth become worn
+away and their remnants cast out in front while development of others
+proceeds behind. The individual teeth are so large and the processes
+of growth and destruction by wear take place so slowly, that not more
+than one or portions of two teeth are ever in place and in use on each
+side of each jaw at one time, and the whole series of changes
+coincides with the usual duration of the animal's life. On the other
+hand the _Dinotherium_, the opposite extreme of the Proboscidean
+series, has the whole of the molar teeth in place and use at one time,
+and the milk molars are vertically displaced by premolars in the
+ordinary fashion. Among Mastodons transitional forms occur in the mode
+of succession as well as in structure, many species showing a vertical
+displacement of one or more of the milk molars, and the same has been
+observed in one extinct species of Elephant (_E. planifrons_) as
+regards the posterior of these teeth."
+
+In the TILLODONTIA the grinding teeth are of Ungulate type, while the
+second incisors are large and grow from persistent pulps, so as to
+resemble those of Rodents.
+
+RODENTIA have a most characteristic and very constant dentition, the
+common dental formula being
+
+ _i_ 1/1 _c_ 0/0 _pm_ (0--1)/(0--1) _m_ 3/3, total 18 or 20.
+
+The incisors always have chisel-like edges and persistent pulps, and
+are separated by a wide diastema from the premolars. Canines are
+always absent, and there are generally three grinding teeth not
+preceded by milk teeth; their surface may be grooved, or may be
+bunodont. Teeth are most numerous in the Duplicidentata (Hares and
+Rabbits), in which the formula is _i_ 2/1 _c_ 0/0 _pm_ 3/2 _m_ 3/3,
+total 28, and fewest in Hydromys and certain other forms, in which
+the formula is _i_ 1/1 _c_ 0/0 _pm_ 0/0 _m_ 2/2, total 12. The hares
+and rabbits are the only rodents which have well developed deciduous
+incisors, though a vestigial milk incisor has been described in the
+Mouse (_Mus musculus_). The last upper molar of _Hydrochaerus_ is very
+complicated, its structure approaching that of the teeth of Elephants.
+
+[Illustration FIG. 84. CARNASSIAL OR SECTORIAL TEETH OF CARNIVORA
+(from FLOWER).
+
+_Upper sectorial teeth_ of I. _Felis_, II. _Canis_, III. _Ursus_. 1.
+anterior, 2. middle, 3. posterior cusp of blade, 4. inner lobe
+supported on distinct root, 5. inner lobe posterior in position and
+without distinct root, characteristic of the Ursidae.
+
+_Lower sectorial teeth._ 1. _Felis_, 2. _Canis_, 3. _Herpestes_. 1.
+anterior, 2. posterior lobe of blade, 3. inner tubercle, 4. heel.]
+
+CARNIVORA have the teeth rooted and markedly diphyodont and
+heterodont. The canines are greatly developed, and the incisors are
+small.
+
+In _CARNIVORA VERA_ the incisors are almost always 3/3. The fourth
+upper premolar and first lower molar are differentiated as carnassial
+teeth (see p. 436), and retain fundamentally the same characters
+throughout the suborder. The upper carnassial (fig. 84, I. II. III.)
+consists of a more or less compressed, commonly trilobed blade borne
+on two roots, with an inner tubercle borne on a third root. The lower
+carnassial has only two roots; its crown consists of a bilobed blade
+with generally an inner cusp, and a heel or talon (fig. 84, 4) behind
+the blade.
+
+The most thoroughly carnivorous type of dentition is seen in the
+Æluroidea, and especially in the cat tribe (Felidae). In the genus
+_Felis_ the dental formula is _i_ 3/3 _c_ 1/1 _pm_ 3/2 _m_ 1/1, total
+30. The incisors are very small, so as not to interfere with the
+action of the large canines, the lower carnassial is reduced to simply
+the bilobed blade (fig. 84, IV), and the cheek teeth are greatly
+subordinated to the carnassial. The extinct _Machaerodus_ has the
+upper canines comparable in size to those of the Walrus.
+
+The Civets and Hyaenas have a dentition allying them closely to the
+cats. The hyaena-like _Proteles_ has, however, the grinding teeth
+greatly reduced.
+
+In the Cynoidea[163] the general dentition is _i_ 3/3 _c_ 1/1 _pm_ 4/4
+_m_ 2/3, total 42. This differs from the regular mammalian dentition
+only in the absence of the last upper molar. The upper carnassial
+tooth (fig. 84, II.) consists of a larger middle and smaller posterior
+lobe with hardly any trace of an anterior lobe. The lower carnassial
+(fig. 84, V.) is typical, consisting of a bilobed blade with inner
+cusp and posterior talon.
+
+The dentition of the Cynoidea is most closely linked with that of the
+Arctoidea by means of fossil forms.
+
+[Illustration FIG. 85. MANDIBLE OF ISABELLINE BEAR (_Ursus
+isabellinus_) × 1/2. (Camb. Mus.)
+
+ 1. condyle.
+ 2. coronoid process.
+ _i_ 1. first incisor.
+ _c._ canine.
+ _pm_ 1, _pm_ 2. first and second premolars.
+ _m_ 1. first molar. The dotted
+ line is pointing to the posterior
+ half of the tooth.
+ This specimen has only
+ three premolars, there
+ should be four.]
+
+In the Arctoidea the dentition is not so typically carnivorous as in
+the Æluroidea and Cynoidea. In the bears, Ursidae, the molars have
+broad flat tuberculated crowns (fig. 85). The dental formula in
+_Ursus_ is _i_ 3/3 _c_ 1/1 _pm_ 4/4 _m_ 2/3, total 42. The upper
+carnassial (fig. 84, III.) differs from that of the Æluroidea and
+Cynoidea in having no inner lobe supported on a third root. In the
+large group of Mustelidae there are generally two molars in the lower
+and one in the upper jaw. The grinding teeth commonly have large,
+flattened, more or less tuberculated crowns, and the upper molar may
+be as large or much smaller than the carnassial.
+
+In the _CREODONTA_ there are no specially differentiated carnassial
+teeth.
+
+[Illustration FIG. 86. LEFT MANDIBULAR RAMUS OF THE SEA LEOPARD
+(_Ogmorhinus leptonyx_) WITH THE ROOTS OF THE TEETH EXPOSED × 1/3.
+(Camb. Mus.)
+
+ 1. condyle.
+ 2. coronoid process.
+ _i_ 3. third incisor.
+ _c._ canine.
+ _pm_ 1, _pm_ 4. first and fourth premolars.
+ _m._ molar.]
+
+In the _PINNIPEDIA_ the dentition differs considerably from that of
+the Carnivora vera. The milk dentition is always vestigial, and the
+teeth are frequently absorbed before birth. There are four premolars
+and one molar, forming an uniform series of cheek teeth, all of which
+except in the Walrus have compressed and pointed, never flattened,
+crowns. There is no special carnassial tooth, and the incisors are
+always fewer than 3/3. In _Otaria_ the dentition is
+
+ _i_ 3/2 _c_ 1/1 _pm_ 4/4 _m_ 1 or 2/1, total 34 or 36.
+
+In the Walrus the upper canines form immense tusks. The other teeth
+are all small and one-rooted, and the molars have flat crowns. In the
+true seals the dentition is strikingly piscivorous, the cheek teeth
+often having accessory cusps (fig. 86).
+
+The INSECTIVORA are diphyodont and heterodont, having well-developed
+rooted teeth. The canines are usually weak, the incisors pointed, and
+those of the two jaws often meet like a pair of forceps. The crowns of
+the molars are characteristically studded with short cusps. Some
+genera, such as _Gymnura_ and the mole, _Talpa_, have the regular
+mammalian dentition. In the hedgehog, _Erinaceus_, the dentition is
+
+ _i_ 3/2 _c_ 1/1 _pm_ 3/2 _m_ 3/3, total 36.
+
+In the genus _Sorex_ (Shrews) the teeth differ in the following two
+marked respects from those of most other Monodelphia, (1) they are
+monophyodont, (2) the lower incisors sometimes become fused to the
+jaws. Most Insectivora have square molar teeth, but in _Potamogale_,
+_Chrysochloris_, _Solenodon_ and the Centetidae the molar teeth are
+triangular in section. Four molars occur in _Centetes_.
+
+In the aberrant genus _Galeopithecus_ the dentition is _i_ 2/3 _c_ 1/1
+_pm_ 2/2 _m_ 3/3, total 34. The upper incisors are placed at some
+distance from the anterior end of the jaw, and the outer upper
+incisors and canines of both jaws have two roots,--a very unusual
+character. The lower incisors are deeply grooved or pectinated in the
+same way as are the lower incisors of _Procavia_. The upper incisors
+and canines of both jaws bear many cusps, and are very similar in
+appearance to the cheek teeth of some Seals.
+
+The dentition of the CHIROPTERA is diphyodont and heterodont, and the
+dental formula never exceeds
+
+ _i_ 2/3 _c_ 1/1 pm 3/3 _m_ 3/3, total 38.
+
+The milk teeth are very slender and have sharp recurved cusps; they
+are quite unlike the permanent teeth. The permanent teeth are of two
+types. In the Insectivorous forms the molar teeth are cusped, and
+resemble those of Insectivora. In the blood-sucking Vampire bat
+_Desmodus_, the teeth are peculiarly modified; the canines and the
+single pair of upper incisors are much enlarged and exceedingly sharp,
+while all the other teeth are much reduced in size.
+
+In the Frugivorous bats the molar teeth have nearly always smooth
+crowns. The dental formula in the chief genus _Pteropus_ is _i_ 2/2
+_c_ 1/1 _pm_ 3/3 _m_ 2/3, total 34.
+
+The PRIMATES have a diphyodont and heterodont dentition, generally of
+an omnivorous type, with cheek teeth adapted for grinding. The
+incisors are generally 2/2, and the molars, except in the Hapalidae,
+are 3/3. In the Lemurs the upper canines are large, and the lower
+incisors slender and directed almost horizontally forwards. The Aye
+Aye, _Chiromys_, has the following singular dentition: _i_ 1/1 _c_ 0/0
+_pm_ 1/0 _m_ 3/3, total 18. The incisors much resemble those of
+rodents having persistent pulps, and enamel only on the anterior face.
+
+In Man and in the Anthropoid and Old World Apes the dental formula is
+always _i_ 2/2 _c_ 1/1 _pm_ 2/2 _m_ 3/3, total 32.
+
+In the Cebidae there is an extra premolar in each jaw bringing the
+number up to 36. In the Hapalidae, as in the Cebidae, there is a third
+premolar, but the molars are reduced to 2/2. Man is the only Primate
+that has the teeth arranged in a continuous series. In all the others
+there is a gap or diastema of larger or smaller size between the
+incisors and canines. In all except man also the canines are enlarged,
+especially in the males.
+
+The Exoskeletal structures of mammals may be summarised in the
+following table:
+
+ I. _Epidermal exoskeletal structures._
+
+ 1. Hairs (_a_) ordinary hair,
+ (_b_) vibrissae and bristles,
+ (_c_) spines of hedgehog, porcupine, _Echidna_,
+ _Centetes_, _Acanthomys_.
+ 2. Scales { of Manidae,
+ { on tails of rats, beavers, &c.
+ 3. Horns of Rhinoceros.
+ 4. Horns of Bovine Ruminants.
+ 5. Nails, claws, hoofs.
+ 6. Spurs of male _Ornithorhynchus_ and _Echidna_.
+ 7. Horny beak and teeth of _Ornithorhynchus_.
+ 8. Horny pads on jaws of Sirenians and Ruminants.
+ 9. Baleen of whales.
+ 10. Enamel of teeth.
+
+ II. _Dermal exoskeletal structures._
+
+ 1. Dentine and cement of teeth.
+ 2. Bony scutes of Armadillos.
+
+
+ENDOSKELETON.
+
+VERTEBRAL COLUMN.
+
+CERVICAL VERTEBRAE.
+
+The cervical vertebrae of all mammals have certain characters in
+common. However long the neck may be, the number of cervical
+vertebrae, with very few exceptions, is seven. Movable ribs are
+generally absent, and if present are small and do not reach the
+sternum. The transverse processes are generally wide but not long, and
+are perforated near the base by the vertebrarterial canals, through
+which the vertebral arteries pass; they generally bear downwardly
+directed inferior lamellae which are sometimes as in the seventh human
+cervical seen to ossify from centres distinct from those forming the
+rest of the transverse process, and are really of the nature of ribs.
+The atlas and axis always differ much from the other vertebrae.
+
+We may pass now to the special characters of the cervical vertebrae in
+the different groups. In MONOTREMES and MARSUPIALS the number of
+cervical vertebrae is always seven. With the exception of the atlas of
+_Echidna_ the cervical vertebrae of Monotremes are without
+zygapophyses. In Monotremes the transverse processes ossify from
+centres distinct from that forming the body, and remain suturally
+connected with the rest of the vertebra until the adult condition is
+reached. The method of the ossification of the atlas in Marsupials
+varies considerably, thus in some forms such as the Wombats
+(_Phascolomys_) there is an unossified gap in the middle of the
+inferior arch of the atlas, which may remain permanently open; in
+_Thylacinus_ this gap is filled up by a distinct heart-shaped piece of
+bone, while in _Didelphys_ and _Perameles_ the atlas is ossified below
+in the same way as in other mammals. In _Notoryctes_ the second to
+sixth cervical vertebrae are ankylosed together.
+
+The cervical vertebrae of the EDENTATA have some remarkable
+peculiarities. In the three-fingered Sloth, _Bradypus_, there are nine
+cervical vertebrae, all except the last of which have their transverse
+processes perforated by the vertebrarterial canals. In a two-fingered
+sloth, _Choloepus hoffmanni_, there are only six cervical vertebrae.
+In the Megatheriidae, Anteaters (Myrmecophagidae), Pangolins
+(Manidae), and Aard Varks (Orycteropodidae), the cervical vertebrae
+are normal, but in the Armadillos (Dasypodidae), and still more in the
+Glyptodonts, several of them are commonly fused together. The fusion
+affects not only the centra, but also the neural arches, so that the
+neural canals form a continuous tube.
+
+In the Glyptodonts there is a complex joint at the base of the neck to
+allow the partial retraction of the head within the carapace. This
+arrangement recalls that in Tortoises.
+
+As a rule the SIRENIA possess seven short cervical vertebrae, not
+fused together and not presenting any marked peculiarities. In
+_Manatus_ however there are only six cervical vertebrae and they are
+very variable.
+
+[Illustration FIG. 87. CERVICAL VERTEBRAE OF A YOUNG FIN WHALE
+(_Balaenoptera musculus_) × 1/10. (Camb. Mus.)
+
+ 1. surface on the atlas for articulation
+ with the occipital condyle of the skull.
+ 2. foramen for exit of the first spinal nerve.
+ 3. upper transverse process.
+ 4. lower transverse process.
+
+In the fresh specimen these two transverse processes are united by
+cartilage, in adult individuals the whole transverse process is
+ossified.
+
+ 5. epiphyses of centrum.
+ 6. neural spine.]
+
+In the CETACEA there are invariably seven cervical vertebrae, but they
+are always very short and are frequently even before birth fused
+together by their centra into one continuous mass (see fig. 67).
+Sometimes the last one or two are free. In the Rorquals
+(_Balaenoptera_) however, the cervical vertebrae are quite separate
+and distinct (fig. 87), and in the fluviatile Odontoceti,
+_Platanista_, _Inia_, and _Pontoporia_, and also in _Beluga_ and
+_Monodon_, though very short they are free. In _Physeter_ the first
+vertebra is free while the others are fused. An odontoid process is
+not commonly present even in Cetaceans with free cervical vertebrae,
+but a very short one occurs in the Rorquals. The cervical vertebrae of
+Rorquals give off on each side two transverse processes (fig. 87, 3
+and 4) which enclose between them a wide space. These processes are
+not completely ossified till the animal is adult.
+
+[Illustration FIG. 88. ATLAS (B) AND AXIS (A) VERTEBRAE OF AN OX (_Bos
+taurus_) × 1/4. (Camb. Mus.)
+
+ 1. neural canal.
+ 2. transverse process.
+ 3. surfaces for articulation with the occipital condyles of the skull.
+ 4. spout-like odontoid process.
+ 5. hypapophysis.
+ 6. anterior opening of the vertebrarterial canal.
+ 7. foramen for the exit of the second spinal nerve.
+ 8. neural spine.
+ 9. postzygapophysis.]
+
+In all UNGULATA the number of cervical vertebrae is seven. Among the
+Artiodactyla two forms of the odontoid process of the axis occurs; in
+the Suina and Tragulina it is conical, in the Ruminantia and Tylopoda
+it is spout-like (fig. 88, 4). The atlas in the Suina and to a less
+extent in the Ruminantia has long flattened transverse processes, and
+the remaining cervical vertebrae are opisthocoelous. Those of the
+Giraffe and Llama (fig. 103) are noticeable for their great length. In
+the Tylopoda the posterior half of the vertebrarterial canal is
+confluent with the neural canal.
+
+The Perissodactyla have remarkably opisthocoelous cervical vertebrae.
+Those of _Macrauchenia_ have the posterior half of the vertebrarterial
+canal confluent with the neural canal as in Tylopoda. In the
+Proboscidea they are short flattened discs slightly opisthocoelous;
+the axis and seventh vertebra and to a less extent the sixth have high
+neural spines.
+
+In the RODENTIA the atlas generally has broad wing-like transverse
+processes, and the axis a large and long neural spine, while the
+odontoid process is much developed. In the Jerboas (_Dipus_) all the
+cervical vertebrae except the atlas are fused together, a condition
+recalling that in armadillos.
+
+In the CARNIVORA the wings of the atlas are well developed (fig. 69,
+A, 1), and it is deeply cupped for articulation with the condyles of
+the skull. The axis has a long odontoid process and a high compressed
+neural spine (fig. 69, B, 4). The third to sixth cervical vertebrae
+have large transverse processes with prominent perforated inferior
+lamellae, whose ventral margins in the third and fourth vertebrae
+diverge as they pass backwards, while in the fifth they are parallel
+and in the sixth convergent. The transverse processes of the seventh
+vertebra have no inferior lamellae and are not perforated.
+Metapophyses are often developed.
+
+In the INSECTIVORA the cervical vertebrae vary considerably. The
+neural spines except in the case of the axis are generally very small
+and in the Shrews and Moles the neural arches are exceedingly slender.
+
+In the CHIROPTERA all the cervical vertebrae are broad and short with
+slender neural arches.
+
+PRIMATES. In Man the cervical vertebrae have short blunt transverse
+processes and small often bifid neural spines. The neural and
+vertebrarterial canals are large. The atlas forms a ring surrounding a
+large cavity, and has a very slender inferior arch and small
+transverse processes. Traces of a pro-atlas have been described in
+_Macacus_ and _Cynocephalus_. The axis has a prominent spine and
+odontoid process and short transverse processes. In most Primates the
+cervical vertebrae are very similar to those of man, but the inferior
+lamellae of the transverse processes are better developed. In the
+Anthropoid Apes the neural spines are as a rule much elongated.
+
+
+THORACO-LUMBAR, OR TRUNK VERTEBRAE.
+
+In the MONOTREMATA there are nineteen thoraco-lumbar vertebrae,
+sixteen (_Echidna_) or seventeen (_Ornithorhynchus_) of which bear
+ribs. The transverse processes are very short and do not articulate
+with the ribs, which are united to the centra only.
+
+In the MARSUPIALIA there are always nineteen thoraco-lumbar vertebrae,
+thirteen of which generally bear ribs. The lumbar vertebrae frequently
+have large metapophyses and anapophyses, these being specially well
+seen in the Kangaroos and Koala (_Phascolarctus_).
+
+The EDENTATA are very variable as regards their trunk vertebrae. The
+two genera of Sloths differ much as regards the number, for while
+_Bradypus_ has only nineteen, fifteen or sixteen of which bear ribs,
+_Choloepus_ has twenty-seven, twenty-four of which are thoracic, and
+bear ribs. In _Bradypus_ a small outgrowth from the transverse process
+articulates with the neural arch of the succeeding vertebra. In both
+genera the neural spines are all directed backwards.
+
+In the Megatheriidae as in the sloths the neural spines are all
+directed backwards, and in the lumbar region additional articulating
+surfaces occur, better developed than are those in _Bradypus_.
+
+In the ant-eaters (Myrmecophagidae) there are seventeen or eighteen
+thoraco-lumbar vertebrae, all of which except two or three bear ribs.
+The posterior thoracic and anterior lumbar vertebrae articulate in a
+very complex fashion, second, third, and fourth pairs of zygapophyses
+being progressively developed in addition to the ordinary ones, as the
+vertebrae are followed back.
+
+In the Armadillos the lumbar vertebrae have long metapophyses which
+project upwards and forwards and help to support the carapace. In
+_Glyptodon_ almost all the thoraco-lumbar vertebrae are completely
+ankylosed together.
+
+In the Manidae there are no additional zygapophyses but the normal
+ones of the lumbar and posterior thoracic regions are very much
+developed, the postzygapophyses being semi-cylindrical and fitting
+into the deep prezygapophyses of the succeeding vertebra.
+
+In the SIRENIA the number of lumbar vertebrae is very small; in the
+dugong there are nineteen thoracic and four lumbar, and in the manatee
+seventeen thoracic and two lumbar.
+
+In the CETACEA the number of thoracic vertebrae varies from nine in
+_Hyperoödon_ to fifteen or sixteen in _Balaenoptera_, and the number
+of lumbar vertebrae from three in _Inia_ to twenty-four or more in
+_Delphinus_. The lumbar vertebrae are often very loosely articulated
+together and the zygapophyses sometimes as in the Dolphins are placed
+high up on the neural spines. The centra are large, short in the
+anterior region but becoming longer behind. The epiphyses are
+prominent, and so are the neural spines and to a less extent the
+metapophyses. The transverse processes are well developed, anteriorly
+they arise high up on the neural arch, but when the vertebral column
+is followed back they come gradually to be placed lower down, till in
+the lumbar region they project from the middle of the centra. This can
+be well traced in the Porpoise (_Phocaena_). In the Physeteridae the
+transverse processes of the anterior thoracic vertebrae are similar to
+those of most Cetacea, but when followed back, instead of shifting
+their position on the vertebrae, they gradually disappear, and other
+processes gradually arise from the point where the capitulum of the
+rib articulates.
+
+UNGULATA. In the Ungulata vera the thoraco-lumbar vertebrae are
+slightly opisthocoelous. The anterior thoracic vertebrae commonly have
+exceedingly high backwardly-projecting neural spines (fig. 89, 1); but
+those of the lumbar and posterior thoracic vertebrae often point
+somewhat forwards so that the spines all converge somewhat to a point
+called the _centre of motion_ (cp. fig. 101). In the Artiodactyla
+there are always nineteen thoraco-lumbar vertebrae, and in the
+Perissodactyla twenty-three.
+
+_Procavia_ sometimes has thirty thoraco-lumbar vertebrae, a greater
+number than occurs in any other terrestrial mammal; twenty-two of
+these are thoracic and eight lumbar. In _Phenacodus_ the convergence
+of the neural spines to a centre of motion is well seen.
+
+[Illustration FIG. 89. FIRST AND SECOND THORACIC VERTEBRAE OF AN OX
+(_Bos taurus_) × 1/3. (Camb. Mus.)
+
+ 1. neural spine.
+ 2. neural canal.
+ 3. prezygapophysis.
+ 4. facet for articulation with the tuberculum of the rib.
+ 5. facet for articulation with the capitulum of the rib.
+ 6. postzygapophysis.
+ 7. foramen for exit of spinal nerve.]
+
+In the Proboscidea there are twenty-three thoraco-lumbar vertebrae, of
+which nineteen or twenty bear ribs.
+
+In the RODENTIA there are generally nineteen thoraco-lumbar vertebrae
+but occasionally the number rises as high as twenty-five. In the Hares
+(Leporidae) the number is nineteen, twelve or thirteen of which are
+thoracic. The anterior thoracic vertebrae have short centra and high
+backwardly-directed neural spines, the lumbar vertebrae have large
+forwardly-and downwardly-directed transverse processes with expanded
+ends. Metapophyses, anapophyses and hypapophyses are all present. In
+the Agouti (_Dasyprocta_) the convergence of the neural spines to a
+centre of motion is very strongly marked.
+
+In the CARNIVORA the trunk vertebrae are nearly always twenty or
+twenty-one in number; in the genera _Felis_ and _Canis_ thirteen of
+these are thoracic and seven lumbar. The anterior thoracic vertebrae
+have long backwardly-projecting neural spines, while the posterior
+thoracic and lumbar vertebrae have shorter and thicker neural spines
+which project slightly forwards. In the Pinnipedia there is no change
+in the direction of the neural spines, and anapophyses are but little
+developed.
+
+In the INSECTIVORA the number of trunk vertebrae varies much from
+nineteen--thirteen thoracic and six lumbar--in _Tupaia_, to
+twenty-four--nineteen thoracic and five lumbar--in _Centetes_. The
+development of the various processes varies in accordance with the
+habits of the animals, being great in the active forms, slight in the
+slowly moving or burrowing forms. In _Talpa_ and _Galeopithecus_ the
+intervertebral discs of the thoraco-lumbar region instead of being
+cartilaginous have ossified forming inter centra, a condition met with
+in very few mammals.
+
+In the CHIROPTERA there are seventeen or eighteen thoraco-lumbar
+vertebrae, eleven to fourteen of which may bear ribs. The development
+of processes is slight.
+
+Among PRIMATES the number of trunk vertebrae is generally nineteen, of
+which twelve to fourteen bear ribs; in man and the Gorilla and
+Chimpanzee the number is, however, seventeen, and in the Orang
+(_Simia_) sixteen. In some of the Lemuroidea there are as many as
+twenty-three or twenty-four. In most cases the neural spines converge
+more or less to a centre of motion, and this is especially marked in
+some of the Lemurs; it does not occur in man and the anthropoid apes.
+
+
+SACRAL AND CAUDAL VERTEBRAE.
+
+At the posterior end of the trunk in all mammals a certain number of
+vertebrae are found fused together forming the sacrum. But of these
+only two or three answer to the definition of true sacral vertebrae in
+being united to the ilia by small ribs. The others which belong to the
+caudal series may be called pseudosacral vertebrae. In different
+individuals of the same species it sometimes happens that different
+vertebrae are attached to the pelvis and form the sacrum. Sometimes
+even different vertebrae are attached to the pelvis at successive
+periods in the life history of the individual. This is owing to a
+shifting of the pelvis and has been especially well seen in man. In
+young human embryos the pelvis is at a certain stage attached to
+vertebra 30, but as development goes on it becomes progressively
+attached to the twenty-ninth, twenty-eighth, twenty-seventh,
+twenty-sixth and twenty-fifth vertebrae. As the attachment to these
+anterior vertebrae is gained, the attachment to the posterior ones
+becomes lost, so that in the adult the pelvis is generally attached to
+vertebrae 25 and 26. But there are no absolutely pre-determined sacral
+vertebrae, as sometimes the pelvis does not reach vertebra 25,
+remaining attached to vertebrae 26 and 27; sometimes it becomes
+attached even to vertebra 24. This shifting of the pelvis is seen in
+_Choloepus_ in a more marked degree even than in man.
+
+Of the MONOTREMATA, _Ornithorhynchus_ has two sacral vertebrae
+ankylosed together, while _Echidna_ has three or four[164].
+
+In MARSUPIALIA as a rule only one vertebra is directly united to the
+ilia, but one or two more are commonly fused to the first. In the
+Wombats there may be as many as four or five vertebrae fused together
+in the sacral region. In _Notoryctes_ there is extensive fusion in the
+sacral region, six vertebrae, owing mainly to the great development of
+their metapophyses, being united with one another, and with the ilia,
+and the greater part of the ischia.
+
+In most EDENTATA there is an extensive fusion of vertebrae in the
+sacral region. This is especially marked in the Armadillos and
+Megatheriidae, and to a less extent in the Sloths and Aard Varks.
+
+In the SIRENIA the vestigial pelvis is attached by ligament to the
+transverse processes of a single vertebra, which hence may be regarded
+as sacral.
+
+In CETACEA there is no sacrum, the vestigial pelvis not being
+connected with the vertebral column.
+
+In most UNGULATA the sacrum consists of one large vertebra united to
+the ilia, and having a varying number of smaller vertebrae fused with
+it behind.
+
+The same arrangement obtains in most RODENTIA, but in the Beavers
+(Castoridae) all the fused vertebrae are of much the same size, the
+posterior ones having long transverse processes which nearly meet the
+ilia.
+
+In CARNIVORA there may be two sacral vertebrae as in the Hyaena, three
+as in the Dog, four or five as in Bears and Seals.
+
+In INSECTIVORA from three to five are united, while in many CHIROPTERA
+all the sacral and caudal vertebrae have coalesced. Among PRIMATES, in
+Man and Anthropoid Apes there are usually five fused vertebrae forming
+the sacrum, but of these only two or three are connected to the ilia
+by ribs. In most of the other Anthropoidea there are two or three
+fused vertebrae, and in the Lemuroidea two to five.
+
+FREE CAUDAL VERTEBRAE. The free caudal vertebrae vary greatly in
+number and character. When the tail is well developed, the anterior
+vertebrae are comparatively short and broad, with well-developed
+neural arches and zygapophyses; but as the tail is followed back, the
+centra gradually lengthen and become cylindrical, and at the same time
+the neural arches and all the processes gradually become reduced and
+disappear, so that the last few vertebrae consist of simple rod-like
+centra. Chevron bones are frequently well-developed.
+
+Of the MONOTREMES _Echidna_ has twelve caudal vertebrae, two of which
+bear irregular chevron bones. In _Ornithorhynchus_ there are twenty or
+twenty-one caudal vertebrae with well-developed hypapophyses, but no
+chevron bones.
+
+In MARSUPIALS there is great diversity as regards the tail. In the
+Wombat and Koala the tail is small and without chevron bones. In most
+other Marsupials it is very long, having sometimes as many as
+thirty-five vertebrae in the prehensile-tailed opossums. In the
+Kangaroos the tail is very large and stout. Chevron bones are almost
+always present, and in _Notoryctes_ are large and expanded.
+
+Most EDENTATES have large tails with well-developed chevron bones. The
+length of the tail varies greatly from the rudimentary condition in
+Sloths to that in the Pangolins, one of which has forty-six to
+forty-nine caudal vertebrae--the largest number in any known mammal.
+Chevron bones are much developed, sometimes they are Y-shaped,
+sometimes as in _Priodon_, they have strong diverging processes. The
+caudal vertebrae of Glyptodonts, though enclosed in a continuous bony
+sheath, have not become ankylosed together.
+
+The SIRENIA have numerous caudal vertebrae with wide transverse
+processes. In the CETACEA also the tail is much developed, and the
+anterior vertebrae have large chevron bones and prominent straight
+transverse processes; the posterior caudal vertebrae, which in life
+are enclosed in the horizontally expanded tail fin, are without
+transverse processes.
+
+In UNGULATA the tail is simple, formed of short cylindrical vertebrae,
+which in living forms are never provided with chevron bones. The
+number of caudal vertebrae varies from four, sometimes met with in
+_Procavia_, to thirty-one in the Elephant. The tail is exceedingly
+long in _Anoplotherium_ and in _Phenacodus_, in which there are thirty
+caudal vertebrae.
+
+In RODENTIA the tail is variable. In the Hares, Guinea pig (_Cavia_)
+and _Capybara_ it is very small, in _Pedetes_ and the Beaver it is
+very long and has well-developed chevron bones.
+
+Most of the CARNIVORA except the Bears and Seals have very long tails,
+the greatest number of vertebrae, thirty-six, being met with in
+_Paradoxurus_. Bears have only eight to ten caudal vertebrae. Chevron
+bones are not often much developed.
+
+In INSECTIVORA the tail is very variable as regards length, the number
+of vertebrae varying from eight in _Centetes_ to forty-three in
+_Microgale_.
+
+In CHIROPTERA the tail is sometimes quite rudimentary, and as in
+_Pteropus_, composed of a few coalesced vertebrae, sometimes it is
+formed of a large number of slender vertebrae.
+
+In PRIMATES also the tail is very variable. In Man all the four caudal
+vertebrae are rudimentary and are fused together, forming the
+_coccyx_. In the Anthropoid apes, too, there are only four or five
+caudal vertebrae. In many monkeys of both the eastern and western
+hemispheres the tail is very long, having thirty-three vertebrae in
+_Ateles_, in which genus it is also prehensile. Chevron bones are
+present in all Primates with well-developed tails. In the Lemuroidea
+the number of caudal vertebrae varies from seven to twenty-nine.
+
+
+FOOTNOTES:
+
+[145] See W.H. Flower, "Remarks on the homologies and notation of the
+teeth in Mammalia," _J. Anat. and Physiol. norm. path._, Vol. III., p.
+262; R. Owen, _Odontography_, London, 1840--45; C.S. Tomes, _Manual of
+Dental Anatomy_, London, 1876. See also H.F. Osborn, "Recent
+researches on succession of teeth in Mammals," _Amer. Natural._,
+XXVII., p. 493, and "Rise of Mammalia in N. America," _Stud. Biol.
+Lab. Columb. Coll._, Zool. I., no. 2.
+
+[146] See E.B. Poulton, _P.R.S._, Feb. 1888, and _Quart. J. Micr.
+Sci._, Vol. XXIX. 1889; also Oldfield Thomas, _P.R.S._, XLVI. (1889).
+
+[147] W.H. Flower, _Phil. Trans._, vol. 156, pp. 631--641, 1867; also
+Oldfield Thomas, _Phil. Trans._, pp. 443--462, 1887.
+
+[148] C. Röse, _Anat. Anz._ VII., p. 639.
+
+[149] W. Kükenthal, _Anat. Anz._ VI., p. 364.
+
+[150] See p. 348.
+
+[151] W. Leche, _Morph. Jahrb._ XX., pp. 113--142 (1893).
+
+[152] E.C. Stirling, _P.Z.S._ 1891, p. 327.
+
+[153] O. Thomas, _P.Z.S._, 1895, p. 870.
+
+[154] F. Ameghino, _Bull. Ac. Argen._ XII. p. 437. According to H.
+Burmeister, _Annal. Mus. Buenos Aires_, III. 401 (1891), enamel does
+not occur, osteodentine having been mistaken for it.
+
+[155] E. Ballowitz, _Arch. Mikr. Anat._ XL. p. 133.
+
+[156] See Oldfield Thomas, _P.R.S._, vol. XLVII., p. 246 (1890).
+
+[157] J. Taeker, "_Fur Kenntniss der Odontogenese bei Ungulaten_."
+Dorpat, 1892.
+
+[158] See p. 345.
+
+[159] See p. 345.
+
+[160] According to H.F. Osborn, _Amer. Natural._, XXVI. p. 763, a
+number of not very closely allied forms have been included under
+_Lophiodon_.
+
+[161] C. Earle, _J. Ac. Philad._, vol. IX., 1892, p. 267.
+
+[162] _Encyclopaedia Britannica_, article _Mammalia_, p. 424.
+
+[163] See T.H. Huxley, "The dental and cranial characters of the
+Canidae," _P.Z.S._, 1880, p. 238.
+
+[164] G.B. Howes, _Journ. of Anat. and Phys._ XXVII., p. 544.
+
+
+
+
+CHAPTER XXIII.
+
+GENERAL ACCOUNT OF THE SKELETON IN MAMMALIA (CONTINUED).
+
+
+THE SKULL AND APPENDICULAR SKELETON.
+
+
+THE SKULL.
+
+MONOTREMATA. In both genera the cranium is thin-walled, has a fairly
+large cavity, and is very smooth and rounded externally. The sutures
+between many of the bones early become obliterated in a manner
+comparable to that in birds, and the facial portion of the skull is
+much prolonged.
+
+In _Echidna_ the face is drawn out into a gradually tapering rostrum,
+formed mainly by the premaxillae, maxillae and nasals. The zygomatic
+arch is very weak, and the palate extends very far back. The tympanic
+forms a slender ring. The mandible is extremely slight, with no
+ascending portion, and but slight traces of the coronoid process and
+angle. The hyoid has a wide basi-hyal and stout thyro-hyals, while the
+anterior cornua are slender, and include ossified epi-hyals and
+cerato-hyals.
+
+In _Ornithorhynchus_ the zygomatic arch is much stouter than in
+_Echidna_. The face is produced into a wide beak, mainly supported by
+the premaxillae, between whose diverging anterior ends there is a
+dumb-bell-shaped bone. The maxillae are flattened below, and each
+bears a large horny tooth, which meets a corresponding structure borne
+on a surface near the middle of the mandible. The mandible is
+considerably stouter than in _Echidna_, but the angle and coronoid
+process are but little developed. The infra-orbital foramen and the
+inferior dental and mental foramina of the mandible are all very
+large.
+
+[Illustration FIG. 90. HALF FRONT VIEW[165] OF THE SKULLS OF A
+TASMANIAN WOLF (_Thylacinus cynocephalus_) (to the left) × 3/8; AND OF
+A HAIRY-NOSED WOMBAT (_Phascolomys latifrons_) (to the right) × 3/8.
+(Camb. Mus.)
+
+ 1. premaxillae.
+ 2. nasal.
+ 3. frontal.
+ 4. infra-orbital foramen.
+ 5. lachrymal.
+ 6. jugal.
+ 7. coronoid process of the mandible.
+ 8. lachrymal foramen.
+ i. 1. first upper incisor.
+ C. canine.]
+
+MARSUPIALIA. The skulls of the various types of the Marsupials
+frequently bear a strong superficial resemblance to those of some of
+the different groups of placental mammals. Thus the skull of the
+Dasyuridae resembles that of the Carnivora, the resemblance being most
+marked between the skulls of _Thylacinus_ and the dog. The skull of
+_Notoryctes_ is strongly suggestive of that of an Insectivore, and
+that of other Marsupials such as the wombat, recalls equally the
+characteristic features of a Rodent's skull. But, however much they
+may differ from one another, the skulls of all Marsupials agree in the
+following respects. (1) The brain cavity, and especially the cerebral
+fossa, has a very small comparative size. (2) The nasals are always
+large, and the mesethmoid is extensively ossified, and terminated by a
+prominent vertical edge. (3) Processes from the jugal and frontal in
+living forms never meet and enclose the orbit, but the zygomatic arch
+is always complete. (4) The jugal always extends back to form part of
+the glenoid fossa. (5) The lachrymal canal opens either external to or
+upon the margin of the orbit, and the nasal processes of the
+premaxillae never quite reach the frontals. (6) The posterior part of
+the palate is commonly pierced by large oval vacuities. (7) The
+tympanic is small and never fused to the bones of the cranium. (8) The
+carotid canal perforates the basisphenoid and not the tympanic bulla.
+(9) The optic foramen and sphenoidal fissure are confluent. (10) In
+every case except _Tarsipes_ the angle of the mandible is more or less
+inflected.
+
+The skull of the extinct _Thylacoleo_ differs from that of all other
+Marsupials in the fact that the postorbital bar is complete. The hyoid
+is constructed on much the same plan in all Marsupials. It consists of
+a small basi-hyal, a pair of broad cerato-hyals, and a pair of strong
+thyro-hyals. The epi-hyals and stylo-hyals are generally unossified.
+
+EDENTATA. In Sloths (Bradypodidae) the sutures become early
+obliterated, the cranial portion of the skull is rather high, and the
+facial portion very short. The lachrymal is very small, and its canal
+opens outside the orbit. The zygomatic arch is incomplete, and the
+jugal (fig. 91, 5) is curiously forked, but in a manner differing in
+the two genera. The premaxillae are very small,--in _Bradypus_ quite
+vestigial. The mandible is well developed, the angle being specially
+marked in _Bradypus_. In _Choloepus_ the symphysial part is drawn out
+in a somewhat spout-like manner (fig. 91, 6). In both genera the
+thyro-hyals are ankylosed with the basi-hyal.
+
+[Illustration FIG. 91. SKULL OF A TWO-FINGERED SLOTH (_Choloepus
+didactylus_) × 1/2. (Camb. Mus.)
+
+ 1. anterior nares.
+ 2. postorbital process of the frontal.
+ 3. coronoid process.
+ 4. angle of the mandible.
+ 5. jugal.
+ 6. spout-like prolongation of the mandible.]
+
+In _Megatherium_ the general appearance of the skull is distinctly
+sloth-like, but the facial portion is more elongated, partly owing to
+the development of a prenasal bone, and the zygomatic arch is
+complete. The mandible is very deep in the middle, and is drawn out
+into a long spout-like process in front.
+
+Anteaters (Myrmecophagidae) have a much modified skull, and this is
+especially the case in the Great Anteater, _Myrmecophaga_. The skull
+is smooth and evenly-rounded, in these respects recalling that of
+_Echidna_, but it is longer and tapers much more gradually than in
+_Echidna_. The occipital condyles are remarkably large. The premaxillae
+are small, and the long rostrum is chiefly composed of the maxillae and
+nasals with the mesethmoid and vomer. The zygomatic arch is
+incomplete, and there is no trace of a separation between the orbit
+and the temporal fossa. The palate is much elongated, the pterygoids
+meeting in the middle line just like the palatines. The mandible is
+very long and slender, there being no definite coronoid process, and a
+short and slight symphysis. The hyoid arch is noticeable for the
+length of the anterior cornu.
+
+In the Armadillos (Dasypodidae) the skull varies a good deal in shape,
+but the facial portion is always tapering and depressed. The zygomatic
+arch is complete. In _Dasypus_ and _Chlamydophorus_ the tympanic bulla
+is well ossified.
+
+In the Glyptodontidae the skull is very short and deep; the zygomatic
+arch is complete, and has a long downwardly projecting maxillary
+process. The mandible is massive, and has a very high ascending
+portion.
+
+In the Manidae the skull is smooth and rounded, the zygomatic arch is
+incomplete, and the orbit is inconspicuous. The palate is long and
+narrow, but the pterygoids do not take part in its formation. The
+mandible is slightly developed and has no angle or coronoid process.
+
+In _Orycteropus_ the zygomatic arch is complete, and there is a small
+postorbital process to the frontal. The mandible is well-developed,
+having a coronoid process and definite ascending portion, and the
+hyoid is well ossified.
+
+SIRENIA. The skull, and especially the brain case of all Sirenia, is
+remarkable for the general density of the component bones, which,
+though often very thick, are without air sinuses. It is noticeable
+also for the roughness of the bones, and the irregular manner in which
+they are united together.
+
+[Illustration FIG. 92. LATERAL VIEW OF THE SKULL OF _Rhytina stelleri_
+× 1/8. (Brit. Mus.)
+
+ 1. frontal.
+ 2. parietal.
+ 3. zygomatic process of the squamosal.
+ 4. squamosal.
+ 5. exoccipital.
+ 6. occipital condyle.
+ 7. pterygoid process of the alisphenoid.
+ 8. jugal.
+ 9. premaxillae.
+ 10. angle of the mandible.
+ 11. maxillae.]
+
+The cranial cavity is decidedly small, the reduction being specially
+noticeable in the cerebral fossa, which is not much larger than the
+cerebellar fossa. The foramen magnum is large, and the dorsal surface
+of the cranium narrow. The zygomatic arch is very strongly developed,
+the squamosal (fig. 92, 4) being especially prominent, and being drawn
+out not only into the zygomatic process, but also into a large
+post-tympanic process which articulates with the exoccipital. At the
+side of the skull between the squamosal, supra-occipital and
+exoccipital, there is a wide vacuity in the cranial wall, partially
+filled up by the very large periotic, which is ankylosed to the
+tympanic, but is not united to any other bones of the skull. The
+foramen lacerum medium is confluent with the foramen lacerum anterius,
+and the two together form an enormous vacuity on the floor of the
+skull, bounded chiefly by the exoccipital, basi-occipital, alisphenoid
+and squamosal. The jugal (fig. 92, 8) is large and in _Manatus_ sends
+up a strong process, which nearly or quite meets the postorbital
+process of the frontal, completing the orbit. In the other Sirenia the
+orbit is completely confluent with the very large temporal fossa. The
+lachrymal in _Manatus_ is very small, but is larger in _Halicore_. The
+premaxillae (fig. 92, 9) are large, but smaller in _Manatus_ than in
+the other genera, in all of which they are curiously bent down in
+front. Their upper margin forms the anterior border of a very large
+aperture lying high on the roof of the skull and extending back for a
+considerable distance. This aperture is formed by the union of the two
+anterior nares. The nasals are quite vestigial or absent, and the
+narial aperture is bounded above by the frontals; in its floor are
+seen the slender vomer and large mesethmoid. The palate is long and
+narrow, and formed mainly by the maxillae; behind it there is a large
+irregular process formed by the union of the palatine, pterygoid, and
+pterygoid plate of the alisphenoid. The mandible is very massive and
+has a very high ascending portion, a rounded angle (fig. 92, 10), and
+a prominent coronoid process; the two rami are firmly ankylosed
+together. The hyoid consists principally of the broad flat basi-hyal;
+the anterior cornua are but slightly ossified, while the thyro-hyals
+are not ossified at all.
+
+CETACEA. The skull in all Cetacea, especially in the Odontoceti, is a
+good deal modified from the ordinary mammalian type.
+
+In the _ARCHAEOCETI_ this modification is less marked than in either
+of the other suborders. The nasals and premaxillae are a good deal
+larger than they are in living forms, and the anterior nares are
+placed further forward. The maxillae do not extend back over the
+frontals, and there is a well-marked sagittal crest.
+
+In the _MYSTACOCETI_ the skull is always quite bilaterally
+symmetrical, and is not so much modified from the ordinary mammalian
+type as in the Odontoceti. The parietals are not, as in the
+Odontoceti, separated by a wide interparietal, but meet; they are,
+however, hidden under the very large supra-occipital. The nasals are
+developed to a certain extent, and the nares, though placed very far
+back and near the top of the head, terminate forwardly-directed narial
+passages. Turbinal bones are also developed to some extent; this fact,
+and the occurrence of a definite though small olfactory fossa
+constituting important distinctions from the Odontoceti. The maxillae
+are large, but do not extend back to cover the frontals as in the
+Odontoceti. The zygomatic process of the squamosal is very large. The
+mandibular rami are not compressed, but are rounded and arched
+outwards, and never meet in a long symphysis.
+
+_ODONTOCETI._ The skull departs widely from the ordinary mammalian
+type. The following description will apply to any of the following
+genera of the Delphinidae, _Phocaena_, _Globicephalus_,
+_Lagenorhynchus_, _Delphinus_, _Tursiops_, _Prodelphinus_, _Sotalia_.
+
+The upper surface of the skull is more or less asymmetrical. The
+cerebral cavity is high, short and broad; and formed mainly by the
+cerebral fossa, the olfactory fossa being entirely absent. The
+supra-occipital (fig. 93, 3) is very large, and forms much of the
+posterior part of the roof of the skull. It has the interparietal
+(fig. 93, 7) fused with it, and completely separates the two
+parietals. The frontal (fig. 93, 10) is large and laterally expanded,
+forming the roof of the orbit, but is almost completely covered by an
+extension of the maxillae. The zygomatic arch is very slender, and is
+mainly formed by a rod-like process from the jugal (fig. 93, 15), the
+zygomatic process of the squamosal being short and stout.
+
+The nasal passages are peculiarly modified, instead of passing
+horizontally forwards above the roof of the mouth, they pass upwards
+and even somewhat backwards towards the top of the skull (fig. 93,
+23). They are bounded laterally by two processes from the premaxillae,
+the left of which is shorter than the right. The nasal cavities are
+narrow and without turbinals and the nasals (fig. 93, 19) are almost
+as much reduced as in Sirenia.
+
+[Illustration FIG. 93. A, LATERAL VIEW, AND B, LONGITUDINAL SECTION OF
+THE SKULL OF A YOUNG CA'ING WHALE (_Globicephalus melas_) × 1/6.
+(Brit. Mus.)
+
+ 1. basi-occipital.
+ 2. exoccipital.
+ 3. supra-occipital.
+ 4. basisphenoid.
+ 5. alisphenoid.
+ 6. parietal.
+ 7. interparietal.
+ 8. presphenoid.
+ 9. orbitosphenoid.
+ 10. frontal.
+ 11. mesethmoid.
+ 12. tympanic.
+ 13. periotic.
+ 14. squamosal.
+ 15. jugal.
+ 16. vomer.
+ 17. palatine.
+ 18. pterygoid.
+ 19. nasal.
+ 20. maxillae.
+ 21. premaxillae.
+ 22. mandible.
+ 23. anterior nares.]
+
+In front of the nasal openings the face is prolonged as a narrow beak
+or rostrum of varying length, formed by the maxillae and premaxillae
+surrounding the vomer and large mesethmoid (fig. 93, 11), which sends
+forwards a long partially cartilaginous process, and is fused behind
+with the presphenoid (fig. 93, 8). The basi-occipital (fig. 93, 1) too
+is fused with the basisphenoid. The foramen rotundum is confluent with
+the sphenoidal fissure, and the foramen ovale with the foramen lacerum
+medium and the foramen lacerum posterius. The palate is mainly formed
+by the maxillae; the premaxillae and palatines (fig. 93, 17), though
+both meet in symphyses, forming very little of it. The pterygoids vary
+in size in the different genera, sometimes as in _Lagenorhynchus_ and
+_Delphinus_ meeting in the middle line, sometimes as in _Phocaena_ and
+_Globicephalus_ (fig. 93, 18) being widely separated. The tympanic and
+periotic are not fused together, and the periotic has generally no
+bony union with the rest of the skull. The mandible is rather slightly
+developed, with the rami straight, compressed and tapering to the
+anterior end. The condyle is not raised at all above the edge of the
+ramus; the angle is rounded and the coronoid process is very small.
+_Platanista_ has a curiously modified skull; the rostrum and mandible
+are exceedingly long and narrow, and arising from the maxillae are two
+great plates of bone which nearly meet above.
+
+In the Physeteridae the skull is raised into a very prominent crest at
+the vertex behind the nares. In front of this in _Hyperoödon_ a pair
+of ridges occur, formed by outgrowths from the maxillae. In the old
+male these ridges reach an enormous size and almost meet in the middle
+line. In _Physeter_, the Sperm whale, these ridges are not developed;
+the maxillae and premaxillae unite with the other bones of the crest
+enclosing an enormous half basin-shaped cavity, at the base of which
+are the very asymmetrical anterior narial apertures.
+
+In all living Cetacea the hyoid has the same general shape, consisting
+firstly of a crescentic bone formed by the fusion of the thyro-hyals
+with the basi-hyal, and secondly of the anterior cornu formed
+principally by the strong stylo-hyal.
+
+UNGULATA. None of the distinctive characters separating the Ungulata
+from the other groups of mammals are drawn from the skull. But in the
+Ungulata vera as opposed to the Subungulata a distinguishing feature
+is found in the fact that the lachrymal and jugal form a considerable
+part of the side of the face, and that the jugal always forms the
+anterior part of the zygomatic arch, the maxillae taking no part in it.
+
+
+UNGULATA VERA.
+
+_ARTIODACTYLA._ The skull in Artiodactyla differs from that in
+Perissodactyla in the fact that the posterior end of the nasal is not
+expanded and there is no alisphenoid canal.
+
+The skulls in the different groups of Artiodactyla differ considerably
+from one another.
+
+[Illustration FIG. 94. A, CRANIUM AND B, MANDIBLE OF A PIG (_Sus
+scrofa_) × 1/5. (Camb. Mus.)
+
+ 1. jugal.
+ 2. postorbital process of the frontal.
+ 3. zygomatic process of the squamosal.
+ 4. supra-occipital.
+ 5. glenoid cavity.
+ 6. occipital condyle.
+ 7. foramen magnum.
+ 8. paroccipital process of the exoccipital.
+ 9. tympanic bulla.
+ 10. pterygoid.
+ 11. anterior palatine foramen.
+ 12. palatal plate of maxillae.
+ 13. coronoid process.
+ 14. mandibular condyle.
+ _i_ 1, _i_ 2, _i_ 3. first, second, and third incisors.
+ _c._ canine.
+ _pm_ 1, _pm_ 2, _pm_ 3, _pm_ 4. first, second, third, and fourth
+ premolars.
+ _m_ 1, _m_ 2, _m_ 3. first, second, and third molars.]
+
+The skull of the Pig[166] will be described as illustrative of the
+skull in the Suina. In the Pig as in most Artiodactyla the face is
+bent sharply down on the basicranial axis, the commencement of the
+vomer being situated below the mesethmoid instead of in front of it as
+in most skulls. The occipital region of the skull is small, and the
+line of junction of the supra-occipital and parietals is raised into a
+prominent occipital crest. The parietal completely fuses at an early
+stage with its fellow, and the exoccipital is drawn out into a long
+paroccipital process (fig. 94, A, 8). The frontal is large and broad
+and drawn out into a small postorbital process. The lachrymal too is
+large and takes a considerable part in forming the side of the face in
+front of the orbit, as does also the jugal, though to a less extent.
+The face is long and tapers much anteriorly. The nasals are long and
+narrow, as are the nasal processes of the premaxillae, which do not
+however reach the frontals. A prenasal ossicle is developed in front
+of the mesethmoid. The palate is long and narrow, the pterygoid (fig.
+94, A, 10) is small, but the pterygoid process of the alisphenoid is
+prominent. The squamosal is small and has the tympanic fused with it;
+the tympanic is dilated below, forming a bulla (fig. 94, A, 9) filled
+with cancellous bone, and above forms the floor of a long
+upwardly-directed auditory meatus. The mandible has a high ascending
+portion and a small coronoid process (fig. 94, B, 13). The hyoid
+differs from that of most Ungulates, the stylo-hyal being very
+imperfectly ossified.
+
+[Illustration FIG. 95. MANDIBLE OF A HIPPOPOTAMUS (_H. amphibius_) ×
+1/7. (Camb. Mus.)
+
+The second incisor of the left side is missing and the crowns of the
+grinding teeth are much worn.
+
+ 1. condyle.
+ 2. coronoid process.
+ 3. mental foramina.
+ _i_ 1, _i_ 2. first and second incisors.
+ _c._ canine.
+ _pm_ 3. third premolar.
+ _m_ 1, _m_ 3. first and third molar.]
+
+In _Hippopotamus_ the skull though essentially like that of the pig is
+much modified in detail. The brain cavity is very small, while the
+jaws are immensely developed. The face contracts in front of the
+orbits and then expands again greatly, to lodge the enormous incisor
+and canine teeth. The postorbital bar is complete or nearly so, and
+the orbits project curiously outwards and slightly upwards; the
+lachrymal is thin and much dilated. The squamosal is drawn out into a
+postglenoid process, and the hamular process of the pterygoid is
+prominent. The tympanic bulla is filled with cancellous bone. The
+mandible is enormously large, the symphysis is long, the angle much
+expanded and drawn out into a process which projects outwards and
+forwards.
+
+Among extinct forms related to the Suina, _Cyclopidius_ is noticeable
+for having large vacuities in the lachrymo-nasal region, while
+_Cotylops_ has the postorbital bar complete; both these forms are from
+the North American Miocene.
+
+In the Tylopoda and Tragulina the skull resembles in most respects
+that of the Ruminants, shortly to be described; but it is allied to
+that of the Suina in having the tympanic bulla filled with cancellous
+bone. The tympanic bulla is better developed in the Tragulina than in
+most Ungulates.
+
+Among Ruminants, the Bovidae, that large group including the Oxen,
+Sheep, and Antelopes, as a rule have the face bent on the basicranial
+axis much as in the Suina. The parietals are generally small and early
+coalesce, the frontals are large and are usually drawn out into horn
+cores, which are however absent in the skulls of some domestic
+varieties of sheep and oxen, and also in some of the earlier extinct
+forms of Bovidae. These horn cores are formed internally of cancellous
+bone, and on them the true epidermal horns are borne. In young animals
+there is a distinct interparietal, but this early fuses with the
+supra-occipital, and in the oxen also with the parietals. The
+occipital crest is generally well marked, but in the genus _Bos_
+becomes merged in a very prominent straight ridge running between the
+two horn cores; this ridge, which contains air cells communicating
+with those in the horn cores, is not nearly so well marked in _Bison_.
+There is often, as in _Gazella_, a vacuity on the side of the face
+between the nasal, frontal, lachrymal, and maxillae, but this is not
+found in oxen or sheep. The premaxillae are small, the nasals are long
+and pointed, and the turbinals are much developed. The Saiga antelope
+has a curiously specialised skull; the nasals are absent or have
+coalesced with the frontals and the anterior nares are enormously
+large. In all Ruminants the lachrymal is large and forms a
+considerable part of the side of the face; it often bears a
+considerable depression, the _suborbital_ or _lachrymal fossa_, well
+seen in most of the smaller antelopes. The postorbital bar is
+complete, and the orbit is prominent and nearly circular. The
+palatines and pterygoids are moderately large, and the pterygoids have
+a backwardly-projecting hamular process. The squamosal is small, but
+has a postglenoid process. The tympanic is not fused to the periotic
+and has a small bulla not filled with cancellous bone. There is a
+large paroccipital process to the exoccipital and the mandible has a
+long slender coronoid process.
+
+In the Cervidae and Giraffidae the face is not bent down on the
+basicranial axis as it is in the Bovidae. The frontals are drawn out,
+not into permanent horn cores as in the Bovidae, but into short
+outgrowths, the pedicels, upon which in the Cervidae long antlers are
+annually developed. These _antlers_ are outgrowths of bone, and are
+covered during development by vascular integument, which dries up and
+peels off when growth is complete. Every year they are detached, by a
+process of absorption at the base, and shed. They may occur in both
+sexes, as in the Reindeer, but as a rule they are found only in the
+male. They are generally more or less branched, and are sometimes of
+enormous size and weight, as in the extinct _Cervus megaceros_. In
+young animals they are always simple, but become annually more and
+more complicated as the animal grows older.
+
+In the Giraffe the frontals bear a small pair of bony cores, which are
+at first distinct, but subsequently become fused to the skull. In the
+allied _Sivatherium_, a very large form from the Indian Pliocene, the
+skull bears two pairs of bony outgrowths, a pair of short conical
+outgrowths above the orbits, and a pair of large expanded outgrowths
+on the occiput.
+
+The opening of the lachrymal canal is commonly double and the
+lachrymal fossa is large in the Cervidae and the Giraffidae except
+_Sivatherium_. The vacuity between the frontal, lachrymal, maxillae,
+and nasal is specially large.
+
+The hyoid of Ruminants is noticeable for the development of the
+anterior cornua, which include stout and short cerato-hyals and
+epi-hyals, long and strong stylo-hyals and large tympano-hyals which
+are more or less imbedded in the tympanics.
+
+_PERISSODACTYLA._ In the skull of Perissodactyles an alisphenoid canal
+is found and the nasals are expanded behind. Among the living animals
+belonging to this group the skull least modified from the ordinary
+type is that in _Rhinoceros_. In this form the skull is considerably
+elongated, the facial portion being very large. The occipital region
+is elevated, but the cranial cavity is small, the boundary line
+between the occipital and parietal regions being drawn out into a
+prominent crest, which is occupied by air cells. There is no
+postorbital process to the frontal, and the orbit is completely
+confluent with the temporal fossa. The nasals are fused together and
+are very strongly developed, extending far forwards, sometimes
+considerably beyond the premaxillae. In some extinct species, such as
+_Elasmotherium_ and the Tichorhine Rhinoceros, _R. antiquitatis_, the
+mesethmoid is ossified as far forwards as the end of the nasals. The
+nasals are arched and bear one or two roughened surfaces to which the
+great nasal horns are attached. The premaxillae are very small and the
+pterygoids are slender. The palate is long, narrow, and deeply
+excavated behind. The postglenoid process of the squamosal is well
+developed, and generally longer than the paroccipital process of the
+exoccipital. The tympanic and periotic are both small and are fused
+together. The condyle of the mandible is very wide, the angle rounded,
+and the coronoid process moderately developed.
+
+In the Titanotheriidae, a family of extinct Perissodactyla from the
+Miocene of North America, the occipital region is much elevated, as is
+also the fronto-nasal region, the nasals (perhaps only in the male)
+bearing a pair of blunt bony outgrowths. Between these two elevated
+regions the skull is much depressed. The cranial cavity is very small,
+the orbit confluent with the temporal fossa, and the zygomatic arch
+massive.
+
+In _Tapirus_ the orbit and temporal fossa are confluent. The nasals
+are small, wide behind and pointed in front, and are supported by the
+mesethmoid; the anterior nares are exceedingly large and their lateral
+boundaries are entirely formed by the maxillae. The postglenoid and
+post-tympanic processes of the squamosal are large. The periotic is
+not fused to the squamosal or to the small tympanic. The mandible is
+large and has the angle much developed and somewhat inflected.
+
+_Palaeotherium_, which lived in early Tertiary times, has a skull much
+like that of the Tapir, especially as regards the nasal bones.
+
+In the Horse and its allies (Equidae) the facial portion of the skull
+is very large as compared with the cranial portion, the nasals and
+nasal cavities being specially large. In the living species of the
+genus _Equus_ there is no fossa between the maxillae and lachrymal, but
+it occurs in some extinct species. The lachrymal and jugal form a
+considerable part of the side of the face; and the orbit though small
+is complete and prominent. The postorbital bar is formed by a strong
+outgrowth from the frontal, which unites with a forward extension of
+the squamosal. The squamosal may extend forwards and form part of the
+wall of the orbit, a very unusual feature, as in most mammals the
+squamosal stops before the postorbital bar. The palate is narrow and
+excavated behind as in _Rhinoceros_; the palatines take very little
+part in its formation. The glenoid surface for the articulation of the
+mandible is very wide. The squamosal gives rise to small postglenoid
+and post-tympanic processes, and the exoccipital to a large
+paroccipital process. The tympanic and periotic are ankylosed
+together, but not to any other bones.
+
+In the SUBUNGULATA, the lachrymal and jugal do not form any
+considerable part of the side of the face, and the maxillae commonly
+takes part in the formation of the zygomatic arch.
+
+_TOXODONTIA._ The skull in the Toxodontia shows several Artiodactyloid
+features, while the manus and pes are of a more Perissodactyloid type.
+The Artiodactyloid features are (1) the absence of an alisphenoid
+canal, (2) the fact that the palate is not excavated behind, and that
+the palatines form a considerable part of it, and (3) the fusion of
+the tympanic to the squamosal and exoccipital, forming the floor of an
+upwardly directed auditory meatus. The frontal has a fairly well
+developed postorbital process, but the orbit is confluent with the
+temporal fossa. The premaxillae is well developed, as is the
+paroccipital process of the exoccipital, especially in _Typotherium_.
+The mandible has a rounded angle and a coronoid process of moderate
+size. In _Typotherium_ the ascending portion is very massive.
+
+_CONDYLARTHRA._ As far as is known the skull of these generalised
+Ungulates is depressed, and is frequently marked by a strong sagittal
+crest. The cranial cavity is small, the cerebral fossa in _Phenacodus_
+being exceptionally small. The orbit is completely confluent with the
+temporal fossa.
+
+_HYRACOIDEA._ The skull of _Procavia_ resembles that of
+Perissodactyles more than that of any other Ungulates, but differs
+strongly in the comparatively small size of its facial portion. The
+posterior portion of the cranium is rather high, the occipital plane
+being nearly vertical. There is a small interparietal. The nasals are
+wide behind, and the zygomatic arch is strongly developed, its most
+anterior part being formed by the maxillae. The jugal and parietal give
+rise to postorbital processes which sometimes meet, but as a rule the
+orbit is confluent with the temporal fossa; it is very uncommon for
+the parietal to give rise to a postorbital process, and even in
+_Procavia_ the frontal often forms part of the process. The
+alisphenoid canal, and postglenoid and paroccipital processes are well
+developed. The tympanic bulla is large and the periotic and tympanic
+are fused together, but not as a rule to the squamosal. The ascending
+portion of the mandible is very high and broad, the angle rounded and
+the coronoid process moderate in size. The hyoid is singular, there is
+a large flat basi-hyal prolonged laterally into two broad flattened
+thyro-hyals. Articulating with its anterior end are two large
+triangular cerato-hyals, which are drawn out into two processes
+meeting in the middle line.
+
+_AMBLYPODA._ In the Uintatheriidae (Dinocerata) the skull has a very
+remarkable character, being long and narrow and drawn out into three
+pairs of rounded protuberances, a small pair on the nasals, a larger
+pair on the maxillae in front of the orbits, and the largest pair on
+the parietals. The cranial cavity, and especially the cerebral fossa,
+is extraordinarily small. The orbit is not divided behind from the
+temporal fossa. The mandible has a prominent angle, and a long curved
+coronoid process; its symphysial portion bears a curious flattened
+outgrowth to protect the great upper canines.
+
+In _Coryphodon_ the skull is of a more normal character, being without
+the conspicuous protuberances. The cranial cavity though very small is
+not so small as in _Uintatherium_.
+
+[Illustration FIG. 96. SKULL OF A YOUNG INDIAN ELEPHANT (_Elephas
+indicus_), SEEN FROM THE RIGHT SIDE, THE ROOTS OF THE TEETH HAVE BEEN
+EXPOSED. × 1/8. (Camb. Mus.)
+
+ 1. exoccipital.
+ 2. parietal.
+ 3. frontal.
+ 4. squamosal.
+ 5. jugal.
+ 6. premaxillae.
+ 7. maxillae.
+ 9. supra-occipital.
+ 13. basi-occipital.
+ 14. postorbital process of the frontal.
+ 15. lachrymal.
+ 16. pterygoid process of the alisphenoid.
+ _i_ 1. incisor.
+ _mm_ 3., _mm_ 4. third and fourth milk molars.
+ _m_ 1. first molar.]
+
+_PROBOSCIDEA._ The character of the skull in the young elephant
+differs much from that in the old animal. In very young individuals
+the skull is of a normal character, and the cranial cavity is
+distinctly large in proportion to the bulk of the skull. But as the
+animal gets older, while its brain does not grow much, the size of its
+trunk and especially of its tusks increases greatly; and consequently
+the skull wall is required to be of very great superficial extent in
+order to afford space for the attachment of the muscles necessary for
+the support of these heavy weights. This increase in superficial
+extent is brought about without much increase in weight of bone by the
+development of an enormous number of air cells in nearly all the bones
+of the skull; sometimes, as in the case of the frontal, separating the
+inner wall of the bone from the outer, by as much as a foot. This
+development of air cells is accompanied by the obliteration of the
+sutures between the various bones. The most noticeable point with
+regard to the cranial cavity is the comparatively large size of the
+olfactory fossa. The supra-occipital (figs. 96 and 97, 9) is
+large--exceedingly large in the adult skull; the parietals (figs. 96
+and 97, 2) are also very large. The frontals send out small
+postorbital processes, but these do not meet processes from the small
+jugal, which forms only the middle part of the slender zygomatic
+arch, the anterior part being formed by the maxillae. The lachrymal
+(fig. 96, 15) is small and lies almost entirely inside the orbit. The
+anterior narial aperture (fig. 97, 8) is wide and directed upwards,
+opening high on the anterior surface of the skull. It is bounded above
+by the short thick nasals and below by the premaxillae. The narial
+passage is freely open, maxillo-turbinals not being developed. The
+palatine is well developed, the pterygoid is small and early fuses
+with the pterygoid process of the alisphenoid. The tympanic is united
+with the periotic but not with the squamosal, and forms a large
+auditory bulla. There are no paroccipital or postglenoid processes.
+The exoccipital is not perforated by the condylar foramen,--a very
+exceptional condition.
+
+[Illustration FIG. 97. LONGITUDINAL SECTION TAKEN RATHER TO THE RIGHT
+OF THE MIDDLE LINE OF THE SKULL OF A YOUNG INDIAN ELEPHANT (_E.
+Indicus_) × 1/8. (Camb. Mus.)
+
+ 8. anterior nares.
+ 10. periotic.
+ 11. palatine.
+ 12. pterygoid.
+ 17. nasal.
+
+Other numbers as in Fig. 96.]
+
+The mandible has a high ascending portion, is rounded off below and
+has no angle. The symphysial portion is long, narrow, and spout-like,
+and the coronoid process is small. The thyro-hyals are ankylosed with
+the basi-hyal, which is connected with the large forked stylo-hyals by
+ligament only.
+
+RODENTIA. The cranial cavity is depressed, elongated, and rather
+small, and the cerebral fossa lies entirely in front of the cerebellar
+fossa. The occipital plane is vertical or directed somewhat backwards,
+and the supra-occipital does not form much of the roof of the cranium.
+The paroccipital processes of the exoccipitals are generally of
+moderate size; in the Capybara (_Hydrochaerus_), however, they are
+very long, and are laterally compressed and directed forwards. The
+parietals are small, and often become completely fused together; there
+is sometimes a small interparietal. The frontals in most genera have
+no trace of a postorbital process; in Squirrels, Marmots and Hares,
+however, one occurs, but in no case does it meet a corresponding
+process from the zygomatic arch, so the orbit and temporal fossa are
+completely confluent. In Hares the postorbital process of the frontal
+is much flattened, and has an irregular margin. The temporal fossa is
+always small, and in _Lophiomys_ is arched over by plates arising
+respectively from the parietal and jugal; a secondary roof is thus
+partially developed in a manner unique among mammals, but carried to a
+great extent in many Chelonia. The nasal bones and cavities are large,
+attaining their maximum development in the Porcupines (fig. 98, 1).
+The premaxillae is always very large, and sends back a long process
+which meets the frontal. The vomer is occasionally found persisting in
+two separate halves, a feature recalling the arrangement in
+Sauropsids. In many Rodents there is an enormous vacuity at the base
+of the maxillary portion of the zygomatic arch. It is sometimes as
+large as the orbit, and attains its maximum development in the
+Capybara and other Hystricomorpha; in the Marmots, Beavers, and
+Squirrels (Sciuromorpha), and in the Hares it is undeveloped. In
+_Lagostomus_ the maxillae bears an upwardly directed plate of bone,
+shutting off from this vacuity a space which is the true infra-orbital
+foramen.
+
+[Illustration FIG. 98. HALF FRONT VIEW OF THE SKULL OF A PORCUPINE
+(_Hystrix cristata_) × 1/2. (Camb. Mus.)
+
+ 1. nasal.
+ 2. maxillo-turbinals.
+ 3. infra-orbital vacuity.
+ 4. maxillae.
+ 5. premaxillae.
+ 6. jugal.
+ _i_ 1. upper incisor.]
+
+The zygomatic arch is always complete, and in many cases the jugal
+extends back to form part at least of the glenoid surface for
+articulation with the mandible. In _Coelogenys_ the jugal and
+maxillary portion of the zygomatic arch is greatly expanded and
+roughened, and the maxillary portion encloses a large cavity. The
+palate in Rodents is narrow, and the space between the incisor and
+molar teeth passes imperceptibly into the sides of the face. The
+anterior palatine foramina form long, rather narrow slits in this
+region. The bony palate between the grinding teeth is sometimes as in
+the Hares very short, sometimes as in the Capybara very long. The
+maxillae extends back beneath the orbit to unite with the squamosal.
+The pterygoid is always small, but sometimes has a well-marked hamular
+process which in _Hystrix_, _Lagostomus_, and some other genera unites
+with the tympanic bulla. The periotic is large, and fused with the
+tympanic, which forms a prominent bulla, and is generally drawn out
+into a tubular meatus. The bulla attains its maximum development in
+_Chinchilla_ and _Dipus_.
+
+The mandible is narrow and rounded in front, the two halves meeting in
+a long symphysis. The angle is generally drawn out into a long
+backwardly-projecting process, which is often pointed and directed
+upwards. In the Hares the angle is rounded. The coronoid process is
+never large.
+
+There are a number of points in which the skull of the Duplicidentata
+(Hares and Rabbits) differs from that of other Rodents. (_a_) The
+sutures between the basi-occipital and basisphenoid, and between the
+basisphenoid and presphenoid remain open throughout life. (_b_) Much
+of the maxillae forming the side of the face in front of the orbit is
+fenestrated. (_c_) The optic foramina are united to form a single
+hole, much as in birds. (_d_) The coronoid process is slightly
+differentiated from the ascending portion of the mandible. The first
+two of these points have been thought to indicate degradation of the
+hares and rabbits as compared with higher mammals.
+
+CARNIVORA[167]. It is characteristic of the skull in Carnivora that
+the glenoid fossa is deep, and the postglenoid process (fig. 75, 23)
+well developed. The condyle of the mandible is much elongated
+transversely. The orbit and temporal fossa in the great majority of
+forms communicate freely, the postorbital bar being incomplete.
+
+_CARNIVORA VERA._ The axis of the facial portion of the skull is a
+direct continuation of that of the cranial portion. The cranial
+cavity though rather depressed is large, and generally long, though in
+Cats it is comparatively short and wide. The occipital plane is nearly
+vertical, and the exoccipitals are developed into fairly prominent
+paroccipital processes. The interparietal is commonly distinct, and
+the parietals unite in a long sagittal suture, which is often
+developed into a crest. The nasals (fig. 73, 4) are well developed,
+especially in Cats, and the nasal processes of the premaxillae do not
+nearly reach the frontals. A considerable part of the palate is formed
+by the palatine, and the maxillary portion is pierced by rather long
+anterior palatine foramina. The pterygoid has a hamular process. The
+zygomatic arch is strong, especially in Cats. Postorbital processes
+are developed on the frontal (fig. 73, 10) and jugal, but never form a
+complete postorbital bar. A carotid canal is well seen in the Ursidae,
+and to a less extent in the Felidae; in the Canidae there is an
+alisphenoid canal (fig. 75, 21).
+
+The auditory bulla differs a good deal in the different groups. In the
+Bears (Ursidae) it is not much inflated, and is most prominent along
+its inner border; it is not closely connected with the paroccipital
+process. In the Cats it is very prominent, and its cavity is almost
+divided by a septum into two parts, the inner of which contains the
+auditory ossicles. The paroccipital process is closely applied to the
+bulla. In the Dogs the bulla is intermediate in character between that
+of the Cats and that of the Bears; it is partially divided by a
+septum, and is moderately expanded.
+
+The mandible is well developed with a prominent angle (fig. 72, 26),
+and a large coronoid process. The hyoid consists of a broad basi-hyal,
+a long many-jointed anterior cornu and short thyro-hyals (fig. 72,
+33).
+
+The skull in the _CREODONTA_ is in most respects allied to that of the
+Canidae, but presents some ursine affinities. The tympanic bulla is
+fairly prominent, but has no well-developed septum. The cranial cavity
+is very small and narrow, the zygomatic arch standing away from it.
+The temporal fossa is of great size.
+
+In the _PINNIPEDIA_ the cranial cavity is large and rounded. The skull
+is much compressed in the interorbital region, and in correlation with
+this compression the ethmo-turbinals are little developed, while the
+maxillo-turbinals are large. The orbit is large, and the temporal
+fossa smaller than in the Carnivora vera. In the Walrus (_Trichechus_)
+the anterior part of the face is distorted by the development of the
+huge canines. The Otariidae have an alisphenoid canal. The tympanic
+bulla is small in _Otaria_, large in the Phocidae, and flattened in
+the Walrus. The hyoid is similar to that in Carnivora vera.
+
+INSECTIVORA. The skull varies much in the different members of the
+order Insectivora, but the following points of agreement are found.
+The cranial cavity is of small size, and is never much elevated. The
+facial part of the skull is generally considerably elongated, and the
+nasals and premaxillae are well developed. The zygomatic arch is
+usually slender or incomplete, and the coronoid process and angle of
+the mandible are commonly prominent.
+
+In some Insectivora, such as _Galeopithecus_, _Tupaia_, and
+_Macroscelides_, the skull shows a higher type of structure than is
+met with in most members of the order. In these genera the cranial
+cavity is comparatively large, and the occipital plane is nearly
+vertical. The zygomatic arch is fairly strong, and the frontal and
+jugal give rise to postorbital processes which nearly or quite
+(_Tupaia_) meet. The tympanic bulla is well developed, and produced
+into a tubular auditory meatus, this being specially well marked in
+_Macroscelides_.
+
+In the other Insectivora the cranial cavity is of smaller comparative
+size, and the orbit and temporal fossa are completely confluent, often
+without any trace of a postorbital bar. The occipital plane commonly
+slopes forwards. In the Hedgehogs (Erinaceidae) and Centetidae the
+tympanic is very slightly developed, forming a small ring. The
+zygomatic arch of Hedgehogs and _Gymnura_ is very slender, the jugal
+being but little developed and the squamosal and maxillae meeting one
+another; in the Centetidae the jugal is absent and the arch is
+incomplete.
+
+The Moles (Talpidae) have an elongated, depressed and rounded skull
+with a very slender zygomatic arch formed by the squamosal and
+maxillae. The nasals are fused together, and the mesethmoid is ossified
+very far forwards. In the Shrews (Soricidae) there is no zygomatic
+arch; the tympanic is ring-like, and the angle of the mandible is very
+prominent. The hyoid has a transversely extended basi-hyal, a long
+anterior cornu with three ossifications, and thyro-hyals which are
+sometimes fused to the basi-hyal.
+
+CHIROPTERA. In the frugivorous Flying Foxes (Pteropidae) the skull is
+elongated, and the cranial cavity is large and arched, though
+considerably contracted in front. There are commonly strong sagittal
+and supra-orbital crests. The parietals take a great part in the
+formation of the walls of the cranial cavity, the supra-occipital and
+frontals being small. The frontal is drawn out into a long postorbital
+process, but the zygomatic arch, which is slender, and formed mainly
+by the squamosal and maxillae, gives rise to only a small postorbital
+process, so that the orbit and temporal fossa are confluent. There is
+no alisphenoid canal, and the tympanics are very slightly connected
+with the rest of the skull. The mandible has a large coronoid process,
+a rounded angle, and a transversely expanded condyle.
+
+In Insectivorous Bats the skull is generally shorter and broader than
+in the Pteropidae. The cranial cavity is large and rounded, and has
+thin smooth walls. The zygomatic arch is slender, and postorbital
+processes are not generally well developed. The premaxillae is
+generally small, sometimes absent. The tympanics are ring-like and are
+not connected with the surrounding bones. The angle of the mandible is
+distinct. The hyoid in most respects resembles that of the
+Insectivora.
+
+PRIMATES. The characters of the skull differ greatly in the two
+suborders of Primates, the Anthropoidea and the Lemuroidea.
+
+In the _LEMUROIDEA_ the general relative proportions of the cranium
+and face are much as in most lower mammals, and the occipital plane
+forms nearly a right angle with the basicranial axis. The postorbital
+processes of the frontals are commonly continued as a pair of ridges
+crossing the roof of the cranium and meeting the occipital crest.
+Though the postorbital bar is complete, the orbit and temporal fossa
+communicate freely below it. The lachrymal canal opens outside the
+orbit, and the lachrymal forms a considerable part of the side of the
+face. The tympanic is developed into a large bulla. The hyoid
+apparatus much resembles that of the Dog.
+
+In the _ANTHROPOIDEA_ the skull differs greatly from that in the
+Lemuroidea. The cranial portion of the skull is very large as compared
+with the facial portion, though the comparative development varies,
+some monkeys, such as the baboons (Cynocephali) having the facial
+portion relatively large. The comparative size of the jaws does not
+vary inversely with the general development of the animal, some of the
+Cercopithecidae having comparatively larger jaws than some of the
+Cebidae. The great size of the cranial part of the skull is mainly due
+to the immense development of the cerebral fossa, which commonly
+completely overlaps the olfactory fossa in front, and the cerebellar
+fossa behind. This development also has the effect of making the
+ethmoidal and occipital planes lie, not at right angles to the
+basicranial axis, but almost in the same straight line with it. This
+is, however, not always the case, as the Howling Monkey (_Mycetes_)
+and also some of the very highest monkeys, the Gibbons (_Hylobates_),
+have the occipital plane nearly vertical to the basicranial axis. In
+adult Man the basi-occipital, exoccipitals and supra-occipital
+coalesce, forming the so-called occipital bone; while the
+basisphenoid, presphenoid, alisphenoids, orbitosphenoids and
+pterygoids form the sphenoid bone. The roof of the skull is partly
+formed by the large supra-occipital and frontals, but mainly by the
+parietals (fig. 99, 1), which in Man are of enormous extent.
+
+[Illustration FIG. 99. HALF FRONT VIEW OF THE SKULLS, _A_ OF AN OLD,
+_B_ OF A YOUNG GORILLA (_Gorilla savagei_) × 1/4. (Camb. Mus.)
+
+ 1. parietal.
+ 2. sagittal crest.
+ 3. frontal.
+ 4. supra-orbital ridge.
+ 5. squamosal.
+ 6. maxillae.
+ 7. external auditory meatus.]
+
+In Man and in most monkeys, at any rate when young (fig. 99, B), the
+roof of the skull is smooth and rounded, but in many forms, such as
+the Baboons, in the adult the supra-orbital and occipital ridges are
+much developed. In the Gorilla this is also the case with the sagittal
+crest (fig. 99, A, 2). The bones of the upper surface of the cranium
+interlock with wavy outlines. The nasals vary much in length, being
+much shorter in man than in most monkeys; they commonly become early
+fused together, as do also the frontals. The vomer is well developed,
+and the ethmo-turbinal always forms part of the boundary of the orbit.
+There are frequently, as in many Lemuroidea, a pair of more or less
+well-marked ridges, crossing the roof of the skull from the
+postorbital processes of the frontals to the occipital crest. The
+orbit is completely encircled by bone, and the alisphenoid assists the
+jugal and frontal in shutting it off from the temporal fossa, leaving
+however a communication between the two as the sphenomaxillary
+fissure. In most cases the frontals meet one another in the middle
+line between the mesethmoid and orbitosphenoid, but in Man, Simia, and
+some Cebidae this does not take place. In nearly all Cebidae the
+parietal and jugal meet one another, separating the frontal and
+alisphenoid on the skull wall; in Man and all Old World monkeys, on
+the other hand, the alisphenoid and frontal meet and separate the
+jugal and parietal. The premaxillae nearly always send back processes
+which meet the nasals. The palate is rather short and both the
+palatine and the premaxillae take a considerable part in its formation.
+The pterygoid plate of the alisphenoid is decidedly large, and there
+is no alisphenoid canal. There is never any great development either
+of the paroccipital process of the exoccipital, or of the postglenoid
+process of the squamosal. The periotic and tympanic are always fused
+together; in Cebidae they form a small bulla, but a bulla is not
+developed in any Old World forms. The periotic is large, especially
+the mastoid portion, which forms a distinct portion of the skull wall
+between the squamosal and exoccipital. In Man and still more in Old
+World monkeys, the external auditory meatus is drawn out into a
+definite tube, whose lower wall is formed by the tympanic; in the
+Cebidae the tympanic is ring-like. The perforation of the periotic by
+the carotid canal is always conspicuous.
+
+The mandible is rather short and broad, and the angle formed by the
+meeting of the two rami is more obtuse than in most mammals. The
+coronoid process is fairly well developed, and the angle is more or
+less rounded. In most Primates the condyle is considerably widened,
+but this is not the case in Man. In _Mycetes_ the mandible is very
+large, its ascending portions being specially developed. The hyoid of
+Primates is remarkable for the large expanded basi-hyal, which is
+generally concave above and convex below. The anterior cornu is never
+well ossified, but the thyro-hyal is always strong. In _Mycetes_ the
+basi-hyal is enormously large, forming a somewhat globular thin-walled
+capsule.
+
+
+AUDITORY OSSICLES.
+
+[Illustration FIG. 100. MALLEUS, STAPES AND INCUS OF _A._ MAN. _B._
+DOG. _C._ RABBIT. (After DORAN) x 1.
+
+ 1. head of malleus.
+ 2. canal of stapes.
+ 3. incus.
+ 4. processus longus (or gracilis).
+ 5. manubrium of malleus.
+ 6. processus brevis.
+ 7. lamella.]
+
+There are in mammals four auditory ossicles forming a chain extending
+from the fenestra ovalis to the tympanic membrane. Three of these, the
+=malleus=, =incus= and =stapes=, are always distinct, while the
+fourth, the =lenticular=, is smaller than the others and is sometimes
+not distinct. The names are derived from human anatomy and indicate in
+the case of the first three a more or less fanciful resemblance
+respectively to a hammer, an anvil and a stirrup. The ossicles are
+homologous as a whole to the hyomandibular of fishes and to the
+columellar chain of Sauropsids and Amphibians. The malleus is
+homologous to the extra-columella of Crocodiles and the stapes to the
+columella. The =malleus= when typically developed consists of a
+rounded _head_ (fig. 100, 1) which bears a surface articulating with
+the incus, and a short _neck_ continued into a process, the
+_manubrium_ (fig. 100, 5), which comes into relation with the tympanic
+membrane. From the junction of the neck and manubrium two processes
+are given off, a _processus longus_ or _gracilis_ (fig. 100, 4), which
+in the embryo is continuous with Meckel's cartilage, and a _processus
+brevis_ (fig. 100, 6). The =incus= generally consists of a more or
+less anvil-shaped portion which articulates with the malleus, and of a
+process which is connected with the stapes by the small =lenticular=.
+The =stapes= is generally stirrup shaped, consisting of a basal
+portion from which arise two _crura_ separated by a space the canal
+through which a branch of the pharyngeal artery runs The lenticular is
+frequently cartilaginous and sometimes is not developed at all.
+
+The above is the arrangement of the auditory ossicles met with in the
+higher Mammalia, but in the lower Mammalia the characters approach
+more nearly to those met with in Sauropsids.
+
+In MONOTREMES the ossicles, though distinctly mammalian in character,
+show a very low type of development. The incus is articulated, or
+often fused, with an outgrowth from the head of the malleus. The
+stapes is very much like a reptilian columella, having a single crus
+with no perforation.
+
+In MARSUPIALS the ossicles are of a low type, but not so low as the
+rest of the skeleton might have led one to expect, and all or almost
+all the points showing a low grade of development may be paralleled
+among the Monodelphia. The lowest Marsupials as regards the ossicles
+are the Peramelidae, whose ossicles are of a frail papery consistence.
+The Didelphyidae on the other hand have the most highly developed
+ossicles, the malleus much resembling that of many Insectivores, and
+the stapes having two definite crura separated by a canal.
+
+In EDENTATES the character of the ossicles varies much. In Sloths the
+stapes approaches that of Sauropsids in its narrowness and the slight
+trace of a canal; this character is however still more marked in
+_Manis_, whose stapes is as Sauropsidan as that of Monotremes, and
+consists of a nearly circular basal plate bearing a column which does
+not show any sign of division into crura. The stapes of other
+Edentates, such as ant-eaters, aard varks, and most armadillos, is of
+a high type and has well-developed crura. _Priodon_ has a lower type
+of stapes than _Dasypus_ and _Tatusia_.
+
+The ossicles of the SIRENIA differ widely from those of all other
+mammals in their great density and clumsy form.
+
+In CETACEA the ossicles are solid, though not so solid as in Sirenia,
+and their details vary much. The malleus is always firmly fused to the
+tympanic by means of the processus longus, and the manubrium is very
+little if at all developed. The incus has the stapedial end greatly
+developed, and the stapes has very thick crura with hardly any canal.
+The ossicles of the Mystacoceti are apparently less specialised than
+are those of the Odontoceti.
+
+The auditory ossicles of the UNGULATA do not present any characters
+common to all the members of the group.
+
+Among Ruminants they are chiefly remarkable for the development of a
+broad lamellar expansion between the head and the processus longus of
+the malleus. In some cases the malleus of the foetus differs
+strikingly from that of the adult. Among Perissodactyla the Rhinoceros
+and Tapir have the malleus of a low type, recalling those of
+Marsupials; while in the Horse the head is well developed, and the
+malleus is of a higher type.
+
+The ossicles of _Procavia_, which recall those of the Equidae, are
+chiefly remarkable for the small size of the body of the incus. In
+Elephants the ossicles are large and massive.
+
+In the RODENTIA (fig. 100, C) the malleus is generally characterised
+by a very broad manubrium. In many genera such as _Bathyergus_, and
+most of the Hystricomorpha such as _Hystrix_, _Chinchilla_ and
+_Dasyprocta_, the malleus and incus are ankylosed together.
+
+CARNIVORA. In Carnivora vera the most striking feature of the malleus
+is the occurrence of a broad lamellar expansion between the head and
+neck and the processus longus. This however does not occur in some
+Viverridae. In the Carnivora vera the incus and stapes are small as
+compared with the malleus, but in the Pinnipedia they are large. In
+the Pinnipedia the auditory ossicles have a very dense consistence,
+and except in the Otariidae are very large. The stapes frequently has
+no canal, or only a very small one.
+
+In INSECTIVORA the characters of the auditory ossicles are very
+diverse. Many forms such as shrews, moles, hedgehogs, and the
+Centetidae have a low type of malleus resembling that of Edentates.
+_Chrysochloris_ has very extraordinary auditory ossicles. The head of
+the malleus is drawn out into a great club-shaped process, the incus
+is long and narrow, and differs much from the ordinary type.
+
+In CHIROPTERA the ossicles and especially the malleus much resemble
+those of shrews. The stapes is always normal in character, never
+becoming at all columelliform.
+
+PRIMATES. In Man and the Anthropoid Apes the malleus has a rounded
+head, a short neck, and the manubrium, a processus longus and a
+processus brevis. The incus consists of an anvil-shaped portion from
+which arises a long tapering process. The stapes has diverging crura
+and consequently a wide canal. The crura in other monkeys do not
+diverge so much as in man and anthropoid apes. The New World monkeys
+have no neck to the malleus.
+
+
+THE STERNUM[168].
+
+In MONOTREMES and most MARSUPIALS the sternum does not present any
+characters of special importance. The presternum is strongly keeled in
+_Notoryctes_.
+
+The sternum in EDENTATES is very variable: in the Sloths it is very
+long, the mesosternum of _Choloepus_ having twelve segments. In the
+ant-eaters and armadillos the presternum is broad and sometimes as in
+_Priodon_ strongly keeled. In _Manis macrura_ the xiphisternum is
+drawn out into a pair of cartilaginous processes about nine inches
+long.
+
+In the SIRENIA the sternum is simple and elongated, and of fairly
+equal width throughout, in the adult it shows no sign of segmentation.
+Its origin from the union of two lateral portions can be well seen in
+_Manatus_.
+
+Two distinct types of sternum are met with in the CETACEA. In the
+Odontoceti the sternum consists of a broad presternum followed by
+three or four mesosternal segments, but with no xiphisternum.
+Indications of the original median fissure can be traced, and are very
+evident in _Hyperoödon_. In the Mystacoceti, on the other hand, the
+sternum consists simply of a broad flattened presternum which is
+sometimes more or less heart-shaped, sometimes cross-shaped. Only a
+single pair of ribs are united to it.
+
+The sternum in UNGULATA is generally long and narrow and formed of six
+or generally seven segments. The presternum is as a rule small and
+compressed, often much keeled, especially in the horse and tapir. The
+segments of the mesosternum gradually widen as followed back and the
+xiphisternum is often terminated by a cartilaginous plate.
+
+In the RODENTIA the sternum is long and narrow and generally has a
+large presternum, and a xiphisternum terminated by a broad
+cartilaginous plate.
+
+In the CARNIVORA, too, the sternum (fig. 76) is long and narrow and
+formed of eight or nine pieces, all of nearly the same size. The
+xiphisternum generally ends in an expanded plate of cartilage.
+
+In INSECTIVORA the sternum is well developed but variable. The
+presternum is commonly large and is sometimes as in the Hedgehog
+(_Erinaceus_) bilobed in front, sometimes as in the Shrew (_Sorex_)
+trilobed. It is especially large in the Mole (_Talpa_) and is expanded
+laterally and keeled below.
+
+In the CHIROPTERA the presternum is strongly keeled and so is
+sometimes the mesosternum.
+
+Among PRIMATES, in Man and the Anthropoid Apes the sternum is rather
+broad and flattened; the mesosternum consists of four segments which
+are commonly fused together and the xiphisternum is imperfectly
+ossified.
+
+
+THE RIBS.
+
+Free ribs are borne as a rule only by the thoracic vertebrae; ribs may
+be found in other regions, especially the cervical and sacral, but
+these are almost always ankylosed to the vertebrae. As a general rule
+the first thoracic rib joins the presternum, while the succeeding ones
+are attached between the several segments of the mesosternum. Some of
+the posterior ribs frequently do not reach the sternum; they may then
+be attached by fibrous tissue to the ribs in front, or may end freely
+(_floating ribs_). There are generally thirteen pairs of ribs, and in
+no case do they have uncinate processes.
+
+In MONOTREMES (fig. 102, B) each rib is divided not into two but into
+three parts, an intermediate portion being interposed between the
+vertebral and sternal parts. The sternal ribs are well ossified, and
+some are very broad and flat. The intermediate portions are
+unossified, those of the anterior ribs are short and narrow, but they
+become longer and wider further back.
+
+In MARSUPIALS there are almost always thirteen pairs of ribs, whose
+sternal portions are very imperfectly ossified. _Notoryctes_ has
+fourteen pairs of ribs, eight of which are floating: the first rib is
+very stout, and is abruptly bent on itself to join the sternum. It has
+no distinct sternal portion. All the other ribs are slender.
+
+Of the EDENTATES the Sloths have very numerous ribs; twenty-four pairs
+occur in _Choloepus_, and half of these reach the sternum. In the
+Armadillos there are only ten or twelve pairs of ribs, but the sternal
+portions are very strongly ossified. The first rib is remarkably broad
+and flat, and is not divisible into vertebral and sternal portions.
+
+In the SIRENIA there are a very large number of ribs noticeable for
+their great thickness and solidity, but not more than three are
+attached to the sternum.
+
+CETACEA. In the Whalebone whales the ribs are remarkable for their
+very loose connection both with the vertebral column and with the
+sternum. The capitula are scarcely developed, and the attachment of
+the tubercula to the transverse processes is loose. The first rib is
+the only one connected with the sternum. In the Toothed whales the
+anterior ribs have capitula articulating with the centra, as well as
+tubercula articulating with the transverse processes; in the posterior
+ones, however, only the tubercula remain. Seven pairs of well-ossified
+sternal ribs generally meet the sternum. In the Physeteridae most of
+the ribs are connected to the vertebrae by both capitula and
+tubercula.
+
+In the UNGULATA the ribs are generally broad and flattened, and this
+is especially the case in the genera _Bos_ and _Bubalus_ (fig. 101,
+6). The anterior ribs are short and nearly straight, and sternal ribs
+are well developed. The Artiodactyla have twelve to fifteen pairs of
+ribs, the Perissodactyla eighteen or nineteen, and _Procavia_ twenty
+to twenty-two. The Elephant has nineteen to twenty-one pairs, seven of
+which may be floating ribs.
+
+[Illustration FIG. 101. SKELETON OF A CAPE BUFFALO (_Bubalus caffer_).
+The left scapula is omitted for the sake of clearness × 1/17. (Brit.
+Mus.)
+
+ 1. premaxillae.
+ 2. nasal.
+ 3. orbit.
+ 4. neural spine of first thoracic vertebra.
+ 5. scapula.
+ 6. rib.
+ 7. femur.
+ 8. patella.
+ 9. tibia.
+ 10. metatarsals.
+ 11. radius.
+ 12. metacarpals.]
+
+In the RODENTIA there are generally thirteen pairs of ribs, which do
+not present any marked peculiarities.
+
+The CARNIVORA have thirteen to fifteen pairs of ribs, whose vertebral
+portions are slender, nearly straight and subcylindrical, while their
+sternal portions are long and imperfectly ossified (fig. 76, 5). There
+is nothing that calls for special remark about the ribs, in either
+INSECTIVORA or CHIROPTERA.
+
+PRIMATES. In Man and the Orang (_Simia_) there are generally twelve
+pairs of ribs; in the Gorilla and Chimpanzee (_Anthropopithecus_), and
+Gibbons (_Hylobates_), there are thirteen, in the Cebidae twelve to
+fifteen, and in the Lemuroidea twelve to seventeen pairs. The first
+vertebral rib is shorter than the others, and the sternal ribs
+generally remain cartilaginous throughout life, though in man the
+first may ossify.
+
+
+APPENDICULAR SKELETON.
+
+
+THE PECTORAL GIRDLE.
+
+By far the most primitive type of the pectoral or shoulder girdle is
+found in the MONOTREMATA. The scapula (fig. 102, A, 1) is long and
+recurved, and has only two surfaces, one corresponding to the
+prescapular[1] fossa, the other to the postscapular[1] and
+subscapular[169] fossae. The coracoid is a short bone attached above
+to the scapula and below to the presternum; it forms a large part of
+the glenoid cavity. In front of the coracoid there is a fairly large
+flattened epicoracoid (fig. 102, 6); there is also a large =T=-shaped
+interclavicle (fig. 102, 4), which is expanded behind and rests on the
+presternum. The clavicles rest on and are firmly united to the
+anterior border of the interclavicle. This shoulder girdle differs
+greatly from that of any other mammals, and recalls that of some
+Lacertilia.
+
+[Illustration FIG. 102. _A_, SIDE VIEW, _B_, DORSAL VIEW OF THE
+SHOULDER GIRDLE AND PART OR THE STERNUM OF THE SPINY ANTEATER
+(_Echidna aculeata_) × 1. (After PARKER.)
+
+ 1. scapula.
+ 2. suprascapula.
+ 3. clavicle.
+ 4. interclavicle.
+ 5. coracoid.
+ 6. epicoracoid.
+ 7. glenoid cavity.
+ 8. presternum.
+ 9. second sternal rib.
+ 10. second vertebral rib.]
+
+In MARSUPIALS, as in all mammals except the Monotremes, the shoulder
+girdle is much reduced; there are no epicoracoids and interclavicle,
+and the coracoid forms simply a small process on the scapula,
+ossifying from a centre separate from that giving rise to the rest of
+the bone. The scapula has a long acromion, and a clavicle is always
+present except in _Perameles_. Unossified remains of the precoracoids
+are found at either end of the clavicle. The scapula of _Notoryctes_
+has a very high overhanging spine, and there is a second strong ridge
+running along the proximal part of the glenoid border.
+
+The shoulder girdle of the EDENTATA shows some very curious
+variations. In _Orycteropus_ the scapula is of very normal form and
+the clavicle is well developed. In the Pangolins and Anteaters the
+scapula is very broad and rounded; there is no clavicle in the
+Pangolins, and generally only a vestigial one in Anteaters. In
+Armadillos, Sloths, and Megatheriidae, the acromion is very long and
+the clavicle is well developed. In the Sloths, _Megatherium_, and
+_Myrmecophaga_, a connection is formed between the coracoid, which is
+unusually large, and the coracoid border of the scapula, converting
+the coraco-scapula notch into a foramen. In _Bradypus_ the clavicle is
+very small, and is attached to the coracoid, which sometimes forms a
+distinct bone[170].
+
+In the SIRENIA the scapula is somewhat narrow and curved backwards:
+the spine, acromion, and coracoid process are moderately developed,
+and there is no clavicle.
+
+CETACEA. In nearly all the Odontoceti the scapula is broad and
+somewhat fan-shaped; the prescapular fossa is much reduced, and the
+acromion and coracoid process form flattened processes, extending
+forwards nearly parallel to one another. Some of the Mystacoceti, such
+as _Balaenoptera_, have a broad, fan-shaped scapula, with a long
+acromion and coracoid process, extending parallel to one another.
+Others, such as _Balaena_, have a higher and narrower scapula, with a
+smaller coracoid process.
+
+In UNGULATA the scapula is always high and rather narrow, and neither
+acromion nor coracoid process is ever much developed. In no adult
+Ungulate except _Typotherium_ is there any trace of a clavicle, but a
+vestigial clavicle has been described in early embryos of sheep[171].
+
+[Illustration FIG. 103. SKELETON OF A LLAMA (_Auchenia glama_) × 1/18.
+(Brit. Mus.)
+
+ 1. hyoid.
+ 2. atlas vertebra.
+ 3. seventh cervical vertebra.
+ 4. scapula.
+ 5. imperfectly ossified suprascapula.
+ 6. olecranon process of ulna.
+ 7. metacarpals.
+ 8. ilium.
+ 9. patella.
+ 10. calcaneum.]
+
+UNGULATA VERA. In the Ruminantia the suprascapular region (fig. 103,
+5) is very imperfectly ossified, and when this is removed the upper
+border of the scapula is very straight (fig. 101, 5). The spine is
+prominent, and generally has a fairly well-marked acromion. In
+_Hippopotamus_ the acromion is fairly prominent, but in the other
+Suina, though the spine is prominent, the acromion is not developed.
+The Perissodactyla have no acromion, but while the Equidae and
+_Hyracotherium_ have the scapula long and slender, with the spine very
+slightly developed, the other living Perissodactyla have the spine
+prominent and strongly bent back at about the middle of its length.
+
+SUBUNGULATA. _Typotherium_ (Toxodontia) differs from all other known
+Ungulates in having well-developed clavicles; its scapula has a strong
+backwardly-projecting process, much like that in _Rhinoceros_.
+
+_Phenacodus_ (Condylarthra), has a curiously rounded scapula, with the
+coracoid and suprascapular borders passing imperceptibly into one
+another. The scapula resembles that of a carnivore more than does that
+of any existing Ungulate.
+
+_Procavia_ has a triangular scapula with a prominent spine and no
+acromion; there is a large unossified suprascapular region.
+
+The scapula in the Proboscidea has a large rounded suprascapular
+border and a narrow, slightly concave glenoid border. The spine is
+large, and has a prominent process projecting backwards from about its
+middle. The spine lies towards the front end of the scapula, so that
+the postscapular fossa is much larger than the prescapular fossa.
+
+In RODENTIA the shoulder girdle is of a rather primitive type. The
+scapula is generally high and narrow, somewhat as in Ruminantia; it
+differs, however, from the Ruminant scapula in having a high acromion,
+which is often, as in the Hares and Rabbits, terminated by a long
+metacromion. The development of the clavicle varies, and sometimes it
+is altogether absent. It is frequently connected by cartilaginous
+bands or ligaments (fig. 104, 7 and 9), on the one hand with the
+scapula, and on the other with the sternum. These unossified bands are
+remains of the precoracoid. Epicoracoidal vestiges of the sternal ends
+of the coracoids (fig. 104, 11) are also often present.
+
+In the CARNIVORA VERA the scapula is large, and generally has rather
+rounded borders. The spine and acromion are well developed, and the
+prescapular and postscapular fossae are nearly equal in size. The
+coracoid is very small, and the clavicle is never completely
+developed, being often absent, as in the Bears and most of their
+allies. In the Seals (Phocidae) the scapula is elongated and curved
+backwards, and has a very concave glenoid border. In the Eared Seals
+(Otariidae) the scapula is proportionally much larger and wider, the
+prescapular fossa being specially large, and being traversed by a
+ridge, which converges to meet the spine.
+
+[Illustration FIG. 104. DORSAL VIEW OF THE STERNUM AND RIGHT HALF OF
+THE SHOULDER-GIRDLE OF _Mus sylvaticus_ × 4. (After PARKER.)
+
+ 1. postscapular fossa.
+ 2. prescapular fossa.
+ 3. spine.
+ 4. suprascapular border unossified.
+ 5. coracoid process.
+ 6. acromion.
+ 7. cartilaginous vestige of precoracoid at scapular end of clavicle.
+ 8. clavicle.
+ 9. cartilaginous vestige of precoracoid at sternal end of clavicle.
+ 10. omosternum.
+ 11. epicoracoid.
+ 12. presternum.
+ 13. first segment of mesosternum.
+ 14. xiphisternum.
+ 15. cartilaginous termination of xiphisternum.
+ 16. 2nd sternal rib.
+ 17. 1st vertebral rib.]
+
+In the INSECTIVORA the shoulder girdle is well developed and, as in
+Rodents, remains are met with of various parts not generally seen in
+mammals. In the Shrews the scapula is long and narrow, and has a
+well-marked spine, whose end bifurcates, forming the acromion and
+metacromion. The clavicle is long and slender, and is connected with
+the sternum and acromion by vestiges of the precoracoid. Considerable
+remains of the sternal end of the coracoid are also found. In
+_Potamogale_, however, there are no clavicles. In the Mole the
+shoulder girdle is greatly developed, and of very remarkable form. The
+scapula is high and very narrow, with the spine and acromion very
+little developed. The other shoulder girdle element is an irregular
+bone, which articulates with the humerus and presternum, and is
+connected by ligaments with the scapula. This bone appears to
+represent both the coracoid and the clavicle, being formed partly of
+cartilage bone, partly of membrane bone.
+
+In the CHIROPTERA the scapula is large and oval, and has a moderately
+high spine and a large acromion. The coracoid process is well
+developed and is often forked. The clavicles are also well developed,
+and vestiges of the precoracoid and of the sternal end of the coracoid
+are often found.
+
+In PRIMATES the clavicle and coracoid process are always well
+developed. In Man and the Gorilla the scapula has a long straight
+suprascapular border, a well-developed coracoid process and spine, and
+a large curved acromion. Vestiges of the precoracoid occur at each end
+of the clavicle. The shape of the scapula varies much in the lower
+Primates.
+
+
+THE UPPER ARM AND FORE-ARM.
+
+In the MONOTREMATA the humerus is short, very broad at each end and
+contracted in the middle. The radius and ulna are stout and of nearly
+equal size, while the ulna has a greatly expanded olecranon.
+
+In the MARSUPIALIA the humerus is generally a strong bone, broad at
+the distal end and having well marked deltoid and supinator ridges,
+which are specially large in _Notoryctes_. An ent-epicondylar or
+supracondylar foramen (fig. 105, 5) is almost always present except in
+_Notoryctes_. The radius and ulna are always distinct and well
+developed, and a certain amount of rotation can take place between
+them. The ulna of _Notoryctes_ has an enormous hooked olecranon which
+causes the bone to be nearly twice as long as the radius.
+
+[Illustration FIG. 105. ANTERIOR SURFACE OF THE RIGHT HUMERUS OF A
+WOMBAT (_Phascolomys latifrons_). (After OWEN.)
+
+ 1. head.
+ 2. greater tuberosity.
+ 3. lesser tuberosity.
+ 4. deltoid ridge.
+ 5. ent-epicondylar (supracondylar) foramen.
+ 6. supinator ridge.
+ 7. external condyle.
+ 8. internal condyle.
+ 9. articular surface for radius.
+ 10. articular surface for ulna.]
+
+EDENTATA. The Sloths have long slender arm bones; the humerus is
+nearly smooth and has a very large ent-epicondylar foramen in
+_Choloepus_, but not in _Bradypus_. The radius and ulna can be
+rotated on one another to a considerable extent. The humerus in all
+other Edentates is very strong and has the points for the attachment
+of muscles much developed, especially in the Armadillos and
+Megatheriidae. An ent-epicondylar foramen is found in all living
+forms. The radius and ulna are well developed, but are not capable of
+much rotation.
+
+In the SIRENIA the humerus is well developed and of a normal
+character. It is expanded at each end and has a prominent internal
+condyle, a small olecranon fossa, and no ent-epicondylar foramen. In
+the Dugong and _Rhytina_ there is a bicipital groove and the
+tuberosities are distinct, but in the Manatee there is no bicipital
+groove, and the tuberosities coalesce. The radius and ulna are about
+equally developed and ankylosed together at both ends.
+
+In the CETACEA the arm bones are very short and thick. The humerus has
+a globular head, and a distal end terminated by two equal flattened
+surfaces to which the radius and ulna are united. There is no
+bicipital groove, and the tuberosities coalesce. The radius and ulna
+are flat expanded bones fixed parallel to one another, but the ulna
+has a definite olecranon. Scarcely any movement can take place between
+them and the humerus, and in old animals the three bones are often
+ankylosed together.
+
+In the UNGULATA VERA the humerus is stout and rather short. The great
+tuberosity is always large and often overhangs the bicipital groove,
+it is especially large in _Titanotherium_ (_Brontops_). There is never
+an ent-epicondylar foramen. The radius is always large at both ends,
+but the condition of the ulna is very variable. Sometimes, as in
+_Tapirus_, _Rhinoceros_, _Macrauchenia_, Suina and Tragulina, the ulna
+is well developed, and quite distinct from the radius; but in most
+forms, although complete, it is much reduced distally, and is fused to
+the radius. Sometimes, as in the Horse and Giraffe, it is reduced to
+the olecranon and to a very slender descending process which does not
+nearly reach the carpus. In the Tylopoda, though the ulna is complete
+and its distal end is often distinct, it has coalesced with the radius
+throughout its whole length; the olecranon is generally very large.
+
+SUBUNGULATA. In the large Condylarthra the humerus has an
+ent-epicondylar foramen, and the radius and ulna are stout bones
+nearly equal in size.
+
+In _Procavia_ the humerus is rather long, and has a very prominent
+greater tuberosity, and a large supra-trochlear fossa, but no
+ent-epicondylar foramen.
+
+In the Proboscidea the humerus is marked by a greatly developed
+supinator ridge, and is very long, longer than the radius and ulna.
+The ulna has a remarkable development, having its distal end larger
+than that of the radius, it has also a larger articular surface for
+the humerus than has the radius.
+
+In RODENTIA the humerus varies much in its development according to
+the animal's mode of life. In the Hares it is long and straight, with
+a small distal end, and a slight deltoid ridge. In the Beaver on the
+other hand the deltoid and supinator ridges are considerably
+developed. There is generally a large supra-trochlear fossa, but no
+ent-epicondylar foramen.
+
+CARNIVORA. In the Carnivora vera the humerus has large tuberosities, a
+prominent deltoid ridge and a deep olecranon fossa. The shaft is
+generally curved, and an ent-epicondylar foramen is often found,
+though not in the Canidae, Hyaenidae, and Ursidae. The radius and ulna
+are never united. The radius (fig. 77, B) has a very similar
+development throughout its whole length, while the ulna has a large
+olecranon (fig. 77, C, 11) and a shaft tapering somewhat towards the
+distal end.
+
+In the Pinnipedia the arm bones are very strongly developed. The
+humerus has a very prominent deltoid ridge, and the proximal end of
+the ulna and distal end of the radius are much expanded.
+
+In the INSECTIVORA the arm bones are well developed, and the radius
+and ulna, though sometimes united, are generally distinct; as a rule
+there is an ent-epicondylar foramen, but this is absent in the
+Hedgehog. The Mole has an extraordinary humerus, very short and
+curved, and much flattened and expanded at both ends. It articulates
+both with the scapula and coraco-clavicle. The ulna has a greatly
+developed olecranon.
+
+In the CHIROPTERA both humerus and radius are exceedingly long and
+slender; the ulna is reduced to little more than the proximal end and
+is fused to the radius. There is no ent-epicondylar foramen.
+
+All PRIMATES have the power of pronation and supination of the
+fore-arm, by the rotation of the distal end of the radius round that
+of the ulna.
+
+In Man and the Anthropoid Apes the humerus is long and straight, and
+has a globular head; neither of the tuberosities, nor the deltoid nor
+supinator ridges are much developed. The olecranon fossa is deep and
+there is no ent-epicondylar foramen. The radius is curved and has a
+narrow proximal, and expanded distal end, the ulna is straighter than
+the radius and has the distal end much smaller than the proximal; the
+olecranon is not much developed.
+
+In the lower Primates, although the radius and ulna are always quite
+separate, the power of pronation and supination is not nearly so great
+as in the higher forms. In most of the Cebidae and Lemurs an
+ent-epicondylar foramen occurs.
+
+
+THE MANUS.
+
+The Manus is divisible into two parts, viz. the carpus or wrist, and
+the hand which is composed of the metacarpals and phalanges. The
+carpal bones are always modified from their primitive arrangement,
+sometimes more, sometimes less. One modification however is always
+found in mammals, viz. the union of carpalia, 4 and 5 to form the
+_unciform_ bone. Two sesamoid bones are commonly developed, one on
+each side of the carpus, the _pisiform_ or one on the ulnar side being
+much the larger and more constant: it has been suggested that these
+represent respectively vestiges of a prepollex and a post-minimus
+digit[172].
+
+One or more of the five digits commonly present may be lost, and
+sometimes all are lost except the third. The terminal or ungual
+phalanges of the digits are commonly specially modified to support
+nails, claws, or hoofs. There are as a rule two small sesamoid bones
+developed on the ventral or flexor side of the metacarpo-phalangeal
+articulations, and sometimes similar bones occur on the dorsal or
+extensor side.
+
+MONOTREMATA. In _Echidna_ the carpus is broad, the scaphoid and lunar
+are united and there is no centrale. The pisiform is large and several
+other sesamoid bones occur. Each of the five digits is terminated by a
+large ungual phalanx. In _Ornithorhynchus_ the manus is more slender,
+but the general arrangement is the same as in _Echidna_.
+
+MARSUPIALIA. The carpus has no centrale and the lunar is generally
+small or absent. Five digits are almost always present. In _Choeropus_
+however the only two functional digits are the second and third, which
+have very long closely apposed metacarpals; the fourth digit is
+vestigial, but has the normal number of phalanges, while the first and
+fifth are absent. The manus in _Notoryctes_ is extraordinarily
+modified, the scaphoid and all the distal carpalia are apparently
+fused, the first, second, and fifth digits are very small, the third
+and fourth, though having only one phalanx apiece, bear each an
+enormous claw. Lying on and obscuring the ventral surface of the manus
+is a large bone, probably a sesamoid.
+
+Among the EDENTATA there is a great diversity in the structure of the
+manus, the centrale is however always wanting, and except in _Manis_
+the scaphoid and lunar are distinct. In the Sloths the manus is very
+long, narrow, and curved, and terminated by two or three long hooked
+claws, borne by the second and third, or the second, third and fourth
+digits. The fifth digit is absent, and the fourth is represented only
+by a small metacarpal. In the Anteaters the third digit is greatly
+developed and bears a long hooked claw. In _Myrmecophaga_ all five
+digits are fairly well though irregularly developed, in _Cycloturus_
+the first, fourth, and fifth, are vestigial. In the Armadillos the
+manus is broad, and has strongly developed ungual phalanges. The
+digits, though almost always five in number, vary much in their
+relative arrangement. In _Dasypus_ they are regular, but are
+remarkably irregular in Priodon. The pollex is absent in Glyptodonts
+and in _Megatherium._ In _Megatherium_ the fifth digit is clawless
+while the second, third, and fourth bear enormous claws. In the
+Manidae the scaphoid and lunar are united; five digits are present,
+the third and fourth being very large, and all being terminated by
+deeply cleft ungual phalanges. In _Orycteropus_ the pollex is absent,
+while the other digits are terminated by pointed ungual phalanges.
+
+In SIRENIA the general structure of the manus is quite of the ordinary
+mammalian type. In _Manatus_ most of the bones of the carpus are
+distinct, but in _Halicore_ many, especially those of the distal row,
+have coalesced. The digits are always five in number and have the
+normal number of flattened phalanges.
+
+In the CETACEA, on the other hand, the manus is much modified by the
+fact that the number of phalanges may be greatly increased above the
+normal number of three, thirteen or fourteen sometimes occurring in
+each digit. These are believed to be duplicated epiphyses. In the
+Mystacoceti the manus remains largely cartilaginous, in the Odontoceti
+it is better ossified, and the phalanges commonly have epiphyses at
+both ends. In _Physeter_ the carpal bones also have epiphyses. The
+carpus generally consists of six bones arranged in two rows of three
+each. Five digits are generally present, but sometimes as in
+_Balaenoptera musculus_, there are four, the third being suppressed.
+Their relative development varies much. The Sperm Whale which till
+recently was placed in the entrance hall of the Natural History Museum
+at South Kensington has one phalanx to the first digit, four to the
+second, five to the third, four to the fourth, and three to the fifth.
+Generally the manus is short and broad, but sometimes, as in
+_Globicephalus_, it is much elongated owing to the great development
+of the second and third digits.
+
+UNGULATA[173]. The manus of the members of this great order is of very
+great classificatory and morphological importance. All the members
+agree in having the scaphoid and lunar distinct, and in almost every
+case the ends of the digits are either encased in hoofs or provided
+with broad flat nails. It is by means of characters derived from the
+manus and pes that the group is subdivided into the Ungulata vera and
+the Subungulata.
+
+In the UNGULATA VERA the manus is never plantigrade, and there are not
+more than four digits, the pollex being almost always completely
+suppressed: in _Cotylops_ among extinct Artiodactyla however a
+vestigial pollex is found. The centrale is absent, and the magnum
+articulates freely with the scaphoid, and is separated from the
+cuneiform by the unciform and lunar. All the bones of the carpus
+interlock strongly, and the axis of the third digit passes through the
+magnum and between the scaphoid and lunar.
+
+There is a very strong distinction between the manus of the suborders
+Artiodactyla and Perissodactyla. In the Artiodactyla the axis of the
+manus passes between the third and fourth digits, which are almost
+equally developed and, except in the Hippopotami and some extinct
+forms such as _Anoplotherium_, have their ungual phalanges flattened
+on their contiguous surfaces.
+
+In all _ARTIODACTYLA_ the third and fourth digits are large, but a
+gradual reduction in the second and fifth can be well traced. Thus in
+the Suina the second and fifth digits, though smaller than the third
+and fourth, are well developed and all four metacarpals are distinct.
+In the Tragulina too all four metacarpals are developed, and in
+_Dorcatherium_ the third and fourth commonly remain distinct as in the
+Suina. In the other Artiodactyla however the third and fourth
+metacarpals are almost always united, though indications of their
+separate origin remain. In some Ruminantia, such as many Deer, the
+second and fifth digits are reduced to minute splint bones attached to
+the proximal end of the fused third and fourth metacarpals, and to
+small hoof-bearing phalanges, sometimes attached to splint-like distal
+vestiges of the metacarpals, sometimes altogether unconnected with any
+other skeletal structures. In some other Ruminants, such as the Sheep
+and Oxen, the only remnants of the second and fifth digits are nodules
+of bone supporting the hoofs, and in others, such as the Giraffe,
+_Anoplotherium commune_, some Antelopes and the Tylopoda, all traces
+of these digits have disappeared. The Camels differ from all living
+Ungulata vera in not having the distal phalanges completely encased in
+hoofs, and from all except the Hippopotami in placing a considerable
+amount of the manus on the ground in walking.
+
+[Illustration FIG. 106. MANUS OF PERISSODACTYLES.
+
+_A._ LEFT MANUS OF _Tapirus_. (After VON ZITTEL.)
+
+_B._ RIGHT MANUS OF _Titanotherium_. (After MARSH.)
+
+_C._ LEFT MANUS OF _Chalicotherium gigantium_. (After GERVAIS.)
+
+ 1. scaphoid.
+ 2. lunar.
+ 3. cuneiform.
+ 4. trapezoid.
+ 5. magnum.
+ 6. unciform.
+ 7. trapezium.
+ II, III, IV, V. second, third, fourth and fifth digits.]
+
+While the manus of the Artiodactyla is symmetrical about a line drawn
+between the third and fourth digits, that of the _PERISSODACTYLA_ is
+symmetrical about a line drawn through the middle of the third digit,
+which is larger than the others and has its ungual phalanx evenly
+rounded and symmetrical in itself. The most reduced manus in the whole
+of the mammalia is found in the Horse and its allies, in which the
+third digit, terminated by a very wide ungual phalanx, is the only one
+functional. Small splint bones representing the second and fourth
+metacarpals are attached to the upper part of the third metacarpal. In
+_Hipparion_[174] and other early horse-like animals the second and
+fourth digits, though very small and functionless, are complete and
+are terminated by small hoofs. In _Rhinoceros_ the second and fourth
+digits are equally developed and nearly as large as the third, and
+reach the ground in walking, a vestige of the fifth is also present.
+In the Tapir (fig. 106, A) and _Hyracotherium_ the fifth digit is
+fully developed but is scarcely functional. In _Titanotherium_
+(_Brontops_) (fig. 106, B) it is nearly as well developed as any of
+the others, and there is little or no difference between the relative
+development of the third and fourth digits.
+
+The Chalicotheriidae[175], though distinctly Perissodactyles in
+various respects such as their cervical vertebrae and teeth, differ
+not only from all other Perissodactyles, but from almost all other
+Ungulates, in the very abnormal character of their manus. For while
+the carpus and metacarpus are like those of ordinary Perissodactyles,
+the phalanges resemble those of Edentates, each second phalanx having
+a strongly developed trochlea, and each distal one being curved,
+pointed and deeply cleft at its termination (fig. 106, C).
+
+The Macraucheniidae, while agreeing with Perissodactyles in having
+only three digits, with the limb symmetrical about a line drawn
+through the middle of the third, have a carpus which approaches
+closely to the subungulate condition, the magnum articulating
+regularly with the lunar, and only to a slight extent with the
+scaphoid.
+
+In the SUBUNGULATA the manus sometimes has five functional digits, and
+a considerable part of it rests on the ground in walking. The bones of
+the carpus retain their primitive relation to one another, the magnum
+articulating with the lunar, but not with the scaphoid. This character
+does not however hold in the Toxodontia, for in most of the animals
+belonging to this group the magnum does articulate with the scaphoid.
+The corner of the scaphoid just reaches the magnum also in Amblypoda.
+
+As far as is known the _TOXODONTIA_ generally have three, sometimes
+five digits to the manus, and the third is symmetrical in itself--a
+Perissodactyloid feature.
+
+In _Phenacodus_ (fig. 107, B) (_CONDYLARTHRA_) all five digits are
+well developed, the pollex being the smallest. The carpal bones retain
+their primitive arrangement, the magnum articulating with the lunar
+and not with the scaphoid. There is no separate centrale.
+
+[Illustration FIG. 107. LEFT MANUS OF
+
+_A. Coryphodon hamatus._ (After MARSH.) × 1/5.
+
+_B. Phenacodus primaevus._ (After COPE.) × 1/3.
+
+_C. Procavia (Dendrohyrax) arboreus._ (After VON ZITTEL.) × 6/7.
+
+ 1. scaphoid.
+ 2. lunar.
+ 3. cuneiform.
+ 4. trapezium.
+ 5. trapezoid.
+ 6. magnum.
+ 7. unciform.
+ 8. centrale.
+ 9. pisiform.
+ I, II, III, IV, V. first, second, third, fourth and fifth digits
+ respectively.]
+
+In the _HYRACOIDEA_ (fig. 107, C) the manus is very similar to that in
+_Phenacodus_, but a centrale is present and the pollex is much
+reduced.
+
+The manus of the _AMBLYPODA_, such as _Coryphodon_ (fig. 107, A) and
+_Uintatherium_, is short and broad, with five well developed digits
+and large carpal bones. The carpals however interlock to a slight
+extent, and the corner of the magnum reaches the scaphoid.
+
+In the _PROBOSCIDEA_ the manus is very short and broad, with large
+somewhat cubical carpals which articulate by very flat surfaces and do
+not interlock at all. All five digits are present, and none of them
+are much reduced in size. The manus in Proboscidea and in _Coryphodon_
+is subplantigrade.
+
+In the Tillodontia the manus is plantigrade and has pointed ungual
+phalanges, in this respect approaching the Carnivora. It differs
+however from that of all living Carnivora in having the scaphoid and
+lunar distinct.
+
+In RODENTIA the manus nearly always has five digits with the normal
+number of phalanges: the pollex may however be very small as in the
+Rabbit, or absent as sometimes in the Capybara. The scaphoid and lunar
+are generally united, and a centrale may be present or absent. In
+_Pedetes caffer_ the radial sesamoid is double and the distal bone
+bears a nail-like horny covering. In _Bathyergus_ the pisiform is
+double. It is upon these facts that the contention for the former
+existence of prehallux and post-minimus digits has partly been based.
+
+In living CARNIVORA the scaphoid, lunar and centrale are always
+united, forming a single bone. All five digits are present, but as a
+rule in Carnivora vera the pollex is small, and in _Hyaena_ is
+represented only by a small metacarpal. Sometimes, as in Cats and
+Dogs, the manus is digitigrade, sometimes, as in Bears, plantigrade.
+The ungual phalanges are large and pointed, and in forms like the
+Cats, whose claws are retractile, they can be folded back into a deep
+hollow on the ulnar side of the middle phalanx; a small radial
+sesamoid is often present.
+
+In Pinnipedia the manus is large and flat and the digits are
+terminated by ungual phalanges which are blunt (sea lions and walrus),
+or slightly curved and pointed (seals). The pollex is nearly or quite
+as long as the second digit, and as a rule the digits then
+successively diminish in size.
+
+The Creodonta differ from living Carnivora in the fact that the
+scaphoid and lunar are usually separate.
+
+In INSECTIVORA the scaphoid and lunar are sometimes united, sometimes
+separate, and a separate centrale is usually present. There are
+generally five digits, but sometimes the pollex is absent. In the Mole
+the manus is greatly developed and considerably modified. It is very
+wide, its breadth being increased by the great development of the
+radial sesamoid which is very large and sickle-shaped. The ungual
+phalanges are also large and are cleft at their extremities.
+
+In the CHIROPTERA the manus is greatly modified for the purpose of
+flight. The pollex is short and is armed with a rather large curved
+claw, the other digits are enormously elongated, the elongation in the
+case of the Insectivorous bats being mainly due to the metacarpals,
+and in the Frugivorous bats to the phalanges. In the Frugivorous bats
+the second digit is clawed as well as the pollex, in other bats this
+claw is always absent, and so is often the ungual phalanx, the middle
+phalanx then tapering gradually to its termination.
+
+In PRIMATES as a rule the manus is moderately short and wide. The
+carpus has the scaphoid and lunar distinct, and generally also the
+centrale; sometimes however, as in Man, the Gorilla, Chimpanzee, and
+some Lemurs, the centrale has apparently fused with the scaphoid.
+There are almost always five well-developed digits, but in the genera
+_Colobus_ and _Ateles_ the pollex is vestigial.
+
+The magnum in man is the largest bone of the carpus. The pisiform also
+is well developed, but there is no radial sesamoid. In Man, the
+Gorilla, Chimpanzee, and Orang, the carpus articulates only with the
+radius, in most Primates it articulates also with the ulna. The third
+digit of the Aye-Aye (_Chiromys_) is remarkable for its extreme
+slenderness.
+
+
+THE PELVIC GIRDLE.
+
+The pelvic girdle in all mammals except the Sirenia and Cetacea
+consists of two halves, usually united with one another at the
+symphysis in the mid-ventral line, and connected near their upper
+ends, with the sacral vertebrae. Each half forms one of the
+_innominate_ bones, and includes at least three separate elements, a
+dorsal bone, the ilium, and two ventral bones, the ischium and pubis.
+Very often a fourth pelvic element, the acetabular or cotyloid bone,
+occurs.
+
+In the MONOTREMATA the pelvis is short and broad, and the pubes and
+ischia meet in a long symphysis. The acetabulum is perforated in
+_Echidna_ as in birds, but not in _Ornithorhynchus_. A pair of
+elongated slender bones project forwards from the edge of the pubes
+near the symphysis; these are sesamoid bones formed by ossifications
+in the tendons of the external oblique abdominal muscles, and are
+generally called _marsupial bones_.
+
+In the MARSUPIALIA the ilia are generally very simple, straight, and
+narrow, while the pubes and ischia are well developed and meet in a
+long symphysis. Marsupial bones are nearly always prominent, but are
+not developed in _Thylacinus_ or _Notoryctes_. The ischium often has a
+well-marked tuberosity and in Kangaroos the pubis bears a prominent
+pectineal process on its anterior border close to the acetabulum. The
+pelvis in _Notoryctes_ differs much from that in all other Marsupials,
+the ilium and ischium being ankylosed with six vertebrae in a manner
+comparable to that of many Edentates.
+
+In the EDENTATA the pelvis is generally well developed, but the
+symphysis is very short. In the Sloths the pelvis is rather weak and
+slender, the obturator foramina are very large and the ischia do not
+meet in a symphysis. In the Megatheriidae the pelvis is exceedingly
+wide and massive, and is firmly ankylosed with a number of vertebrae.
+In the Armadillos, Glyptodonts, Anteaters, and Pangolins it is much
+developed and firmly united to the vertebral column by both the ilia
+and the ischia. In _Orycteropus_ however the ischium does not become
+united to the vertebral column, and the pubis generally has a strongly
+developed pectineal process.
+
+In the SIRENIA the pelvis is quite vestigial. In the Dugong it
+consists on each side of two slender bones, one of which represents
+the ilium and the other the ischium and pubis; the two bones are
+placed end to end and are commonly fused together. The ilium is
+attached by ligament to the transverse process of one of the
+vertebrae. In the Manatee each half of the pelvis is represented by a
+triangular bone connected by ligaments with its fellow and with the
+vertebral column. In neither Manatee nor Dugong is there any trace of
+an acetabulum but one can be made out in _Halitherium_.
+
+In the CETACEA the pelvis is even more vestigial than in the Sirenia,
+consisting simply of a pair of small straight bones which probably
+represent the ischia, and lie parallel to and below the vertebral
+column at the point where the development of chevron bones commences.
+
+In UNGULATA VERA the pelvis is generally rather long and narrow. The
+ilium is flattened and expanded in front (fig. 103, 8), but becomes
+much narrower and more cylindrical before reaching the acetabulum.
+Both pubis and ischium contribute to the symphysis which is often very
+long. The ischia are large and have prominent tuberosities, especially
+in Artiodactyles. In most Ruminantia there is a deep depression, the
+supra-acetabular fossa above the acetabulum, but this is not found in
+the Suina or Tylopoda.
+
+SUBUNGULATA. In _Procavia_ the pelvis is long and narrow, and bears
+resemblance to that in Artiodactyles.
+
+The Proboscidea have a very large pelvis set nearly at right angles to
+the vertebral column; the ilium is very wide, having expanded iliac[1]
+and gluteal[1] surfaces, and a narrow sacral[176] surface. The pubes
+and ischia are rather small, but both meet their fellows in the
+symphysis. _Uintatherium_ (suborder Amblypoda) also has a large and
+vertically placed pelvis (fig. 108) with a much expanded ilium. The
+pelvis however differs from that of the Proboscidea in the fact that
+the ischia do not meet in a ventral symphysis.
+
+In many RODENTIA the ilia have their gluteal, iliac, and sacral
+surfaces of nearly equal extent; in the Hares, however, the gluteal
+and iliac surfaces are confluent. The pubes and ischia are always well
+developed and sometimes, as in the Hares, the acetabular bone also. In
+these animals the pubis does not take part in the formation of the
+acetabulum, and the ischium bears on its outer side a well-marked
+ischial tuberosity.
+
+In the CARNIVORA the pelvis is long and narrow. The iliac surfaces
+(fig. 78, A, 5) are very small and the sacral large; the crest or
+supra-iliac border is formed by the union of the sacral and gluteal
+surfaces. The symphysis is long and includes part of both pubis and
+ischium. The ischial tuberosity (fig. 78, A, 10) is often well marked,
+and sometimes as in _Viverra_ the acetabular bone is distinct. In the
+Pinnipedia the pelvic symphysis is little developed, or sometimes not
+developed at all, and the obturator foramina are remarkably large.
+
+In some INSECTIVORA such as _Galeopithecus_, there is a long pelvic
+symphysis, in others such as _Erinaceus_ and _Centetes_, it is very
+short, in others again such as _Talpa_ and _Sorex_, there is no pelvic
+symphysis. The acetabular bone is exceptionally large in _Talpa_ and
+_Sorex_.
+
+In CHIROPTERA the pelvis is small and narrow, and in the great
+majority of cases the two halves do not meet in a ventral symphysis.
+The pubis has a strongly developed pectineal process, which
+occasionally unites with a process from the ilium enclosing a large
+pre-acetabular foramen.
+
+PRIMATES. In Man and the Anthropoid Apes the pelvis is very large and
+wide, and the ilium has much expanded iliac and gluteal surfaces. The
+symphysis is rather short and formed by the pubis alone. The
+acetabulum is deep and the obturator foramen large, and there is
+frequently a well-marked ischial tuberosity. In the lower Anthropoidea
+the ilium is long and narrow and has a small iliac surface. The
+ischial tuberosities are large in the old world monkeys.
+
+[Illustration FIG. 108. LEFT ANTERIOR AND POSTERIOR LIMB AND LIMB
+GIRDLE OF _Uintatherium mirabile_. The anterior limb is to the left,
+the posterior to the right × 1/10. (From casts, Brit. Mus.)
+
+ 1. ilium.
+ 2. head of femur.
+ 3. great trochanter.
+ 4. patella.
+ 5. fibula.
+ 6. tibia.
+ 7. second digit of pes.
+ 8. ungual phalanx of fifth digit of pes.
+ 9. calcaneum.
+ 10. postscapular fossa.
+ 11. prescapular fossa.
+ 12. coracoid process.
+ 13. humerus.
+ 14. radius.
+ 15. ulna.
+ 17. unciform.
+ 18. cuneiform.
+ 20. lunar.
+ 21. first metacarpal.
+ 22. fifth metacarpal.]
+
+
+THE THIGH AND SHIN.
+
+In the MONOTREMATA the femur is short, rather narrow in the middle,
+and expanded at each end. The great and lesser trochanters are large
+and about equally developed, but there is no third trochanter. The
+fibula is very large and is expanded at its proximal end, forming a
+flattened plate much resembling an olecranon. The patella is well
+developed.
+
+In the MARSUPIALIA there is no third trochanter to the femur, the
+fibula is well developed but not the patella as a general rule.
+_Notoryctes_ has a femur with a prominent ridge extending some little
+way down the shaft from the great trochanter; the tibia has a
+remarkably developed crest, and the fibula has its proximal end much
+expanded and perforated; there is an irregularly shaped patella
+closely connected with the proximal end of the tibia.
+
+EDENTATA. In the Sloths the leg bones are all long and slender. The
+femur has no third trochanter, and the fibula is complete and nearly
+equal in size to the tibia. In the Megatheriidae the leg bones are
+extraordinarily massive, the circumference of the shaft of the femur
+in _Megatherium_ equalling or exceeding the length of the bone. There
+is no third trochanter in _Megatherium_. In most of the remaining
+Edentata the leg bones are strongly developed. The femur in the
+Armadillos and Aard Varks has a strong third trochanter, and the
+tibia and fibula are both large and are commonly ankylosed together at
+either end. The limb bones are very massive also in the Glyptodonts.
+
+SIRENIA. In no living Sirenian is there any trace of a hind limb, but
+in _Halitherium_ a vestigial femur is found, which articulates with
+the pelvis by a definite acetabulum.
+
+[Illustration FIG. 109. LEFT FEMUR OF AN OX (_Bos taurus_) (to the
+left) AND OF A SUMATRAN RHINOCEROS (_R. sumatrensis_) (to the right).
+× 1/6. (Camb. Mus.)
+
+ 1. head.
+ 2. great trochanter.
+ 3. lesser trochanter.
+ 4. third trochanter.
+ 5. shaft.
+ 6. condyles.]
+
+In the Mystacoceti among the CETACEA small nodules of bone or
+cartilage occur connected with the vestigial pelvis, and may represent
+the femur and tibia. No trace of the skeleton of the hind limb is
+known in the Odontoceti.
+
+In the UNGULATA VERA the femur is noticeable for the size of the great
+trochanter (fig. 109, 2); there is no definitely constricted neck
+separating the head from the rest of the bone, and the lesser
+trochanter (fig. 109, 3) is not very prominent. All Perissodactyles
+except the Chalicotheriidae show a strongly marked third trochanter,
+but this is absent in all known Artiodactyles. The development of the
+fibula in general corresponds to that of the ulna. In _Rhinoceros_,
+_Macrauchenia_, _Tapirus_ and the Suina it is distinct and fairly well
+developed; in the Tragulina on the other hand it is vestigial, being
+reduced to the proximal end only. In the Ruminantia and Tylopoda also,
+it is much reduced forming merely a small bone attached to the distal
+end of the tibia, sometimes, as in the Red deer a slender vestige of
+the proximal end also is preserved quite detached from the distal
+portion; in the Horse this proximal portion is all that there is found
+of the fibula. The progressive diminution of the fibula can be well
+seen in the series of forms that are regarded as the ancestors of the
+Horse. The patella of the Ungulata vera is well ossified, but
+fabellae[177] are not usually found.
+
+SUBUNGULATA. Of the Toxodontia, _Toxodon_ has no third trochanter
+while _Typotherium_ and _Astrapotherium_ have one. In the Condylarthra
+the femur has well-marked lesser and third trochanters, and the fibula
+and patella are well developed. In the Hyracoidea there is a slight
+ridge on the femur in the place of the third trochanter, the fibula is
+complete, but is generally fused to the tibia at its proximal end.
+
+Of the Amblypoda, _Coryphodon_ has a third trochanter, but
+_Uintatherium_ has none; in this respect, in the vertical position and
+general appearance (fig. 108) of the limb, and in the articulation of
+the fibula with the calcaneum, the leg of _Uintatherium_ closely
+approaches that of the Proboscidea.
+
+In the Proboscidea the femur is very long and straight, the
+development of trochanters is slight, and the fibula though slender is
+complete and articulates with the calcaneum.
+
+A third trochanter is found in the Tillodontia.
+
+In RODENTIA the femur is variable, the great trochanter is generally
+large and so sometimes is the third as in the Hares. In most Rodents
+as in the Beaver the fibula is distinct, sometimes as in the Hares it
+is united distally with the tibia. The patella is well developed, and
+so too are the fabellae as a general rule.
+
+CARNIVORA. In the Carnivora vera the femur (fig. 79, A) is generally
+rather straight and slender, and has a very distinct head. The fibula
+(fig. 79, C) is always distinct and there is generally a considerable
+interval between it and the tibia. Fabellae (fig. 79, 7) are commonly
+present.
+
+In the Pinnipedia the femur is short, broad and flattened, having a
+prominent great trochanter. The fibula is nearly as large as the
+tibia, and the two bones are generally ankylosed together at their
+proximal ends.
+
+The Creodonta differ from all living Carnivores in having a femur with
+a third trochanter.
+
+In the INSECTIVORA a third trochanter is sometimes developed. The
+fibula is sometimes distinct, sometimes fused distally with the tibia,
+thus differing from that of a Carnivore.
+
+In CHIROPTERA the femur is straight, slender and rather short, with a
+small but well-developed head. The fibula may be well developed or
+quite vestigial or absent. Owing to the connection of the hind limb
+with the wing membrane the knee joint is directed backwards.
+
+In PRIMATES the femur is rather long and slender, having a nearly
+spherical head and large great trochanter. The tibia and fibula are
+always distinct and well developed. Fabellae are not found in the
+highest forms but are generally present in the others.
+
+
+THE PES.
+
+The skeleton of the pes is in most respects a counterpart of that of
+the manus. Just as in the manus if one digit is absent it is the
+pollex, so in the pes it is the hallux. But while in the manus the
+third digit is always well developed, however much the limb may be
+modified, in the pes any of the digits may be lost. In all mammals the
+tibiale and intermedium fuse to form the _astragalus_, and the fourth
+and fifth tarsalia to form the _cuboid_. Sesamoid bones are
+considerably developed. In almost every case the phalanges and first
+metatarsal have epiphyses only on their proximal ends, while the
+remaining four metatarsals have epiphyses only on their distal ends.
+
+In the MONOTREMATA all the usual tarsal bones are distinct, and the
+five digits have the normal number of phalanges. Several sesamoid
+bones are developed, the most important one, found only in the male,
+being articulated to the tibia and bearing the curious horny spur. The
+ungual phalanges of the pes like those of the manus, are deeply cleft
+at their extremities. In the Echidnidae the pes is turned outwards and
+backwards in walking.
+
+In the MARSUPIALIA the pes is subject to great modifications, but in
+every case the seven usual tarsal bones are distinct. In the
+Didelphyidae the foot is broad, all five digits are well developed,
+and the hallux is opposable to the others. In the Dasyuridae the foot
+is narrow, and the hallux may be very small, or as in _Thylacinus_
+completely absent. In _Notoryctes_ the pes is much less abnormal than
+the manus, and all five digits have the usual number of phalanges. The
+fifth metatarsal has a curious projecting process, and there is a
+large sesamoid above the hallux. In the Wombats (Phascolomyidae) the
+foot is short and broad, the digits are all distinct, and the hallux
+is divaricated from the others.
+
+In the remaining marsupials the second and third metacarpals and
+digits are very slender, and are enclosed within a common integument.
+This condition is known as _syndactylism_, and its effect is to
+produce the appearance of one toe with two claws. In the Kangaroos
+(Macropodidae) the pes is very long and narrow, owing to the
+elongation of the metacarpals. The fourth digit is greatly developed,
+the fifth moderately so, while the hallux is absent, and the second
+and third digits are very small. The Peramelidae have the foot
+constructed on the same plan as in the Kangaroos, and in one genus
+_Choeropus_ the same type of foot is carried to a greater extreme than
+even in the Kangaroos. Thus the fourth digit is enormously developed,
+the second and third are small, and the fifth smaller still, while the
+hallux is absent. In the Phalangers and Koalas though the second and
+third toes are very slender, the hallux is well developed and
+opposable.
+
+EDENTATA. In the Sloths the pes much resembles the manus, being long
+and narrow, but in both genera the second, third and fourth digits are
+well developed. Most of the other Edentates have a but little modified
+pes with the normal number of tarsal bones and the complete series of
+digits. In _Cycloturus_ however the hallux is vestigial and it is
+absent in Glyptodonts. _Megatherium_ has a greatly modified pes, the
+hallux is absent, and the second digit vestigial, while the third is
+very large, having an enormous ungual phalanx. The calcaneum too is
+abnormally large.
+
+No trace of the pes occurs in either SIRENIA or CETACEA.
+
+In the UNGULATA the pes like the manus is subject to much variation
+and is of great morphological importance.
+
+In the UNGULATA VERA the pes is never plantigrade and never has more
+than four digits, the hallux being absent. The cuboid always
+articulates with the astragalus, and the tarsal bones strongly
+interlock. As was the case also with the manus, the pes is formed on
+two well-marked types characteristic respectively of the Artiodactyla
+and Perissodactyla.
+
+_ARTIODACTYLA._ Just as in the manus, the third and fourth digits are
+well and subequally developed; their ungual phalanges have the
+contiguous sides flat, and the axis of the limb passes between them,
+and between the cuboid and navicular. The astragalus has both the
+proximal and distal surfaces pulley-like, and articulates with the
+navicular and cuboid by two facets of nearly equal size. The calcaneum
+articulates with the lower end of the fibula if that bone is fully
+developed.
+
+In the Suina four toes are developed, and though in the Peccaries the
+third and fourth metatarsals are united, they are all distinct in most
+members of the group, as are all the tarsal bones. In the Hippopotami
+the four digits are of approximately equal size, and the middle ones
+do not have the contiguous faces of their ungual phalanges flattened.
+
+In the Tragulina the cuboid, navicular, and two outer cuneiforms are
+united forming a single bone; all four metatarsals are complete and
+the two middle ones are united. In the Tylopoda and _Anoplotherium
+commune_ only the third and fourth digits are developed, their
+metatarsals are free distally, but are elsewhere united. In the
+Ruminantia the cuboid and navicular are always united and so are the
+second and third cuneiforms, while in _Cervulus_ all four bones are
+united together. The third and fourth metatarsals in Ruminants are
+always united in the same way as are the third and fourth metacarpals,
+while the second and fifth are always wanting. In Deer the second and
+fifth digits are usually each represented by three small phalanges,
+but in the Giraffe and most Bovidae the bones of these digits are
+wanting.
+
+[Illustration
+
+ FIG. 110. _A._ LEFT PES OF A TAPIR (_Tapirus americanus_). × 1/6.
+ _B._ RIGHT PES OF A RHINOCEROS (_R. sumatrensis_). × 1/8.
+ _C._ (CAST OF) RIGHT PES OF _Hipparion gracile_. × 1/7.
+ _D._ RIGHT PES OF A HORSE (_Equus caballus_). × 1/10. (All Camb. Mus.)
+
+ 1. calcaneum.
+ 2. astragalus.
+ 3. navicular.
+ 4. cuboid.
+ 5. external cuneiform.
+ 6. middle cuneiform.
+ 7. internal cuneiform.]
+
+In the _PERISSODACTYLA_ the pes like the manus is symmetrical about a
+line drawn through the third digit; this line when continued passes
+through the external cuneiform, navicular and astragalus. The
+astragalus has its distal portion abruptly truncated, and the facet by
+which it articulates with the cuboid is much smaller than that by
+which it articulates with the navicular. The calcaneum does not
+articulate with the fibula. The tarsus in _Macrauchenia_ like the
+carpus differs from that of other Perissodactyles and resembles that
+of Subungulates in having the bones arranged in lines with little or
+no interlocking. The calcaneum resembles that of Artiodactyles in
+having a small facet for articulation with the fibula. _Tapirus_ (fig.
+110, A), _Rhinoceros_ (fig. 110, B) and _Titanotherium_ have a short
+and broad foot with the usual tarsal bones and three well-developed
+digits,--a number never exceeded by any Perissodactyle. From this
+tridactylate limb a series of stages is exhibited by various extinct
+forms leading gradually to the condition met with in the Horse (fig.
+110, D) in which the third toe is greatly developed, while the second
+and fourth are reduced to slender metatarsals attached to the proximal
+half of the third metatarsal.
+
+In _Chalicotherium_ and _Agriochoerus_ the pes has the same abnormal
+characters as the manus, the digits being clawed and the ungual
+phalanges in _Chalicotherium_ deeply cleft.
+
+In the SUBUNGULATA the pes is sometimes plantigrade and
+pentedactylate, the cuboid sometimes does not articulate with the
+astragalus, and the tarsal bones sometimes do not interlock.
+
+In _Typotherium_ (_TOXODONTIA_) the hallux is absent and the other
+four digits are well developed; in _Toxodon_ and _Nesodon_ the pes is
+tridactylate. The tarsal bones have the regular Subungulate
+arrangement, the cuboid not articulating with the astragalus. The
+calcaneum articulates with the fibula as in Artiodactyles. The
+astragalus in most forms, but not in _Astrapotherium_, resembles that
+of the Ungulata vera in having a grooved proximal surface.
+
+In _Phenacodus_ (_CONDYLARTHRA_) the tarsus is very little modified,
+five digits are present, the first and fifth being small and not
+reaching the ground.
+
+In _Procavia_ only the three middle digits are present with a vestige
+of the fifth metacarpal.
+
+In the _AMBLYPODA_ the pes (fig. 108) is very short and broad, all
+five digits are functional, and at any rate in _Coryphodon_
+plantigrade, the hallux being the smallest. The astragalus is very
+flat, and the tarsals interlock to a slight extent, the cuboid
+articulating with both calcaneum and astragalus.
+
+The pes in the _PROBOSCIDEA_ much resembles that in the Amblypoda, but
+differs in that the astragalus does not articulate with the cuboid,
+the tarsals not interlocking at all.
+
+In the RODENTIA the structure of the foot is very variable. In Beavers
+the foot is very large, all five digits being well developed; the
+fifth metatarsal articulates with the outer side of the fourth
+metatarsal, and not with the cuboid, and there is a large sesamoid
+bone on the tibial side of the tarsus. In the Rats, Porcupines and
+Squirrels, there are five digits, in the Hares only four, and in the
+Capybara and some of its allies only three. In the Jerboa (_Dipus_) a
+curious condition of the pes is met with, as it consists of three very
+long metatarsals fused together and bearing three short toes, each
+formed of three phalanges. _Lophiomys_ differs from all other Rodents
+in having the hallux opposable.
+
+CARNIVORA. In the Carnivora vera the pes is regular and shows little
+deviation from the normal condition. All the usual tarsal bones are
+present, but sometimes as in the Dogs, Cats, and Hyaenas, the hallux
+is vestigial. Sometimes as in the Bears the pes is plantigrade,
+sometimes as in the Cats and Dogs it is digitigrade. In this respect
+and in the character of the ungual phalanges, the pes closely
+corresponds with the manus. In the Sea Otter (_Latax_) the foot is
+large and flattened and approaches in character that of the
+Pinnipedia.
+
+In the Pinnipedia the pes differs much from that in the Carnivora
+vera. In the Seals in which the foot cannot be used for walking, and
+is habitually directed backwards, the first and fifth digits are much
+longer and stouter than any of the others. In the Sea Lions which can
+use the pes for walking, the digits are all of nearly the same length,
+and in the Walrus the fifth is somewhat the longest.
+
+In the INSECTIVORA the pes is almost always normal, and provided with
+five digits.
+
+In the CHIROPTERA the pes is pentedactylate, and the digits are
+terminated by long curved ungual phalanges. In some genera the toes
+have only two phalanges. The calcaneum is sometimes produced into a
+long slender process which helps to support the membrane between the
+leg and the tail.
+
+Among the PRIMATES Man has the simplest form of pes. In Man all five
+digits are well developed, the hallux being considerably the largest.
+Sesamoid bones occur only under the metatarso-phalangeal joint of the
+hallux.
+
+In the other Primates the internal cuneiform has a saddle-shaped
+articulating surface for the hallux, which is obliquely directed to
+the side of the foot and opposable to the other digits. Two sesamoid
+bones are usually developed below each metatarso-phalangeal joint, and
+one below the cuboid. The second digit in Lemurs, and all except the
+hallux in _Chiromys_ have pointed ungual phalanges; in all other cases
+the ungual phalanges are flat. In some of the Lemuroidea, especially
+_Tarsius_, the tarsus is curiously modified by the elongation of the
+calcaneum and navicular.
+
+
+FOOTNOTES:
+
+[165] The figure was drawn from a photograph and the size of the jaws
+relatively to the cranium is exaggerated.
+
+[166] See W.K. Parker, "On the Structure and Development of the Skull
+in the Pig." _Phil. Trans._ pp. 289-336, 1874.
+
+[167] See W.H. Flower, "On the value of the characters of the base of
+the cranium in the classification of the order Carnivora." _P.Z.S._
+1869, p. 5.
+
+[168] See W.K. Parker, _Monograph of the shoulder-girdle and sternum
+of the Vertebrata_, _Ray Soc._ 1868.
+
+[169] See p. 405.
+
+[170] See E. Lydekker, _P.Z.S._ 1895, p. 172.
+
+[171] See H. Wincza, _Morph. Jahr._ XVI. p. 647, 1890.
+
+[172] See K. Bardeleben, _P.Z.S._, 1889, p. 259.
+
+[173] See E. Cope, "The origin of the foot structures of Ungulata,"
+_Journ. of Philad. Acad._ 1874. H.F. Osborn, "The evolution of the
+Ungulate foot," _T. Amer. Phil. Soc._ 1889.
+
+[174] See O.C. Marsh, various papers including "Fossil horses in
+America," _Amer. Natural._ 1874; "Polydactyl horses," _Amer. J. Sci._
+1879 and 1892. M. Pavlow, "Le développement des Equidés," _Bul. Soc.
+Moscou_, 1887, and subsequent papers in the same. Osborn and Wortman,
+"On the Perissodactyls of the White River beds," _Bull. Amer. Mus._
+Dec. 23rd, 1895.
+
+[175] See H.F. Osborn, _Chalicotherium and Macrotherium, Amer.
+Natural._ 1889--91--92.
+
+[176] See p. 409.
+
+[177] See p. 412.
+
+
+
+
+LIST OF AUTHORS REFERRED TO.
+
+
+ Abbott, E.C., 112
+
+ Ameghino, F., 351, 424
+
+ Andrews, C.W., 299
+
+
+ Balfour, F.M., 16
+
+ Ballowitz, E., 424
+
+ Bardeleben, K., 504
+
+ Bateson, W., 50, 344
+
+ Baum, H., 374
+
+ Baur, G., 27, 189, 190, 344, 346
+
+ Beneden, P.J. van, 353
+
+ Benham, W.B., 51
+
+ Bettany, G.T., 16, 87, 154
+
+ Boulenger, G.A., 169
+
+ Brandt, J.F., 352
+
+ Bridge, T.W., 123
+
+ Brühl, C.B., 210
+
+ Burmeister, H., 351, 424
+
+
+ Cope, E.D., 135, 199, 204, 351, 359, 361, 363, 368
+
+ Credner, H., 135
+
+
+ Dean, B., 63, 104
+
+ Dobson, G.E., 369, 370
+
+
+ Earle, C., 432
+
+ Ecker, A., 151
+
+ Ellenberger, W., 374
+
+
+ Flower, W.H., 28, 42, 351, 420, 422, 434
+
+ Fritsch, A., 135
+
+ Fürbringer, M., 295
+
+
+ Gadow, H., 40, 112, 190, 295, 343, 350
+
+ Gegenbaur, C., 127
+
+ Gervais, P., 353
+
+ Günther, A.C.L.G., 70, 104
+
+
+ Haslam, G., 151
+
+ Hasse, C., 112, 113
+
+ Haswell, W.A., 127
+
+ Hertwig, O., 169
+
+ Hoffmann, C.K., 190, 202, 210
+
+ Howes, G.B., 164, 451
+
+ Hubrecht, A.A.W., 104
+
+ Hulke, J.W., 192, 204
+
+ Hurst, C.H., 71, 297
+
+ Hutton, F.W., 299
+
+ Huxley, T.H., 11, 13, 133, 135, 191, 210, 295, 297, 334, 343, 351,
+ 374, 437
+
+
+ Kirkaldy, J.W., 51
+
+ Klein, E., 11
+
+ Kölliker, A., 9
+
+ Kükenthal, W., 349, 422
+
+
+ Lankester, E. Ray, 51
+
+ Leche, W., 344, 423
+
+ Lindsay, B., 336
+
+ Lydekker, R., 36, 42, 190, 195, 495
+
+
+ Macbride, E.W., 50
+
+ Marsh, O.C., 204, 209, 299, 348, 361, 364, 365, 508
+
+ Marshall, A.M., 71, 151
+
+ Masterman, A.T., 51
+
+ Meyer, H. v., 135
+
+ Miall, L.C., 135, 243
+
+ Mivart, St G., 369
+
+ Morgan, C. Lloyd, 11
+
+
+ Newton, E.T., 283
+
+
+ Osborn, H.F., 348, 420, 429, 508
+
+ Owen, R., 191, 204, 210, 297, 348, 351, 420
+
+
+ Parker, T.J., 83, 96, 299
+
+ Parker, W.K., 16, 24, 53, 87, 154, 173, 200, 243, 465, 489
+
+ Pavlow, M., 358, 508
+
+ Pollard, H.B., 119
+
+ Poulton, E.B., 422
+
+ Pycraft, W.P., 297
+
+
+ Ridewood, W.G., 106, 164
+
+ Röse, C., 422
+
+
+ Sagemehl, M., 104
+
+ Schäfer, E., 11
+
+ Scott, W.B., 368
+
+ Seeley, H.G., 191, 212
+
+ Selenka, E., 40, 295
+
+ Shufeldt, R., 123
+
+ Smith, E. Noble, 11
+
+ Stirling, E.C., 423
+
+ Swirski, G., 103
+
+
+ Taeker, J., 427
+
+ Thomas, O., 349, 362, 370, 422, 424, 425
+
+ Tomes, C.S., 420
+
+ Traquair, R.H., 55, 58
+
+
+ Vogt, C., 297
+
+
+ Wiedersheim, R., 25, 134, 136
+
+ Wincza, H., 358, 495
+
+ Woodward, A. Smith, 34, 54, 58, 62, 127, 210
+
+ Wortman, J.L., 508
+
+ Wray, R.S., 303
+
+
+ Zittel, K.A. v., 36, 205, 212
+
+
+
+
+INDEX.
+
+
+ Every reference is to the page: words in italics are names of
+ genera or species; figures in italics indicate that the
+ reference relates to systematic position; figures in thick type
+ refer to an illustration; _f._ = and in following page or
+ pages; _n._ = note.
+
+
+ Aard Vark, _44_, _352_;
+ femur, 517;
+ sacrum, 452;
+ teeth, 425;
+ see _Orycteropus_
+
+ Aard wolf, _48_;
+ see _Proteles_
+
+ Abdominal ribs, crocodile, 260;
+ reptiles, 286
+
+ Abdominal shield, turtle, 215
+
+ _Acanthias_, _32_;
+ calcification of vertebrae, 114;
+ pectoral fins, 130
+
+ _Acanthodes_, _32_, _64_
+
+ Acanthodii, _32_;
+ general characters, 64;
+ spines, 106
+
+ _Acanthomys_, _47_;
+ spines, 417
+
+ Acanthopterygii, _34_
+
+ Accipitres, _41_
+
+ Acetabular bone, 25, 513;
+ dog, 409 f.;
+ frog, 165
+
+ Acetabulum, 25;
+ crocodile, 266;
+ dog, 409;
+ duck, 324;
+ frog, 165;
+ newt, 149;
+ turtle, 235
+
+ _Acipenser_, _32_, 117;
+ exoskeleton, 67;
+ distribution, 66;
+ pectoral fins, 131;
+ plates, 104;
+ skull, 121, =122=;
+ spinal column, 112
+
+ Acipenseridae, _32_
+
+ Acrodont, defined, 199;
+ teeth of reptiles, 273
+
+ _Acrodus_, _32_;
+ teeth, 109
+
+ Acromion, dog, 405
+
+ _Actinotrocha_, _30_;
+ organ regarded as double notochord, 51
+
+ Ad-digital quill, duck, 303
+
+ Adjutant, _41_;
+ clavicles, 338
+
+ Ægithognathous, 335
+
+ Æluroidea, _48_, _369_;
+ teeth, 437
+
+ Æpyornis, _40_;
+ tibio-tarsus, 341
+
+ Æpyornithes, _40_, _299_
+
+ Aftershaft, 328
+
+ _Agama_, _38_;
+ teeth, 273
+
+ Agamidae, _38_;
+ premaxillae, 284
+
+ Aglossa, _36_
+
+ Agouti, _48_;
+ see _Dasyprocta_
+
+ Agriochoeridae, _45_
+
+ _Agriochoerus_, _45_;
+ pes, 525
+
+ Ala spuria, duck, 304
+
+ Alcidae, _42_;
+ see Auks
+
+ Alisphenoid, 19;
+ crocodile, 247;
+ duck, 317;
+ dog, 386
+
+ _Alligator_, _39_, _210_, _212_;
+ hyoid, =285=;
+ limbs, =264=;
+ pectoral girdle, =262=;
+ pelvis and sacrum, =267=;
+ scutes, 271;
+ skull, =245=, =248=, =253=
+
+ Alligatoridae, 39
+
+ _Alytes_, _36_;
+ fronto-parietal fontanelle, 179;
+ vertebrae, 172
+
+ Amblypoda, _47_;
+ general characters, 363;
+ manus, 510;
+ pes, 525;
+ skull, 473;
+ teeth, 433;
+ thigh and shin, 519
+
+ _Amblystoma_, _35_;
+ skull, 175
+
+ American monkeys, _373_;
+ see _Cebidae_
+
+ American vultures, _41_;
+ vomers, 335
+
+ _Amia_, _33_;
+ distribution, 66;
+ exoskeleton, 67;
+ pectoral fin, 131;
+ scales, 105;
+ skull, 123;
+ tail, 115, 117;
+ vertebrae, 114
+
+ Amiidae, _33_
+
+ _Ammocoetes_, _31_, _55_
+
+ Amphibia, _35_;
+ anterior limb, 185;
+ exoskeleton, 168;
+ general characters, 133;
+ hyoid apparatus, 180;
+ pectoral girdle, 184;
+ pelvic girdle, 187;
+ posterior limb, 188;
+ ribs, 182;
+ skull, 173;
+ sternum, 182;
+ teeth, 169;
+ vertebral column, 170
+
+ Amphicoelous, defined, 14
+
+ _Amphioxus_, _30_;
+ skeleton, 51 f.;
+ spinal column, 112
+
+ _Amphisbaena_, _38_, 272;
+ loss of limbs, 289
+
+ Amphisbaenidae, _38_, _200_;
+ pectoral girdle, 288;
+ skull, 277;
+ vertebral column, 275
+
+ Amphitheriidae, _43_
+
+ _Amphiuma_, _35_, _135_;
+ manus, 187;
+ pes, 188;
+ skull, 174
+
+ Amphiumidae, _35_
+
+ Anacanthini, _33_
+
+ Anal shield, turtle, 215
+
+ _Anas_, _41_;
+ _A. boschas_, see Duck
+
+ Ankylosis, defined, 12
+
+ Angel fish, _32_
+
+ Angler, attachment of teeth, 107
+
+ Anguidae, _38_
+
+ _Anguilla_, _33_;
+ see Eel
+
+ _Anguis_, _38_;
+ loss of limbs, 289;
+ scutes, 271
+
+ Angular, 22;
+ cod, 100;
+ crocodile, 258;
+ duck, 319;
+ salmon, 94;
+ turtle, 231
+
+ Angulo-splenial, frog, 161
+
+ Ankle joint, duck, 327;
+ reptiles, 294
+
+ Anomodontia, _36_
+
+ Anoplotheriidae, _45_
+
+ _Anoplotherium_, _45_;
+ manus, 506;
+ pes, 523;
+ tail, 454;
+ teeth, 428
+
+ _Anser_, _41_
+
+ Anseres, _41_;
+ aftershaft, 329;
+ claws, 330
+
+ Anseriformes, _41_
+
+ Anteaters, _352_;
+ absence of teeth, 424;
+ manus, 505;
+ pectoral girdle, 495;
+ pelvis, 513;
+ skull, 458;
+ thoraco-lumbar vertebrae, 447;
+ Spiny --, _43_;
+ Great and Two-toed --, _44_
+
+ Antelope, _359_;
+ manus, 507;
+ Four-horned A., _46_
+
+ Anterior limb, 26;
+ Amphibia, 185;
+ birds, 338;
+ crocodile, 263;
+ dog, 405;
+ duck, 322;
+ frog, 164;
+ newt, 147;
+ reptiles, 290;
+ turtle, 232
+
+ Anthropoid apes, _373_;
+ arm-bones, 503;
+ pelvis, 515
+
+ Anthropoidea, 49;
+ general characters, 372;
+ sacrum, 452;
+ skull, 482;
+ teeth, 441
+
+ _Anthropopithecus_, _49_;
+ ribs, 493
+
+ Antiarcha, _31_;
+ general characters, 55
+
+ Antibrachium, see fore-arm
+
+ _Antilocapra_, _46_;
+ horns, 417
+
+ Antilocapridae, _46_
+
+ Antitrochanter, duck, 325
+
+ Antlers, 8, 358;
+ Cervidae, 469
+
+ Antorbital process, 18
+
+ Anura, _36_;
+ general characters, 136;
+ hyoid apparatus, 180;
+ pelvis, 187;
+ posterior limb, 188;
+ skull, 179;
+ sternum, 182;
+ vertebrae, 172
+
+ _Apatornis_, _40_;
+ vertebrae, 332
+
+ Apteria, 328
+
+ Apteryges, _40_
+
+ _Apteryx_, _40_, _299_;
+ aftershaft, 329;
+ anterior nares, 333;
+ claws, 330;
+ foot, 342;
+ manus, 338;
+ pectineal process, 341;
+ pectoral girdle, 338;
+ pneumaticity of skeleton, 331;
+ _A. oweni_, pelvic girdle and sacrum, =340=
+
+ Aqueductus vestibuli, dogfish, 74
+
+ Arcade:
+ infratemporal--, crocodile, 255;
+ _Sphenodon_, 283;
+ inner --, duck, 318;
+ outer --, duck, 318;
+ supratemporal --, crocodile, 257;
+ reptiles, 281
+
+ Archaeoceti, _44_;
+ general characters, 356;
+ skull, 461;
+ teeth, 426
+
+ _Archaeopteryx_, _40_, _297_;
+ claws, 330;
+ fibula, 341;
+ mandible, 335;
+ metatarsals, 342;
+ pelvis, 341;
+ ribs, 336;
+ sacrum, 333;
+ skull, 333;
+ tail, 333;
+ teeth, 330;
+ wing, 338
+
+ Archaeornithes, _40_;
+ characters, 297
+
+ _Archegosaurus_, _35_, _136_;
+ palatines, 177
+
+ Archipterygium, _Ceratodus_, 127;
+ Ichthyotomi, 62
+
+ Arcifera, _36_, 185
+
+ Arctoidea, _48_, 369;
+ teeth, 438
+
+ _Ardea_, _41_;
+ see Heron
+
+ Ardeae, _41_
+
+ Arm, see fore-arm and upper arm
+
+ Armadillo, _44_, _352_;
+ cervical vertebrae, 443;
+ femur, 517;
+ humerus, 501;
+ lumbar vertebrae, 447;
+ manus, 505;
+ pectoral girdle, 495;
+ pelvis, 513;
+ ribs, 491;
+ sacrum, 452;
+ scales, 417;
+ scutes, 419;
+ skull, 459;
+ teeth, 424
+
+ Armour plates, 8
+
+ Arthrodira, _34_;
+ characters, 70
+
+ Articular, 22;
+ cod, 100;
+ crocodile, 258;
+ duck, 319;
+ newt, 144;
+ salmon, 94;
+ turtle, 231
+
+ Artiodactyla, _45_;
+ characters, 358;
+ manus, 506;
+ odontoid process, 445;
+ pes, 522;
+ ribs, 491;
+ skull, 465;
+ teeth, 427;
+ thoraco-lumbar vertebrae, 448
+
+ Asses, _360_
+
+ _Asterolepis_, _31_, _55_
+
+ Asterospondyli, 114
+
+ Astragalus, 27;
+ crocodile, 268;
+ dog, 414;
+ mammals, 521
+
+ Astrapotheriidae, _46_
+
+ _Astrapotherium_, _46_, 361;
+ dental formula, 432;
+ femur, 519;
+ pes, 525
+
+ _Ateles_, _49_;
+ pollex, 512;
+ tail, 454
+
+ Atlantosauridae, _38_
+
+ Atlas, 15;
+ crocodile, 240;
+ dog, =379=, 380;
+ duck, 309;
+ ox, =445=;
+ turtle, 219
+
+ Attachment of teeth, 4;
+ in fish, 107
+
+ _Auchenia_, _45_;
+ see Llama
+
+ Auditory aperture or meatus:
+ external --, crocodile, 250;
+ dog, 402;
+ turtle, 228;
+ internal --, crocodile, 246, 251;
+ dog, 392;
+ turtle, 228
+
+ Auditory capsule, 20;
+ cod, 96;
+ crocodile, 250;
+ dog, 390;
+ dogfish, 74;
+ frog, 156;
+ newt, 143;
+ turtle, 227
+
+ Auditory ossicles, crocodile, 251;
+ dog, 393;
+ duck, 320;
+ mammals, 485 f.;
+ turtle, 228
+
+ Auks, _42_;
+ thoracic vertebrae, 332
+
+ Autostylic, 61, 119
+
+ Aves, _40_;
+ characters, 295;
+ see Birds
+
+ Axial skeletal rods, 50
+
+ Axial skeleton, crocodile, 239;
+ cod, 83;
+ dog, 377;
+ dogfish, 72;
+ duck, 307;
+ frog, 152;
+ newt, 138;
+ turtle, 218
+
+ Axis vertebra, crocodile, 241;
+ dog, 380;
+ duck, 309;
+ turtle, 220
+
+ Axolotl, _35_;
+ see _Siredon_
+
+ Aye Aye, _49_;
+ see _Chiromys_
+
+
+ _Babirussa_, _45_;
+ dental formula, 428
+
+ Baboon, _49_;
+ see _Cynocephalus_
+
+ _Balanoglossus_, _30_, 50
+
+ _Balaena_, _44_, _357_;
+ scapula, 495;
+ _B. mysticetus_ baleen, 419
+
+ Balaenidae, _44_
+
+ Balaenoidea, _44_;
+ general characters, 356
+
+ _Balaenoptera_, _44_, 357;
+ manus, 506;
+ thoracic vertebrae, 448;
+ scapula, 495;
+ _B. musculus_, cervical vertebrae, =444=
+
+ Baleen, 3, 418
+
+ _Balistes_, _33_;
+ teeth, 111
+
+ Balistidae, _33_
+
+ Ball and socket joints, 13
+
+ Bandicoot, _43_
+
+ Barb, 302
+
+ Barbule, 303
+
+ Barramunda, _34_;
+ see _Ceratodus_
+
+ Basalia, dogfish, 79
+
+ Basibranchial, dogfish, 78;
+ cod, 101;
+ duck, 320;
+ newt, 145;
+ salmon, 95
+
+ Basi-branchiostegal, cod, 101;
+ salmon, 95
+
+ Basicranial axis, 19;
+ dog, 384
+
+ Basidorsalia, dogfish, 72
+
+ Basi-hyal, dogfish, 78;
+ dog, 399;
+ duck, 320
+
+ Basilar plate, 17
+
+ Basilingual plate, Anura, 180;
+ crocodile, 259;
+ frog, 161;
+ turtle, 231
+
+ Basi-occipital, 19;
+ crocodile, 246;
+ cod, 97;
+ dog, 386;
+ duck, 315;
+ salmon, 89;
+ turtle, 224
+
+ Basipterygium, cod, 103;
+ dogfish, 82
+
+ Basisphenoid, 19;
+ crocodile, 247;
+ dog, 386;
+ salmon, 91;
+ turtle, 225
+
+ Bastard wing, duck, 304
+
+ Bathyerginae, palate, 366
+
+ _Bathyergus_, _47_;
+ auditory ossicles, 488;
+ manus, 511
+
+ Batoidei, _32_, 64
+
+ _Batrachoseps_, _35_;
+ teeth, 169
+
+ Bats, claws, 418;
+ Horseshoe bats, _49_;
+ see Chiroptera
+
+ _Bdellostoma_, _31_, _55_;
+ teeth, 57
+
+ Beak, 3;
+ birds, 329;
+ duck, 302;
+ _Siren_, 168;
+ tadpoles of Anura, 168;
+ turtle, 215
+
+ Bears, _48_, _369_;
+ manus, 511;
+ pes, 526;
+ sacral vertebrae, 452;
+ skull, 479;
+ Isabelline -- mandible, =438=
+
+ Beaver, _47_;
+ fibula, 520;
+ humerus, 502;
+ pes, 526;
+ sacrum, 452;
+ tail, 454
+
+ _Belodon_, _39_, 211;
+ frontals, 277;
+ palate, 281;
+ vertebrae, 275
+
+ Bichir, _33_;
+ see _Polypterus_
+
+ Bicipital groove, dog, 405
+
+ Bilophodont, defined, 345;
+ teeth of Tapiridae, 429
+
+ _Bipes_, _38_;
+ limbs, 289
+
+ Birds, anterior limb, 338;
+ endoskeleton, 331 f.;
+ exoskeleton, 328 f.;
+ general characters, 295;
+ hyoid, 336;
+ pectoral girdle, 336;
+ pelvic girdle, 339;
+ posterior limb, 341;
+ ribs, 336;
+ skull, 333;
+ sternum, 336;
+ teeth, 330;
+ vertebral, 332
+
+ _Bison_, _46_;
+ occipital crest, 468
+
+ Blind snake, _38_;
+ see _Typhlops_
+
+ Blind worm, _38_;
+ see _Anguis_
+
+ Boidae, _38_
+
+ _Bombinator_, _36_;
+ vertebrae, 172
+
+ Bone, development of, 10 f.
+
+ Bone cells, 10
+
+ Bony Ganoids, fins, 105;
+ pelvic fin, 132;
+ ribs, 126;
+ skull, 123;
+ vertebral column, 114;
+ see _Holostei_
+
+ Border:
+ alveolar --, of dog's jaw, 398;
+ coracoid, glenoid, and suprascapular -- of dog's scapula, 405
+
+ _Bos_, _46_;
+ occipital crest, 468;
+ ribs, 491;
+ see Ox
+
+ Bottlenose, _44_;
+ see _Hyperoödon_
+
+ Bovidae, _46_;
+ pes, 523;
+ skull, 468
+
+ Bow-fin, _33_;
+ see _Amia_
+
+ Brachial ossicles, cod, 103
+
+ Brachium; see upper arm
+
+ Brachydont, defined, 345;
+ teeth of Ungulates, 429 f.
+
+ _Brachycephalus_, _36_;
+ bony plates of, 168
+
+ Brain case, crocodile, 245;
+ dog, 384;
+ duck, 314;
+ frog, 154;
+ newt, 140;
+ turtle, 224
+
+ Bradypodidae, _43_;
+ see Sloths
+
+ _Bradypus_, _43_;
+ cervical vertebrae, 443;
+ pectoral girdle, 495;
+ skull, 457;
+ thoraco-lumbar vertebrae, 447
+
+ Branchial arches, Amphibia, 180 f.;
+ cod, 101;
+ dogfish, 78;
+ fish, 120 f.;
+ newt, 145;
+ salmon, 95;
+ -- basket, Marsipobranchii, 38;
+ -- skeleton, _Amphioxus_, 52;
+ Balanoglossus, 50
+
+ _Branchiosaurus_, _35_;
+ branchial arches, 180
+
+ Branchiostegal rays, cod, 100
+
+ _Brontops_, _46_;
+ see _Titanotherium_
+
+ _Brontosaurus_, _38_, _207_;
+ sternum, 288
+
+ _Bubalus_, _46_;
+ ribs, 491;
+ see Buffalo
+
+ Buccal skeleton, _Amphioxus_, 52
+
+ _Buceros_, _42_;
+ see Hornbill
+
+ Buckler, of Labyrinthodonts, 168, 184
+
+ Buffalo, _46_;
+ Cape --, skeleton of, =492=
+
+ _Bufo_, _36_;
+ hyoid, 182;
+ jaws, 169;
+ _B. viridis_, carpus, 186
+
+ Bufonidae, _36_
+
+ Bunodont, defined, 345;
+ teeth of Ungulata, 427 f.
+
+ Buno-selenodont, defined, 432
+
+
+ Caeciliidae, _35_
+
+ _Caiman_, _39_;
+ _C. latirostris_ hyoid, =285=,
+ limbs, =264=,
+ lateral view of skull, =248=,
+ palatal view of cranium and mandible, =245=,
+ longitudinal section of skull, =253=,
+ pectoral girdle, =262=,
+ pelvic girdle and sacrum, =267=;
+ _C. sclerops_, scutes, 271
+
+ Ca'ing whale, _45_;
+ see _Globicephalus_
+
+ _Calamoichthys_, _33_;
+ distribution, 66
+
+ Calamus, 302
+
+ Calcaneum, 27;
+ crocodile, 268;
+ dog, 414
+
+ Calcar, of frog, 167
+
+ _Callorhynchus_, _32_, _66_;
+ teeth, 110
+
+ Camel, _45_, _359_;
+ manus, 507;
+ teeth, 428
+
+ Camelidae, _45_
+
+ _Camelus_, _45_;
+ see Camel
+
+ Camptosauridae, _39_
+
+ Canal:
+ alisphenoid --, dog, 402;
+ carotid --, duck, 315;
+ Eustachian --, crocodile, 247;
+ dog, 402;
+ duck, 316;
+ interorbital --, dogfish, 76
+
+ Canaliculi, 10
+
+ Canidae, _48_;
+ humerus, 502;
+ skull, 479;
+ see Dog
+
+ Canine, 344;
+ dog, 376 f.
+
+ _Canis_, _48_;
+ thoraco-lumbar vertebrae, 450;
+ see Dog
+
+ _Capitosaurus_, _35_;
+ skull, =176=
+
+ Capybara, _48_;
+ manus, 511;
+ pes, 526;
+ skull, 476;
+ tail, 454
+
+ Carapace, Chelonia, 271;
+ _Dermochelys_, 272;
+ Glyptodonts, 419;
+ Green turtle, 215;
+ Loggerhead turtle, =216=
+
+ Carcharidae, _32_
+
+ Carina sterni, duck, 321
+
+ Carinatae, _40_;
+ general characters, 300;
+ quadrate, 334
+
+ Carnassial teeth, 368;
+ carnivora, =436=;
+ dog, 376 f.
+
+ Carnivora, _48_;
+ arm bones, 502;
+ auditory ossicles, 488;
+ cervical vertebrae, 446;
+ general characters, 367;
+ manus, 511;
+ pelvis, 515;
+ pes, 526;
+ ribs, 493;
+ sacral vertebrae, 452;
+ skull, 478;
+ sternum, 490;
+ tail, 454;
+ teeth, 437;
+ thigh and shin, 520;
+ thoraco-lumbar vertebrae, 450
+
+ Carnivora vera, _48_;
+ general characters, 368;
+ scapula, 497
+
+ Carp, _33_;
+ pharyngeal teeth, 111
+
+ Carpo-metacarpus, duck, 324
+
+ Carpus, 26;
+ crocodile, 265;
+ dog, 408;
+ duck, 323;
+ frog, 164;
+ newt, 147;
+ turtle, 233
+
+ Cartilage, structure of, 10
+
+ Cartilaginous ganoids, cranium, 121;
+ pelvic fin, 132;
+ spinal column, 112;
+ see Chondrostei
+
+ Cassowary, _40_, _299_;
+ aftershaft, 328;
+ bony crest, 334;
+ claws, 330;
+ pelvic girdle and sacrum, =340=;
+ secondaries, 329
+
+ _Castor_, _47_;
+ see Beaver
+
+ Castoridae, _47_
+
+ _Casuarius_, _40_;
+ see Cassowary
+
+ Cataphracti, _34_
+
+ Cat, _48_, _369_;
+ hallux, 526;
+ manus, 511;
+ skull, 479
+
+ Cat-fish, _33_
+
+ Cathartae, _41_
+
+ _Cathartes_, _41_;
+ see American vulture
+
+ Caudal fin, Cetacea, 453;
+ fish, 116;
+ -- vertebrae, crocodile, 243;
+ cod, 85;
+ dog, 383;
+ duck, 312;
+ general characters, 16;
+ newt, 140;
+ turtle, 222
+
+ _Cavia_, _48_;
+ tail, 454
+
+ Caviidae, _48_
+
+ Cebidae, _49_, _373_;
+ ribs, 493;
+ skull, 484;
+ teeth, 441
+
+ _Coenolestes_, _43_, 424
+
+ Cement, 5
+
+ _Centetes_, _49_;
+ caudal vertebrae, 454;
+ pelvic symphysis, 515;
+ spines, 417;
+ teeth, 440;
+ thoraco-lumbar vertebrae, 450
+
+ Centetidae, _49_;
+ auditory ossicles, 488;
+ skull, 480
+
+ Centrale, 27;
+ see Carpus and Tarsus
+
+ Centre of motion, 448
+
+ Centrum, 14
+
+ _Cephalaspis_, _31_, _55_
+
+ Cephalic shield, armadillos, 419
+
+ _Cephalochordata_, _30_, 51
+
+ _Cephalodiscus_, _30_, 50
+
+ _Ceratodus_, _34_, _70_;
+ branchial arches, 124;
+ cranium, =125=;
+ skeleton, =128=;
+ skull, 117, 124;
+ spinal column, 113;
+ teeth, 111
+
+ Cerato-branchial, cod, 101;
+ dogfish, 78;
+ duck, 320;
+ salmon, 95
+
+ Cerato-hyal, 23;
+ cod, 100;
+ dog, 399;
+ dogfish, 78;
+ salmon, 95
+
+ _Ceratophrys_, _36_;
+ bony plates of, 168;
+ teeth, 170
+
+ _Ceratops_, _39_;
+ see _Polyonax_
+
+ Ceratopsia, _39_;
+ characters, 209;
+ premaxillae, 284
+
+ Ceratopsidae, _39_
+
+ _Ceratosaurus_, _38_, 208;
+ supratemporal fossae, 283;
+ _C. nasicornis_, skeleton, =206=
+
+ Cercopithecidae, _49_, _373_
+
+ Cervical ribs, crocodile, 260;
+ reptiles, 285
+
+ Cervical vertebrae, crocodile, 239;
+ dog, 380;
+ duck, 307;
+ general characters, 15;
+ mammals, 442;
+ turtle, 219
+
+ Cervidae, _46_;
+ skull, 469
+
+ _Cervulus_, _46_;
+ pes, 523
+
+ _Cervus_, 46;
+ _C. megaceros_, antlers, 469
+
+ _Cestracion 32_;
+ calcification of vertebrae 114;
+ external branchial arches 121;
+ pectoral fin 130;
+ skull =118=;
+ suspensorium 119;
+ teeth 109;
+ vertebral column 114
+
+ Cestraciontidae _32_
+
+ Cetacea _44_, 522;
+ arm bones 501;
+ auditory ossicles 487;
+ caudal vertebrae 453;
+ cervical vertebrae 444;
+ characters 353;
+ exoskeleton 416 f.;
+ hind limb 518;
+ manus 505;
+ pectoral girdle 495;
+ pelvis 514;
+ position of limbs 28;
+ ribs 491;
+ skull 461 f.;
+ sternum 489;
+ teeth 426;
+ thoraco-lumbar vertebrae 448
+
+ Cetiosauridae _38_
+
+ _Chalcides_ _38_;
+ limbs 289
+
+ Chalicotheriidae _46_;
+ femur 519;
+ manus 509
+
+ _Chalicotherium_ _46_;
+ femur 360;
+ pes =508=, 525;
+ teeth 432
+
+ _Chamaeleon_ _38_, 199 f.;
+ epipubis 293;
+ ilia 291;
+ manus 291;
+ skull 278
+
+ Chamaeleonidae _38_
+
+ Charadriidae _42_
+
+ Charadriiformes _42_
+
+ _Chauna_ _41_;
+ interorbital septum 333;
+ ribs 336;
+ _C. derbiana_, spurs 330
+
+ _Chelone_ _37_, _194_;
+ plastron 271, =218=;
+ see Turtle
+
+ Chelonia _37_;
+ beaks 271;
+ carapace 271;
+ general characters 193;
+ humerus 290;
+ limbs 290;
+ palate 281;
+ pectoral girdle 288;
+ pelvic girdle 291;
+ skull 277 f.;
+ tarsus 293;
+ vertebrae 275 f.
+
+ Chelonidae _37_
+
+ Chelydae _37_
+
+ _Chelydra_ _37_;
+ carpus 26, 291
+
+ Chelydridae _37_
+
+ _Chelys_ _37_, _195_
+
+ Chersidae _37_
+
+ Chevron bones 16;
+ crocodile 243;
+ mammals 453 f.;
+ reptiles 276
+
+ Chevrotain _45_, _359_;
+ teeth 429
+
+ _Chimaera_ _32,_ _66_;
+ attachment of fins 130;
+ pelvic girdle 127;
+ skull =65=;
+ teeth 110
+
+ Chimaeridae _32_
+
+ Chimaeroidei, general characters 65
+
+ Chimpanzee _49_;
+ carpus 512;
+ ribs 493;
+ thoraco-lumbar vertebrae 450
+
+ _Chinchilla_ _47_;
+ auditory ossicles 488
+
+ Chinchillidae _47_
+
+ Chiromyidae _49_
+
+ _Chiromys_ _49,_ _372_;
+ manus 512;
+ pes 527;
+ teeth 441
+
+ Chiroptera _49_;
+ auditory ossicles 488;
+ arm bones 503;
+ cervical vertebrae 446;
+ general characters 370;
+ manus 512;
+ pelvis 515;
+ pes 527;
+ sacrum 452;
+ shoulder girdle 499;
+ skull 481;
+ sternum 490;
+ tail 454;
+ teeth 440;
+ thigh and shin 520;
+ thoraco-lumbar vertebrae 450
+
+ _Chirotes_ _38_;
+ limbs 289
+
+ _Chlamydophorus_ _44,_ _272_;
+ scutes 419;
+ skull 459
+
+ _Chlamydoselache_ _31_;
+ branchial arches 121
+
+ _Choeropus_ _43_;
+ manus 504;
+ pes 522
+
+ _Choloepus_ _43_;
+ ribs 491;
+ shifting of pelvis 451;
+ skull =458=;
+ sternum 489;
+ thoraco-lumbar vertebrae 447;
+ _C. hoffmanni_ cervical vertebrae 443
+
+ Chondrocranium, salmon 87
+
+ Chondroid tissue, _Balanoglossus_ 50
+
+ Chondrostei _32_;
+ fins 105;
+ general characters 67;
+ teeth 110;
+ see Cartilaginous ganoids
+
+ Chordal sheath, _Amphioxus_ 52
+
+ Chrysochloridae _49_
+
+ _Chrysochloris_ _49_;
+ auditory ossicles 488;
+ claws 418;
+ teeth 440
+
+ _Ciconia_ _41_;
+ see Stork
+
+ Ciconiiformes _41_
+
+ Cingulum 376
+
+ Civet _48,_ _369_;
+ teeth 437
+
+ _Cladoselache_ _31,_ _63_; fin 129
+
+ Clasper 132;
+ dogfish 82
+
+ Clavicle 25;
+ birds 338;
+ cod 102;
+ duck 322;
+ dog 405;
+ fish 126;
+ frog 163;
+ mammals 494 f.;
+ reptiles 289
+
+ Claws 3;
+ birds 330;
+ crocodile 237;
+ dog 374;
+ duck 302;
+ mammals 417;
+ turtle 215
+
+ Clupeidae, _33_
+
+ _Clupeus_, _33_
+
+ Cnemial crest, dog, 412;
+ duck, 326
+
+ _Coccosteus_, _34_, _70_
+
+ Coccyx, man, 454
+
+ Cochliodontidae, _31_
+
+ _Cochliodus_, _31_;
+ dental plates, 109
+
+ Cod, _33_;
+ appendicular skeleton, 101 f.;
+ cranium, 96;
+ mandibular and hyoid arches, =99=;
+ median fins, 86;
+ pectoral girdle and fin, =102=;
+ ribs, 86;
+ skull, 96 f.;
+ vertebral column, 83 f.
+
+ _Coelogenys_, _48_;
+ zygomatic arch, 477
+
+ _Coenolestes_, _43_, 424
+
+ Coffer-fish, _33_;
+ see, _Ostracion_
+
+ _Colobus_, _49_;
+ pollex, 512
+
+ Colubridae, _38_
+
+ Columbae, _42_
+
+ Columbidae, _42_
+
+ Columella, crocodile, 251;
+ duck, 320;
+ frog, 157;
+ turtle, 228
+
+ Columella cranii, 200 n;
+ see epipterygoid
+
+ Colymbi, _40_
+
+ Colymbiformes, _40_
+
+ Compsognathidae, _38_
+
+ _Compsognathus_, _38_, 208
+
+ Condylar ridge, duck, 326
+
+ Condyle of humerus, dog, 406;
+ of mandible, dog, 398
+
+ Condylarthra, _47_;
+ femur, 519;
+ general characters, 361;
+ manus, 509;
+ skull, 472;
+ teeth, 432
+
+ Contour feather, duck, 303
+
+ Copula, 23
+
+ _Coracias_, _42_;
+ see Roller
+
+ Coraciae, _42_
+
+ Coraciiformes, _42_
+
+ Coracoid, 25;
+ cod, 103;
+ crocodile, 263;
+ duck, 322;
+ frog, 163;
+ Monotremata, 493;
+ newt, 147;
+ reptiles, 288;
+ turtle, 232
+
+ Coracoid groove, duck, 321
+
+ Cormorant, _41_;
+ foot, 342;
+ skull, 335
+
+ Cornu, see hyoid
+
+ Cornua trabeculae, 18
+
+ _Coryphodon_, _47_;
+ femur, 519;
+ manus, 510;
+ pes, 525;
+ skull, 473;
+ teeth, 433;
+ _C. hamatus_, manus, =510=
+
+ Coryphodontidae, _47_
+
+ Costal plate, turtle, 215;
+ -- process, duck, 321;
+ -- shield, turtle, 214
+
+ Cotyloid bone, 25, 513;
+ see Acetabular bone
+
+ Cotylopidae, _45_
+
+ _Cotylops_, _45_;
+ pollex, 506;
+ skull, 468
+
+ Coverts, 306, 328
+
+ Cranium, 18;
+ cod, 96 f.;
+ crocodile, 244 f.;
+ development of, 16 f.;
+ dog, 384 f.;
+ dogfish, 73 f.;
+ duck, 314;
+ frog, 154 f.;
+ newt, 140 f.;
+ turtle, 222 f.
+
+ Cranio-facial axis, dog, 384
+
+ Creodonta, _48_;
+ carpus, 512;
+ femur, 520;
+ general characters, 368;
+ skull, 479;
+ teeth, 439
+
+ Cribriform plate, dog, 388, 400
+
+ Crocodile, _210_, _212_;
+ anterior limb, 263;
+ exoskeleton, 237;
+ pectoral girdle, 262;
+ pelvic girdle, 266;
+ posterior limb, 268;
+ ribs and sternum, 259;
+ skeleton, 237 f.;
+ skull, 243 f.;
+ tarsus, 293;
+ teeth, 238;
+ vacuities in surface of cranium, 256;
+ vertebral column, 239
+
+ Crocodilia, _39_;
+ general characters of, 210;
+ palate, 281;
+ skull, 277 f.;
+ succession of teeth, 274;
+ teeth, 273
+
+ Crocodilidae, _39_
+
+ _Crocodilus_, _39_;
+ _C. palustris_, sternum and associated bones, =261=;
+ late thoracic and first sacral vertebrae, =242=;
+ _C. vulgaris_, cervical vertebrae, =239=
+
+ Crossopterygii, _33_;
+ general characters, 68
+
+ Crotalidae, _38_
+
+ _Crotalus_, _38_;
+ jaws 280;
+ see Rattlesnake
+
+ Crows, _42_
+
+ Crura of stapes, dog, 393
+
+ Cruro-tarsal, ankle joint, 345
+
+ Crus, 26;
+ crocodile, 268;
+ dog, 412;
+ duck, 326;
+ frog, 166;
+ newt, 149;
+ turtle, 235
+
+ Crusta petrosa, 5
+
+ _Cryptobranchus_, _35_, 135;
+ skull, 175;
+ _C. lateralis_, sacral vertebrae, 171
+
+ Cryptodira, _37_;
+ characters, 194
+
+ Ctenoid scales, 8, 60, 105
+
+ Cubitals, 303 f.
+
+ Cuboid, 27;
+ dog, 415
+
+ Cuckoo, foot, 342
+
+ Cuculi, _42_
+
+ Cuculiformes, _42_
+
+ Cuneiform bones, 27;
+ dog, 414 f.
+
+ _Cyclodus_, _38_;
+ see _Tiliqua_
+
+ Cycloid scales, 8, 60, 105;
+ cod, 83
+
+ _Cyclopidius_, _45_;
+ skull, 468
+
+ Cyclospondyli, 114
+
+ Cyclostomata, _31_;
+ general characters, 53
+
+ _Cycloturus_, _44_;
+ hallux, 522;
+ manus, 505
+
+ _Cygnus_, _41_;
+ see Swan
+
+ _Cynocephalus_, _49_;
+ cervical vertebrae, 446;
+ skull, 482
+
+ Cynoidea, _48_, _369_;
+ dental formula, 437
+
+ _Cynognathus_, _36_;
+ occipital condyle, 277;
+ teeth, 273
+
+ Cyprinidae, _33_
+
+ _Cyprinus_, _33_;
+ see Carp
+
+ Cypseli, _42_
+
+ Cypselidae, _42_;
+ see Swifts
+
+ Cystignathidae, _36_
+
+
+ _Dactylopterus_, _34_;
+ pectoral fins, 131
+
+ Dasypodidae, _44_
+
+ _Dasyprocta_, _48_;
+ auditory ossicles, 488;
+ thoraco-lumbar vertebrae, 450
+
+ Dasyproctidae, _48_
+
+ _Dasypus_, _44_;
+ manus, 505;
+ skull, 459;
+ stapes, 487;
+ teeth, 424
+
+ Dasyuridae, _43_, _350_;
+ dentition, 423;
+ pes, 521;
+ skull, 456
+
+ Deer, _359_;
+ manus, 507;
+ pes, 523;
+ Chinese water --, _46_,
+ see _Hydropotes_;
+ Musk --, _46_,
+ see _Moschus_;
+ Red -- fibula, 519
+
+ Delphinidae, _45_
+
+ _Delphinus_, _45_, 357;
+ lumbar vertebrae, 448;
+ skull, 462 f.
+
+ Deltoid ridge, crocodile, 263;
+ dog, 406;
+ frog, 164
+
+ _Dendrohyrax_, 363
+
+ Dental formula, regular, 344, 422;
+ Anthropoidea, 441;
+ _Astrapotherium_, 432;
+ _Babirussa_, 428;
+ Camel, 428;
+ _Chiromys_, 441;
+ Chiroptera (many), 440;
+ Cynoidea, 437;
+ _Dinotherium_, 434;
+ Dog, 376;
+ Duplicidentata, 435;
+ _Elephas_, 434;
+ _Erinaceus_, 440;
+ _Felis_, 437;
+ _Galeopithecus_, 440;
+ _Hippopotamus_, 427;
+ Horse, 430;
+ _Hydromys_, 436;
+ Hyracoidea, 362;
+ Macropodidae, 423;
+ _Manatus_, 425;
+ _Notoryctes_, 423;
+ _Otaria_, 439;
+ _Procavia_, 432;
+ _Pteropus_, 441;
+ Rodentia (most), 435;
+ Ruminantia, 429;
+ _Squalodon_, 427;
+ _Sus_, 428;
+ Tapiridae, 429;
+ _Thylacinus_, 423;
+ _Uintatherium_, 433;
+ _Ursus_, 439;
+ _Zeuglodon_, 426
+
+ Dentary, 22;
+ crocodile, 258;
+ cod, 100;
+ duck, 320;
+ frog, 161;
+ newt, 144;
+ salmon, 94;
+ turtle, 230
+
+ Dentine, 5
+
+ Derbian Screamer, spurs, 330
+
+ Dermal exoskeleton, crocodile, 237;
+ fish, 105;
+ mammals, 419;
+ reptiles, 271;
+ turtle, 215
+
+ Dermo-supra-occipital, Labyrinthodontia, 177;
+ _Polypterus_, 122
+
+ Dermochelydidae, _37_
+
+ _Dermochelys_, _37_, _194_, 214, 270;
+ carapace and plastron, 272
+
+ Dermoptera, _48_;
+ general characters, 370
+
+ Derotremata, _35_
+
+ _Desmodus_, _49_;
+ teeth, 441
+
+ Desmognathous, 319, 335
+
+ Development of bone, 10;
+ of cranium, 16;
+ of teeth, 7
+
+ _Dicynodon_, _36_, _192_;
+ beak, 271;
+ supratemporal fossa, 283;
+ teeth, 273
+
+ Didelphia, _43_;
+ general characters, 349
+
+ Didelphyidae, _43_, _350_;
+ auditory ossicles, 486;
+ pes, 521;
+ teeth, 423
+
+ _Didelphys_, _43_;
+ atlas, 443;
+ teeth, 422
+
+ _Didus_, _42_;
+ see Dodo
+
+ Digitigrade, defined, 358 n.
+
+ Digits, _26_;
+ see Manus and Pes
+
+ _Dimetrodon_, _36_;
+ thoracic vertebrae, 276
+
+ _Dinichthys_, _34_, _70_
+
+ Dinocerata, 364;
+ see Uintatheriidae
+
+ Dinornithes, _40_, _299_;
+ see Moas
+
+ Dinosauria, _38_;
+ general characters, 204;
+ humerus, 290;
+ ischium, 291;
+ pectoral girdle, 288;
+ pes, 293;
+ pre-orbital vacuity, 284;
+ ribs, 285;
+ vertebrae, 275 f.
+
+ Dinotheriidae, _47_
+
+ _Dinotherium_, _47_, 365;
+ dental formula, 434;
+ teeth, 345
+
+ _Diodon_, _33_;
+ beaks, 111;
+ _D. hystrix_, scales, 105
+
+ Diphycercal tail, 60, 116
+
+ Diphyodont, defined, 7, 344
+
+ _Diplacanthus_, _32_, _64_
+
+ Dipneumona, _34_
+
+ Dipnoi, _34_;
+ general characters, 69;
+ pelvic fins, 131;
+ skull, 124;
+ spinal column, 113;
+ tail, 116;
+ teeth, 111
+
+ Dipodidae, _47_
+
+ Diprotodont, 423
+
+ Diprotodontia, _43_;
+ characters, 350
+
+ Dipteridae, _34_, _70_;
+ cranium, 124;
+ tail, 117;
+ teeth, 111
+
+ _Dipus_, _47_;
+ cervical vertebrae, 446;
+ pes, 526
+
+ Discoglossidae, _36_
+
+ _Discoglossus_, _36_;
+ ribs, 182;
+ vertebrae, 172
+
+ Distal, defined, 23 n.
+
+ Divers, _40_;
+ thoracic vertebrae, 332
+
+ _Docidophryne_, _36_;
+ shoulder girdle and sternum, =183=
+
+ Dodo, _42_;
+ wing, 338
+
+ Dog, _48_;
+ arm bones, =407=;
+ anterior limb, 405;
+ atlas and axis, =379=;
+ cranium, 384, =389=, =396=;
+ dentition, =375=;
+ innominate bone, =410=;
+ leg bones, =411=;
+ manus, 408, =413=, 511;
+ pectoral girdle, 404;
+ pelvic girdle, 409;
+ pes, =413=, 414;
+ posterior limb, 412;
+ ribs, 402;
+ second lumbar vertebra, =382=;
+ second thoracic vertebra, =382=;
+ skull, 383, =387=;
+ sternum, =403=, 404;
+ vertebral column, 378
+
+ Dogfish, _64_;
+ cranium, 73;
+ exoskeleton, 71;
+ median fins, 79;
+ pectoral girdle and fin, 79;
+ pelvic girdle, 81;
+ pelvic fin, 81;
+ ribs, 73;
+ skull, 73, =75=;
+ vertebral column, 72;
+ visceral skeleton, 77;
+ Spotted and Spiny --, _32_
+
+ Dolphin, _45_, _357_;
+ lumbar vertebrae, 448;
+ Gangetic --, _45_,
+ see _Platanista_
+
+ Donkey, skull, =431=
+
+ _Dorcatherium_, _45_;
+ manus, 507
+
+ Dorsal vertebra, 16
+
+ Dorsal shield, crocodile, 238
+
+ Down feathers, 306
+
+ _Draco_, _38_;
+ ribs, 286
+
+ Dromaeognathous, 335
+
+ _Dromaeus_, _40_, _299_;
+ see Emeu
+
+ Duck, _41_, 334;
+ beak, 329;
+ claws, 330;
+ cranium, =313=;
+ exoskeleton, 302;
+ pectoral girdle, 321;
+ pelvic girdle, 324, =311=, =325=;
+ pes, 327;
+ posterior limb, 326;
+ ribs, 320;
+ skull, 312, =312=, =313=;
+ sternum, 321;
+ vertebral column, 307;
+ wing, 322, =304=, =305=
+
+ Duckbill, _43_;
+ see _Ornithorhynchus_
+
+ Dugong, _44_;
+ humerus, 501;
+ pelvis, 514;
+ thoraco-lumbar vertebrae, 448;
+ see _Halicore_
+
+ Duplicidentata, _48_, 366;
+ dental formula, 435;
+ skull, 478
+
+
+ Eagles, 335
+
+ Eared Seals, _369_;
+ scapula, 498;
+ see Otariidae
+
+ _Echidna_, _43_;
+ caudal vertebrae, 453;
+ manus, 504;
+ pelvis, 513;
+ sacral vertebrae, 451;
+ shoulder-girdle and sternum, =494=;
+ skull, 455;
+ spines, 417;
+ spur, 418;
+ thoraco-lumbar vertebrae, 447
+
+ Echidnidae, _43_;
+ pes, 521
+
+ Ectethmoid, 21 n.
+
+ Ectocondylar ridge, dog, 406
+
+ Edentata, _43_;
+ auditory ossicles, 487;
+ arm bones, 500;
+ caudal vertebrae, 453;
+ cervical vertebrae, 443;
+ manus, 504;
+ pectoral girdle, 495;
+ pes, 522;
+ pelvis, 513;
+ ribs, 491;
+ sacrum, 452;
+ skull, 457;
+ sternum, 489;
+ teeth, 424;
+ thigh and shin, 517;
+ thoraco-lumbar vertebrae, 447
+
+ Eel, _33_;
+ scales, 105
+
+ Elasmobranchii, _31_;
+ cranium, 118 f.;
+ clasper, 132;
+ general characters, 61;
+ pelvic fins, 131;
+ ribs, 125;
+ teeth, 109;
+ vertebral column, 113 f.;
+ visceral arches, 120
+
+ _Elasmotherium_, _46_;
+ mesethmoid, 470
+
+ Elephant, _47_;
+ auditory ossicles, 487;
+ caudal vertebrae, 453;
+ ribs, 491;
+ skull, 473 f., =474= and =475=;
+ tusks, 420;
+ see also Proboscidea
+
+ Elephantidae, _47_
+
+ _Elephas_, _47_, _364_;
+ dental formula, 434;
+ _E. planifrons_, 435;
+ see Elephant
+
+ _Elginia_, _36_;
+ skull, 191, 283
+
+ Embolomerous, 172
+
+ Emeu, _40_, _299_;
+ aftershaft, 328;
+ claws, 330
+
+ Enamel, 4;
+ -- cap, 7;
+ -- organ, 7
+
+ Endochondral ossification, 11
+
+ Endoskeleton, Amphibia, 170;
+ birds, 331 f.;
+ cod, 83 f.;
+ crocodile 239, f.;
+ dog, 377 f.;
+ dogfish, 71 f.;
+ duck, 306 f.;
+ fish, 112 f.;
+ frog, 151 f.;
+ mammals, 442 f.;
+ newt, 138 f.;
+ reptiles, 275 f.;
+ turtle, 218 f.
+
+ Engystomatidae, _36_
+
+ Entoplastron, turtle, 217
+
+ Epanorthidae, _43_, _350_
+
+ Epi-branchial, cod, 101;
+ dogfish, 78;
+ salmon, 94
+
+ Epicoracoid, 25;
+ frog, 163;
+ turtle, 232;
+ Monotremes, 493;
+ vestiges of in Rodentia, 497
+
+ _Epicrium_, _35_;
+ orbit, 179
+
+ Epidermal exoskeleton, birds, 328;
+ crocodile, 237;
+ dog, 374;
+ duck, 302;
+ mammals, 416;
+ reptiles, 270;
+ turtle, 214
+
+ Epi-hyal, cod, 100;
+ dog, 399;
+ salmon, 94
+
+ Epi-otic, 20;
+ cod, 96;
+ crocodile, 250;
+ Labyrinthodontia, 177;
+ reptiles, 278;
+ salmon, 89;
+ turtle, 227
+
+ Epiphysis, 11
+
+ Epiplastron, turtle, 217
+
+ Epiprecoracoid, Amphibia, 184;
+ turtle, 232
+
+ Epipterygoid, Lacertilia, 200;
+ reptiles, 278
+
+ Epipubis, crocodile, 267;
+ newt, 149;
+ turtle, 235
+
+ Episternum, 217;
+ frog, 163
+
+ Equidae, _46_;
+ mane, 416;
+ scapula, 496;
+ skull, 471
+
+ _Equus_, _46_;
+ see Horse
+
+ Erinaceidae, _49_
+
+ _Erinaceus_, _49_;
+ dental formula, 440;
+ pelvic symphysis, 515;
+ presternum, 490;
+ see Hedgehog
+
+ Esocidae, _33_
+
+ Esox, _33_;
+ attachment of teeth, 107
+
+ Ethmoid, 394;
+ see median ethmoid
+
+ Ethmoidal plane, 390;
+ -- region, 21
+
+ Ethmo-palatine ligament, dogfish, 77
+
+ Ethmo-turbinal, dog, 395
+
+ _Euchirosaurus_, _35_;
+ vertebrae, 171
+
+ Eustachian canal, see Canal
+
+ Eusuchia, _39_;
+ general characters, 212
+
+ Eutheria, _43_;
+ general characters, 351
+
+ _Exocaetus_, _33_;
+ pectoral fins, 131
+
+ Exoccipital, 19;
+ cod, 97;
+ crocodile, 246;
+ dog, 386;
+ duck, 314;
+ frog, 154;
+ newt, 141;
+ salmon, 89;
+ turtle, 224
+
+ Exoskeleton, 2;
+ Amphibia, 168;
+ birds, 328;
+ crocodile, 237;
+ dog, 374;
+ dogfish, 71;
+ duck, 302;
+ fish, 104;
+ ganoids, 66;
+ mammals, 442;
+ reptiles, 270;
+ turtle, 214
+
+ Extensor side, defined, 29
+
+ Extra-branchial, dogfish, 79
+
+ Extra-columella, crocodile, 251;
+ turtle, 228
+
+
+ Fabella, dog, 412
+
+ _Falco_, _41_
+
+ Falcon _41_, 335
+
+ Falconiformes, _41_
+
+ Feathers, 3, 328;
+ duck, 302
+
+ Felidae, _48_;
+ claws, 418
+
+ _Felis_, _48_;
+ dental formula, 437;
+ thoraco-lumbar vertebrae, 450
+
+ Femoral shield, turtle, 215
+
+ Femur, 26;
+ crocodile, 268;
+ dog, 412;
+ duck, 326;
+ frog, 166;
+ mammals, 517 f.;
+ newt, 149;
+ ox and rhinoceros, =518=;
+ turtle, 235
+
+ Fenestra ovalis, crocodile, 250 f.;
+ dog, 392;
+ duck, 316;
+ frog, 157;
+ turtle, 228;
+ -- rotunda, dog, 392;
+ duck, 316
+
+ Fenestral recess, duck, 316
+
+ Fibula, 26;
+ crocodile, 268;
+ dog, 412;
+ duck, 327;
+ frog, 166;
+ newt, 149;
+ turtle, 235
+
+ Fibulare, 27;
+ see Tarsus
+
+ File-fish, _33_;
+ see _Balistes_
+
+ Filoplume, 306
+
+ Finches, _42_
+
+ Fins, fish, 115;
+ caudal --, Cetacea, 453;
+ cod, 87;
+ fish, 116;
+ Ichthyosauria, 195;
+ median --, cod, 86;
+ dogfish, 79;
+ pectoral --, cod, 103;
+ dogfish, 79;
+ pelvic --, cod, 103;
+ dogfish, 81
+
+ Fin-rays, 105, 115;
+ cod, 83, 103;
+ dogfish, 79
+
+ Firmisternia, _36_, 185
+
+ Fish, appendicular skeleton, 126;
+ endoskeleton, 112 f.;
+ exoskeleton, 104;
+ general characters, 60 f.;
+ paired fins, 127 f.;
+ ribs, 125 f.;
+ skull, 117 f.;
+ spinal column, 112 f.;
+ teeth, 106 f.
+
+ Fissipedia, _48_;
+ general characters, 368
+
+ Flamingo, 335
+
+ Flexor side, defined, 29
+
+ Floating ribs, dog, 402;
+ mammals, 490
+
+ Flower, Sir W.H., on succession of teeth in elephants, 434
+
+ Flying-fish, _33_;
+ -- fox, _49_, _371_,
+ skull, 481,
+ see _Pteropus_;
+ -- gurnard, _34_,
+ see _Dactylopterus_;
+ -- lemur, _48_,
+ see _Galeopithecus_;
+ -- lizard, _38_,
+ see _Draco_
+
+ Fontanelle, salmon, 89;
+ anterior --, dogfish, 74;
+ frog, 154;
+ posterior --, frog, 154
+
+ Foot, crocodile, 269;
+ dog, 414;
+ frog, 167;
+ newt, 149 f.;
+ turtle, 236
+
+ Foramen:
+ anterior palatine --, dog, 401;
+ condylar --, dog, 401;
+ -- cordiforme, reptiles, 292;
+ ect-epicondylar --, _Sphenodon_, 290;
+ ent-epicondylar --, 191 n.;
+ Carnivora vera, 502;
+ Cebidae, 503;
+ _Choloepus_, 500;
+ Condylarthra, 362, 502;
+ Creodonta, 368;
+ Insectivora, 503;
+ Lemurs, 503;
+ Marsupials, 500;
+ reptiles, 290;
+ external mandibular --, crocodile, 258;
+ inferior dental --, dog, 399;
+ infra-orbital --, dog, 401;
+ Rodents, 477;
+ ilio-sciatic --, duck, 325;
+ internal mandibular --, crocodile, 258;
+ internal orbital --, dog, 401;
+ interparietal --, Labyrinthodontia, 173, 177;
+ reptiles, 277;
+ -- lacerum anterius, dog, 388, 400;
+ -- lacerum medium, dog, 402;
+ -- lacerum posterius, dog, 392, 401;
+ lachrymal --, dog, 394, 401;
+ -- magnum, cod, 97;
+ crocodile, 257;
+ dog, 386, 402;
+ dogfish, 76;
+ duck, 314;
+ frog, 154;
+ newt, 141;
+ salmon, 89;
+ turtle, 224;
+ mental --, dog, 399;
+ obturator --, duck, 326;
+ dog, 410;
+ ophthalmic --, dogfish, 74;
+ optic --, dog, 400;
+ dogfish, 74;
+ orbitonasal --, dogfish, 74;
+ -- ovale, crocodile, 249;
+ dog, 400;
+ pneumatic --, duck, 323;
+ pneumogastric --, dogfish, 76;
+ posterior palatine --, dog, 401;
+ postglenoid --, dog, 402;
+ pre-acetabular --, Chiroptera, 515;
+ -- rotundum, dog, 400;
+ stylomastoid --, dog, 392 f., 400;
+ thyroid --, dog, 410;
+ trigeminal --, duck, 316;
+ -- triosseum, duck, 322
+
+ Fore-arm, 26;
+ crocodile, 265;
+ dog, 406;
+ duck, 323;
+ frog, 164;
+ newt, 147;
+ turtle, 233
+
+ Fossa:
+ cerebellar --, dog, 392;
+ cerebral --, dog, 392;
+ digital --, dog, 412;
+ floccular --, dog, 392;
+ infratemporal --, see lateral temporal --;
+ lachrymal --, Ruminants, 469;
+ lateral temporal --, crocodile, 257;
+ _Sphenodon_, 283;
+ olecranon --, dog, 406;
+ prescapular --, dog, 405;
+ postscapular --, dog, 405;
+ post-temporal --, _Sphenodon_, 283;
+ pterygoid --, crocodile, 257;
+ suborbital --, Ruminants, 469;
+ supra-acetabular --, Ruminants, 514;
+ supratemporal --, crocodile, 249, 256;
+ reptiles, 283;
+ supra-trochlear --, dog, 406;
+ temporal --, dog, 398;
+ trochanteric --, dog, 412
+
+ Fowl, _41_, 335;
+ claws, 330;
+ skeleton, =301=
+
+ Fox, _48_
+
+ Frigate bird, _41_;
+ clavicles, 338
+
+ Frog, anterior limb, 164;
+ cranium, =155=, =157=;
+ hyoid apparatus, 161;
+ pelvic girdle, 165;
+ posterior limb, 166;
+ shoulder-girdle and sternum, =183=;
+ skull, 154 f., =155=, =159=;
+ teeth, 151;
+ vertebral column, 152;
+ Common --, Edible --, Fire-bellied --, Green-tree --,
+ Horned --, Midwife --, Painted -- and Toad --, _36_;
+
+ Frontal, 19;
+ cod, 96;
+ crocodile, 249;
+ dog, 388;
+ duck, 314;
+ newt, 141;
+ salmon, 91;
+ turtle, 225;
+ -- segment, crocodile, 249;
+ dog, 388;
+ turtle, 225
+
+ Fronto-parietal, frog, 156
+
+ Frugivorous bats, manus 512;
+ see Pteropidae
+
+ Fulcra 67;
+ _Polypterus_ 106
+
+ Furcula 296; duck 322
+
+
+ Gadidae _33_
+
+ _Gadus_ _33_;
+ see Cod
+
+ Galeopithecidae _48_
+
+ _Galeopithecus_ _48_, _370_;
+ dental formula 440;
+ inter centra 450;
+ pelvic symphysis 515;
+ skull 480
+
+ _Galesaurus_ _36_, _192_; teeth 273
+
+ _Galeus_, _32_;
+ occipital joint 118
+
+ Galli _41_
+
+ Galliformes _41_
+
+ _Gallus_ _41_;
+ _G. bankiva_ skeleton =301=
+
+ Gannet, _41_; wing =339=
+
+ Ganoid scales 8, 60, 104
+
+ Ganoidei _32_;
+ general characters and distribution 66;
+ pectoral girdle 126;
+ pelvic fins 132;
+ teeth 110;
+ skull 121 f.;
+ spinal column 112 and 114
+
+ Garialidae _39_
+
+ _Garialis_ _39_, _212_
+
+ Garial 210
+
+ Gar pike 33;
+ see _Lepidosteus_
+
+ _Gavialis_ _39_
+
+ _Gazella_ _46_
+
+ Gazelle _46_;
+ skull 468
+
+ Geckonidae _37_;
+ see Gecko
+
+ Gecko _37_;
+ epipubis 293;
+ parietals 277;
+ supratemporal fossa 283;
+ vertebrae 275
+
+ Gibbon _49_;
+ ribs 493;
+ skull 482
+
+ Gill-rays, dogfish 78;
+ salmon 95
+
+ _Giraffa_ _46_
+
+ Giraffe _46_, _359_;
+ cervical vertebrae 445;
+ manus 507;
+ pes 523;
+ ulna 501
+
+ Giraffidae _46_; skull 469
+
+ Girdle bone, frog 156
+
+ Glenoid cavity 25;
+ crocodile 263;
+ dog 405;
+ duck 322;
+ frog 162;
+ newt 146;
+ turtle 232;
+ --fossa, dog 394
+
+ Globe-fish _33_
+
+ _Globicephalus_ _45_;
+ cervical vertebrae 354;
+ manus 506;
+ skull 463
+
+ Gluteal surface of ilium, dog 410
+
+ _Glyptodon_ _44_;
+ carapace 419;
+ cervical vertebrae 443;
+ caudal vertebrae 453;
+ manus 505;
+ pelvis 513;
+ pes 522;
+ teeth 425;
+ thoraco-lumbar vertebrae 447
+
+ Glyptodontidae _44_, _352_;
+ skull 459;
+ see also _Glyptodon_
+
+ Gnathostomata _31_, _59_
+
+ Golden mole _49_;
+ see Chrysochloris
+
+ Goniopholidae _39_
+
+ _Goniopholis_ _39_;
+ vertebrae 275
+
+ Goose 334;
+ beak 329;
+ Spur-winged--41
+
+ _Gorilla_ _49_;
+ carpus 512;
+ ribs 493;
+ scapula 499;
+ skull =483=;
+ thoraco-lumbar vertebrae 450
+
+ Gruidae _41_;
+ see Cranes
+
+ Gruiformes _41_
+
+ Guinea-pig _48_;
+ tail 454
+
+ Gular shield, turtle 215
+
+ Gulls _42_, 335;
+ aftershaft 328
+
+ Gymnodontidae _33_;
+ beaks 111
+
+ Gymnophiona _35_;
+ branchial arches 180;
+ general characters 136;
+ ribs 182;
+ scales 168;
+ skull 177;
+ teeth 169;
+ vertebrae 172
+
+ _Gymnura_ _49_;
+ teeth 440;
+ zygomatic arch 481
+
+ _Gypogeranus_ _41_;
+ claws 330
+
+ _Gyrinophilus_ _35_;
+ vertebral column 171
+
+
+ Haddock _33_
+
+ Hadrosauridae _39_
+
+ _Hadrosaurus_ _39_;
+ skull 284
+
+ Hag or hag-fish _31_, 54 f.
+
+ Hair 3;
+ dog 374;
+ mammals 416
+
+ _Halicore_ _44_, _352_;
+ manus 505;
+ skull 460;
+ teeth 425;
+ see Dugong
+
+ Halicoridae _44_
+
+ Halitheriidae _44_
+
+ _Halitherium_ _44_, 352;
+ femur 518;
+ pelvis 514;
+ teeth 425
+
+ Hallux 26;
+ dog 415;
+ duck 327;
+ frog 167
+
+ Hamular process, dog 397
+
+ Hand, crocodile 266;
+ dog 408 f.;
+ duck 324;
+ frog 165;
+ newt 147;
+ turtle 233
+
+ _Hapale_ _49_
+
+ Hapalidae _49_, 372 f.;
+ teeth 441
+
+ Hare _48_, 366;
+ acetabular bone 515;
+ dental formula 435;
+ femur 520;
+ humerus 502;
+ pelvis 515;
+ scapula 497;
+ skull 476;
+ tail 454;
+ thoraco-lumbar vertebrae 449;
+ Cape jumping --, _47_;
+ see _Pedetes_
+
+ _Harpagus_, _41_;
+ serrated beak, 334
+
+ _Harriotta_, _32_, _66_
+
+ _Hatteria_, _37_, _197_;
+ see _Sphenodon_
+
+ Haversian canals, 10;
+ -- system, 10
+
+ Hawks, beak of, 330
+
+ Hedgehog, _49_, _370_;
+ auditory ossicles, 488;
+ humerus, 503;
+ presternum, 490;
+ skull, 480;
+ spines, 417;
+ see _Erinaceus_
+
+ Hemichordata, _30_, _50_
+
+ _Heptanchus_, _31_;
+ branchial arches, 63, =120=;
+ vertebrae, 114
+
+ Herbivorous dentition, 427, 430
+
+ Heron, _41_, 335;
+ interorbital septum, 333;
+ powder down feathers, 329
+
+ Herring, _33_
+
+ _Hesperornis_, _40_, _299_;
+ caudal vertebrae, 333;
+ clavicles, 338;
+ teeth, 330;
+ wing, 338
+
+ Heterocercal tail, 60, 116
+
+ Heterodont, 7
+
+ Heterostraci, _31_;
+ general character, 54
+
+ _Hexanchus_, _31_;
+ branchial arches, 63, 121
+
+ Hinge joint, 13
+
+ _Hipparion_, _46_;
+ manus, 508;
+ pes, =524=
+
+ Hippopotamidae, _45_
+
+ _Hippopotamus_, _45_, _359_;
+ dental formula, 427;
+ hair, 416;
+ mandible, 467;
+ manus, 506;
+ pes, 523;
+ scapula, 496;
+ skull, 467;
+ teeth, 345
+
+ Hoatzin, _41_;
+ see _Opisthocomus_
+
+ Holocephali, _32_, 65, 104;
+ clasper, 132;
+ spinal column, 113;
+ tail, 116;
+ teeth, 109
+
+ Holoptychiidae, _33_
+
+ _Holoptychius_, _33_;
+ scales, 105
+
+ Holostei, _33_;
+ general characters, 68;
+ teeth, 110;
+ see Bony Ganoids
+
+ Hominidae, _49_, _373_
+
+ _Homo_, _49_;
+ see Man
+
+ Homocercal tail, 60, 69, 117;
+ codfish, 87
+
+ Homodont, defined, 7
+
+ Hoofs, 3, 418
+
+ Hoopoe, _42_, 335
+
+ _Hoplopterus_, spur, 330
+
+ Hornbill, _42_, 331;
+ bony crest, 334;
+ interorbital septum, 333
+
+ Horns, 3, 417
+
+ Horny plates on palate, 418;
+ -- teeth, Lampreys, 4;
+ Myxinoids, 57;
+ _Ornithorhynchus_, 4
+
+ Horse, _46_, _360_;
+ fibula, 519;
+ malleus, 487;
+ manus, 507;
+ pes, =524=;
+ skull, 471;
+ teeth, 345, 430;
+ ulna, 501
+
+ Howling monkey, _49_;
+ see _Mycetes_
+
+ Humerals, duck, 303 f.
+
+ Humeral shield, turtle, 215
+
+ Humerus, 26;
+ crocodile, 263;
+ dog, 405;
+ duck, 323;
+ frog, 164;
+ newt, 147;
+ turtle, 232;
+ wombat, =500=
+
+ Humming-birds, _42_, 335
+
+ Humpbacked whale, _44_, _357_
+
+ _Hyaena_, _48_, _369_;
+ hallux, 526;
+ pollex, 511;
+ sacral vertebrae, 452;
+ teeth, 437
+
+ Hyaenidae, _48_;
+ humerus, 502
+
+ _Hyaenodon_, _48_, _368_
+
+ Hyaenodontidae, _48_
+
+ Hyaline cartilage, 10
+
+ _Hydrochaerus_, _48_;
+ teeth, 437;
+ see Capybara
+
+ _Hydromys_, _47_;
+ dental formula, 436
+
+ Hydrophidae, _38_;
+ scales, 270
+
+ _Hydropotes_, _46_;
+ canines, 429
+
+ _Hyla_, _36_;
+ fronto-parietal fontanelle, 179;
+ sternum, 184
+
+ Hylidae, _36_
+
+ _Hylobates_, _49_;
+ ribs, 493;
+ skull, 482
+
+ Hyoid, 21;
+ alligator, =285=;
+ Amphibia, 180;
+ birds, 336;
+ cod, 100;
+ crocodile, 259;
+ dogfish, 77;
+ dog, 399;
+ duck, 320;
+ frog, 161;
+ newt, 144;
+ reptiles, 284;
+ salmon, 94;
+ turtle, 231, =285=
+
+ Hyomandibular, 23;
+ cod, 100;
+ dogfish, 78;
+ salmon, 94
+
+ _Hyomoschus_, _45_
+
+ Hyoplastron, turtle, 217
+
+ Hyostylic, 61, 119
+
+ _Hyotherium_, _45_;
+ teeth, 427
+
+ _Hyperodapedon_, _37_, _198_;
+ premaxillae, 284
+
+ _Hyperoödon_, _44_;
+ skull, 464;
+ sternum, 489;
+ thoracic vertebrae, 448
+
+ Hypo-branchial, cod, 101;
+ dogfish, 78
+
+ Hypo-hyal, cod, 100;
+ salmon, 95
+
+ Hypo-ischium, Lacertilia, 292
+
+ Hypoplastron, turtle, 217
+
+ Hyporachis, 328
+
+ _Hypsilophodon_, _39_;
+ predentary bone, 284
+
+ Hypsodont, defined, 345, 429
+
+ Hypural bone, cod, 85
+
+ Hyracidae, _47_
+
+ Hyracoidea, _47_;
+ femur, 519;
+ general characters, 362;
+ manus, 510;
+ nails, 418;
+ skull, 472;
+ teeth, 432
+
+ _Hyracotherium_, _46_;
+ manus, 508;
+ scapula, 496
+
+ _Hyrax_, _47_, _363_;
+ see _Procavia_
+
+ Hystricidae, _47_
+
+ Hystricomorpha, _47_;
+ auditory ossicles, 488
+
+ _Hystrix_, _47_;
+ auditory ossicles, 488;
+ see Porcupine
+
+
+ Ichthyodorulites, 106
+
+ Ichthyoidea, _35_;
+ general characters, 134
+
+ Ichthyopsida, _31_;
+ general characters, 59
+
+ Ichthyopterygium, 130
+
+ _Ichthyornis_, _40_;
+ mandible, 335;
+ pelvis, 341;
+ teeth, 330;
+ vertebrae, 332
+
+ Ichthyornithiformes, _40_, 300
+
+ Ichthyosauria, _37_;
+ general characters, 195;
+ ribs, 285
+
+ Ichthyosauridae, _37_
+
+ _Ichthyosaurus_, _37_, _197_;
+ limbs, 290;
+ palatines, 281;
+ pectoral girdle, 288;
+ position of limbs, 28;
+ skull, =196=;
+ teeth, 273;
+ vertebral column, 275
+
+ Ichthyotomi, _31_;
+ general characters, 62;
+ fins, 130 f.;
+ tail, 116
+
+ _Iguana_, _38_;
+ teeth, 273
+
+ Iguanidae, _38_;
+ zygosphenes, 200, 276
+
+ _Iguanodon_, _39_, 208 f.;
+ jaws, 292;
+ predentary, 284;
+ sternum, 288;
+ teeth, 272 f.;
+ vertebrae, 275
+
+ Iguanodontidae, _39_
+
+ Iliac surface of ilium, dog, 410
+
+ Ilium, 25;
+ crocodile, 266;
+ dog, 409;
+ duck, 325;
+ frog, 165;
+ mammals, 513 f.;
+ newt, 149;
+ reptiles, 291;
+ turtle, 235
+
+ Incisors, dog, 376 f.;
+ mammals, 344
+
+ Incus, dog, 393;
+ man, dog and rabbit, =485=
+
+ Infra-marginal shield, turtle, 215
+
+ Infra-pharyngeal bone, cod, 101
+
+ _Inia_, _45_;
+ cervical vertebrae, 444;
+ lumbar vertebrae, 448
+
+ Innominate bone, dog, 409;
+ mammals, 513
+
+ Insectivora, _48_;
+ arm bones, 503;
+ auditory ossicles, 488;
+ cervical vertebrae, 446;
+ general characters, 369 f.;
+ manus, 512;
+ pelvis, 515;
+ pes, 527;
+ sacrum, 452;
+ shoulder-girdle, 499;
+ skull, 480;
+ sternum, 490;
+ tail, 454;
+ teeth, 440;
+ thigh and shin, 520;
+ thoraco-lumbar vertebrae, 450
+
+ Insectivora vera, _49_;
+ general characters, 370
+
+ inter centra, 15;
+ _Galeopithecus_, 370;
+ Ichthyosauria, 195;
+ Labyrinthodontia, 172;
+ _Sphenodon_, 198;
+ _Talpa_, 450
+
+ Interclavicle, 25;
+ crocodile, 263;
+ Monotremata, 494;
+ reptiles, 289
+
+ Intercondylar notch, dog, 412
+
+ Intergular shield, turtle, 215
+
+ Interhyal, cod, 100
+
+ Intermedium, 27;
+ see Carpus and Tarsus
+
+ Intermuscular bones, cod, 86
+
+ Internasal septum, dogfish, 76
+
+ Interorbital septum, birds, 333;
+ crocodile, 247;
+ duck, 317;
+ reptiles, 277
+
+ Interspinous bones, cod, 86
+
+ Intertarsal ankle joint, 190
+
+ Intervertebral discs, 15, 378
+
+ Ischial tuberosity, dog, 411
+
+ Ischium, 25;
+ crocodile, 266;
+ dog, 410;
+ duck, 325;
+ frog, 165;
+ newt, 149;
+ turtle, 235
+
+ _Ischyodus_, _32_, _66_
+
+ Ivory, 5
+
+
+ Jacana, _42_;
+ see _Parra_
+
+ _Jacare_, _39_;
+ scutes, 271
+
+ Jaws, 21;
+ cod, 98 f.;
+ crocodile, 252 f.;
+ dog, 395 f.;
+ dogfish, 77;
+ duck, 317 f.;
+ frog, 158 f.;
+ newt, 143 f.;
+ salmon, 93 f.;
+ turtle, 229 f.
+
+ Jerboa, _47_;
+ cervical vertebrae, 446;
+ pes, 526
+
+ Joints, kinds of, 13
+
+ Jugal, 22;
+ crocodile, 255;
+ dog, 398;
+ duck, 318;
+ turtle, 229
+
+ Jugulares, 132
+
+
+ Kangaroo, _43_;
+ dental formula, 423;
+ lumbar vertebrae, 447;
+ pectineal process, 513;
+ pes, 522;
+ tail, 453;
+ teeth, 345
+
+ Kestrel, claws, 330
+
+ Killer, _45_;
+ see _Orca_
+
+ Kiwi, _40_;
+ see _Apteryx_
+
+ Knee-cap, see patella
+
+ Koala, _43_;
+ lumbar vertebrae, 447;
+ pes, 522;
+ tail, 453
+
+ Kükenthal, W., on teeth of Cetacea, 426;
+ on teeth of Marsupials, 422
+
+
+ Labial cartilage, dogfish, 77;
+ _Squatina_, 119
+
+ Labridae, _33_
+
+ _Labrus_, _33_;
+ see Wrasse
+
+ Labyrinthodontia, _35_;
+ buckler, 168;
+ general characters, 135;
+ interparietal foramen, 173;
+ pelvis, 187;
+ ribs, 182;
+ skull, 176;
+ teeth, 169
+
+ Lacertilia, _37_;
+ general characters, 199;
+ pectoral girdle, 288;
+ skull, 277;
+ vertebrae, 275
+
+ Lachrymal, 20;
+ cod, 97;
+ crocodile, 251;
+ dog, 394;
+ duck, 317;
+ salmon, 93
+
+ Lacunae, 10
+
+ _Lagenorhynchus_, _45_;
+ skull, 462
+
+ _Lagostomus_, _47_;
+ maxillae, 477
+
+ Lambdoidal crest, duck, 315
+
+ Lamella of malleus, dog, 393
+
+ Lamnidae, _32_
+
+ Lamprey, _31_, 55 f.
+
+ Lancelet, _30_;
+ see _Amphioxus_
+
+ Laridae, _42_;
+ see Gulls
+
+ Larks, _42_
+
+ Larvacea, _30_;
+ notochord, 51
+
+ _Latax_, _48_;
+ pes, 526
+
+ Lateral ethmoid, 21;
+ cod, 97;
+ salmon, 89 f.
+
+ Leathery turtle, _37_;
+ see _Dermochelys_
+
+ Lemuroidea, _49_;
+ caudal vertebrae, 454;
+ general characters, 372;
+ nails, 418;
+ ribs, 493;
+ sacrum, 452;
+ skull, 482;
+ thoraco-lumbar vertebrae, 450;
+ see Lemurs
+
+ Lemurs, carpus, 512;
+ pes, 527;
+ teeth, 441;
+ see Lemuroidea
+
+ Lenticular, 485;
+ dog, 393
+
+ _Lepidosiren_, _34_, _70_;
+ branchial arches, 125;
+ fins, 130
+
+ Lepidosteidae, _33_
+
+ _Lepidosteus_, _33_;
+ attachment of teeth, 108;
+ distribution, 66;
+ pectoral fins, 131;
+ scales, 67, 104;
+ skull, 123;
+ tail, 117;
+ vertebrae, 68
+
+ _Lepidotus_, _33_;
+ teeth, 110
+
+ Leporidae, _48_
+
+ Lepospondyli, _35_
+
+ _Lepus_, _48_;
+ see Hare
+
+ _Leptoptilus_, _41_;
+ see Adjutant
+
+ _Lialis_, _37_, 289
+
+ Limbs, general account, 26;
+ modifications in position of, 28;
+ reptiles, 289
+
+ Llama, _45_, 359;
+ cervical vertebrae, 445;
+ skeleton, =496=;
+ teeth, 428
+
+ Limicolae, _42_
+
+ Lingual apparatus, lampreys, 58;
+ myxinoids, 57
+
+ Lion, _48_
+
+ _Loemanctus longipes_, shoulder girdle and sternum, =287=
+
+ Loggerhead turtle, carapace, =216=
+
+ _Lophiodon_, _46_;
+ teeth, 345, 429
+
+ Lophiodontidae, _46_
+
+ Lophiomyidae, _47_
+
+ _Lophiomys_, _47_;
+ pes, 526;
+ skull, 476
+
+ _Lophius_, attachment of teeth, 107
+
+ Lower jaw, see Mandible
+
+ Lumbar vertebrae, 16;
+ crocodile, 242;
+ dog, 378 f.;
+ duck, 311
+
+ Lunar, 27;
+ dog, 408
+
+
+ _Macacus_, _49_;
+ cervical vertebrae, 446
+
+ _Machaerodus_, _48_;
+ upper canines, 437
+
+ _Macrauchenia_, _46_, 358;
+ calcaneum, 360;
+ cervical vertebrae, 445;
+ fibula, 519;
+ tarsus, 523;
+ ulna, 501
+
+ Macraucheniidae, _46_, 509
+
+ Macropodidae, _43_, _350_;
+ dental formula, 423;
+ pes, 522
+
+ _Macropus_, _43_;
+ see Kangaroo
+
+ Macroscelidae, _49_
+
+ _Macroscelides_, _49_;
+ skull, 480
+
+ Magnum, 27;
+ see Carpus
+
+ Malar, 22;
+ see jugal
+
+ Malleus, dog, 393;
+ man, dog and rabbit, =485=
+
+ Mammalia, _42_;
+ auditory ossicles, 485;
+ cervical vertebrae, 442;
+ exoskeleton, 416;
+ general characters, 343;
+ manus, 503;
+ Mesozoic --, 348;
+ pectoral girdle, 493;
+ pelvic girdle, 512;
+ pes, 521;
+ ribs, 490;
+ sacral and caudal vertebrae, 451;
+ skull, 455;
+ sternum, 489;
+ thigh and shin, 517;
+ thoraco-lumbar vertebrae, 447
+
+ Man, _49_;
+ arm bones, 503;
+ auditory ossicles, 488;
+ caudal vertebrae, 454;
+ cervical vertebrae 446;
+ pelvis, 515;
+ pes, 527;
+ ribs, 493;
+ scapula, 499;
+ skull, 483;
+ sternum, 490;
+ teeth, 441
+
+ Manatee, _44_;
+ see _Manatus_
+
+ Manatidae, _44_
+
+ _Manatus_, _44_;
+ cervical vertebrae, 444;
+ dental formula, 425;
+ humerus, 501;
+ manus, 505;
+ pelvis, 514;
+ skull, 460;
+ sternum, 489;
+ teeth, 345;
+ thoraco-lumbar vertebrae, 448
+
+ Mandible, birds, 335;
+ cod, 100;
+ crocodile, 258;
+ dog, 398;
+ duck, 319;
+ frog, 160;
+ Hippopotamus, =467=;
+ Isabelline bear, =438=;
+ newt, 144;
+ salmon, 94;
+ turtle, 230
+
+ Manidae, _44_;
+ see _Manis_
+
+ _Manis_, _44_;
+ auditory ossicles, 487;
+ manus, 504;
+ scales, 3, 417;
+ skull, 459;
+ _M. macrura_ xiphisternum, 489;
+ see Pangolin
+
+ Manubrium of malleus 486;
+ dog, 393;
+ -- sterni, dog 404
+
+ Manus, 26;
+ crocodile, 265;
+ dog, 408, =413=;
+ duck, 323;
+ frog, 164;
+ mammalia, 503;
+ newt, 147;
+ Perissodactyles, =508=;
+ turtle, 233
+
+ Marginal plate, turtle 216;
+ -- ray, 131;
+ -- shield, turtle, 214
+
+ Marmoset, _49_, 372 f.
+
+ Marmot, frontals, 476
+
+ Marsipobranchii, _31_, _53_;
+ spinal column, 56
+
+ Marsupial bones, 513
+
+ Marsupial mole, 43;
+ see _Notoryctes_
+
+ Marsupialia, _43_;
+ arm bones, 499;
+ auditory ossicles, 486;
+ cervical vertebrae, 443;
+ caudal vertebrae, 453;
+ general characters, 349;
+ manus, 504;
+ pectoral girdle, 494;
+ pelvis, 513;
+ pes, 521;
+ ribs, 491;
+ sacral vertebrae, 451;
+ skull, 456;
+ teeth, 422;
+ thigh and shin, 517;
+ thoraco-lumbar vertebrae, 447
+
+ _Mastodon_, _47_, _365_;
+ teeth, 434
+
+ _Mastodonsaurus_, _35_, _136_;
+ pelvis, 187
+
+ Mastoid portion of periotic, dog, 391
+
+ Maxilla, 22;
+ cod, 98;
+ crocodile, 254;
+ dog, 397;
+ duck, 318;
+ frog, 159;
+ newt, 144;
+ turtle, 229
+
+ Maxillo-mandibular arch, 21
+
+ Maxillo-palatine, duck, 318
+
+ Maxillo-turbinal, dog, 395
+
+ Meatus, external auditory --, crocodile, 250;
+ dog, 393;
+ turtle, 228;
+ internal auditory --, crocodile, 251;
+ dog, 392, 400;
+ turtle, 228
+
+ Meckel's cartilage, 22;
+ cod, 100;
+ dogfish, 77;
+ salmon, 94
+
+ Median ethmoid, 21;
+ cod, 98;
+ Gymnophiona, 179;
+ salmon, 91;
+ -- fin, Amphibia, 52;
+ cod, 86;
+ dogfish, 79
+
+ Megachiroptera, _49_;
+ general characters, 371
+
+ _Megalobatrachus_, _35_, _135_;
+ carpus, 186;
+ skull, 175
+
+ Megalosauridae, _38_
+
+ _Megalosaurus_, _38_, _208_
+
+ _Megapodius_, spur, 330
+
+ _Megaptera_, _44_, _357_
+
+ Megatheriidae, _44_, 352;
+ humerus, 501;
+ leg bones, 517;
+ pelvis, 513;
+ sacrum, 452;
+ teeth, 424;
+ thoraco-lumbar vertebrae, 447
+
+ _Megatherium_, _44_;
+ femur, 517;
+ manus, 505;
+ pectoral girdle, 495;
+ pes, 522;
+ skull, 458
+
+ Megistanes, _40_, _299_
+
+ Membranous cranium, 17
+
+ Menobranchidae, _35_
+
+ _Menobranchus_, _35_, _135_;
+ carpus, 185;
+ pes, 188;
+ skull, 174;
+ teeth, 169
+
+ _Menopoma_, _35_;
+ see _Cryptobranchus_
+
+ Mento-meckelian, 22;
+ frog, 161;
+ reptiles, 284
+
+ Merganser, _41_;
+ beak, 329
+
+ _Mergus_, _41_
+
+ Merrythought, duck, 322
+
+ Mesethmoid, 20;
+ dog, 390;
+ duck, 317
+
+ _Mesoplodon_, _44_;
+ teeth, 427
+
+ Mesopterygium, 79
+
+ Mesosauridae, _37_
+
+ _Mesosaurus_, _37_
+
+ Mesosternum, dog, 404
+
+ Metacarpal quill, duck, 303
+
+ Metacarpo-digital, duck, 303
+
+ Metacarpus, 26;
+ see Manus
+
+ Metacromion, hares and rabbits, 497
+
+ Meta-pterygium, 79
+
+ Metatarsus 26;
+ see Pes
+
+ Metatheria, _43_;
+ general characters, 349
+
+ _Metriorhynchus_, _39_, 278
+
+ Microchiroptera, _49_;
+ general characters, 371
+
+ _Microgale_, _49_;
+ caudal vertebrae, 454
+
+ Mid-digital quill, duck, 303
+
+ Milk-teeth, 344;
+ dog, 377;
+ horse, 430
+
+ Moa, _40_, _299_;
+ aftershaft, 328;
+ pectoral girdle, 336;
+ wing, 338
+
+ Molar teeth, 344;
+ dog, 376 f.
+
+ Mole, _49_, _370_;
+ auditory ossicles, 488;
+ cervical vertebrae, 446;
+ humerus, 503;
+ manus, 512;
+ presternum, 490;
+ shoulder girdle, 499;
+ skull, 481;
+ teeth, 440;
+ Golden --, _49_;
+ see _Chrysochloris_;
+ Marsupial --, _43_;
+ see _Notoryctes_
+
+ _Molge_, _35_, _135_;
+ see Newt
+
+ Monitor, _38_;
+ see _Varanus_
+
+ Monkey, _49_, _373_;
+ see under Primates
+
+ Monodelphia, _43_;
+ characters of, 351
+
+ _Monodon_, _45_, _357_;
+ see Narwhal
+
+ Monophyodont, defined, 7, 344
+
+ Monopneumona, _34_
+
+ Monotremata, _42_;
+ arm bones, 499;
+ auditory ossicles, 486;
+ caudal vertebrae, 453;
+ cervical vertebrae, 443;
+ general characters, 346;
+ manus, 504;
+ pectoral girdle, 493;
+ pelvis, 513;
+ pes, 521;
+ ribs, 490;
+ sacral vertebrae, 451;
+ skull, 455;
+ sternum, 489;
+ teeth, 422;
+ thigh and shin, 517;
+ thoraco-lumbar vertebrae, 447
+
+ _Morosaurus_, _38_, _207_;
+ pes, 294
+
+ _Mosasaurus_, _38_, _204_
+
+ _Moschus_, _46_;
+ canines, 429
+
+ Mouse, _47_;
+ teeth, 437;
+ see _Mus_
+
+ Mud-fish, _34_
+
+ Multituberculata, _43_, _348_
+
+ Muntjac, _46_;
+ see _Cervulus_
+
+ Muraenidae, _33_
+
+ Muridae _47_
+
+ _Mus_, _47_;
+ _M. musculus_, teeth, 437;
+ _M. sylvaticus_, sternum and shoulder girdle, 498
+
+ Musk deer, _46_;
+ canines, 429
+
+ Mustelidae, _48_;
+ teeth 439
+
+ _Mycetes_, 49;
+ hyoid, 485;
+ mandible, 484;
+ skull, 482
+
+ Myliobatidae, _32_;
+ teeth, 109
+
+ Myomorpha, _47_
+
+ _Myrmecobius_, teeth, 423
+
+ _Myrmecophaga_, _44_;
+ manus 505;
+ pectoral girdle, 495;
+ skull, 458
+
+ Myrmecophagidae, _44_, 424;
+ see Anteaters
+
+ Mystacoceti, _44_;
+ general characters, 356;
+ hind limb, 518;
+ manus, 505;
+ pectoral girdle, 495;
+ skull, 461;
+ teeth, 426
+
+ _Myxine_, _31_, 55;
+ fins, 115;
+ notochordal sheath, 9
+
+ Myxinoidei, _31_, 55
+
+
+ Nails, 3;
+ Amphibia, 168;
+ mammals, 417
+
+ Nares:
+ anterior --, crocodile, 252, 257;
+ dog, 401;
+ duck, 317;
+ newt, 143;
+ turtle, 225, 229;
+ posterior --, crocodile, 257;
+ dog, 402;
+ duck, 318;
+ frog, 158;
+ newt, 143;
+ turtle, 230
+
+ Narial cavity, salmon, 89;
+ -- passage, crocodile, 254;
+ dog, 395;
+ -- septum, dog, 401
+
+ Narwhal, _45_, _357_;
+ teeth, 427
+
+ Nasal 21;
+ crocodile, 252;
+ dog, 394;
+ duck, 317;
+ frog, 158;
+ newt, 143;
+ turtle, 228;
+ -- capsule, 20;
+ cod, 97;
+ crocodile, 252;
+ dog, 394;
+ dogfish, 74;
+ frog, 158;
+ newt, 143;
+ turtle, 228;
+ -- cavity, dog, 388;
+ -- fossae, salmon, 89;
+ -- horns, rhinoceros, 3
+
+ Navicular, 27;
+ dog, 414
+
+ Neornithes, _40_;
+ general characters, 298
+
+ _Nesodon_, _46_, _361_;
+ pes, 525;
+ teeth, 432
+
+ Nesodontidae, _46_
+
+ Neural arch, 14;
+ -- plate, turtle, 215;
+ -- spine, 14
+
+ Neuromere, defined, 112
+
+ Newt, _35_;
+ anterior limb, 147;
+ hyoid apparatus or visceral arches, 144, =181=;
+ pelvic girdle, 149;
+ ribs, 145;
+ shoulder girdle, =146=;
+ skull, 140;
+ sternum, 145;
+ vertebral column, 138
+
+ Notidanidae, _31_;
+ calcification of vertebrae, 114;
+ pectoral fins, 130;
+ vertebral column, 113;
+ visceral arches, 63, 119 f.
+
+ Notochord, _Amphioxus_ 52;
+ Balanoglossus, 50;
+ dogfish, 72;
+ Tunicates, 51
+
+ _Nothosaurus_, _37_, _193_;
+ supratemporal fossae, 283
+
+ Nothosauridae, _37_
+
+ _Notoryctes_, _43_;
+ arm bones, 500;
+ caudal vertebrae, 453;
+ cervical vertebrae, 443;
+ claws, 418;
+ dental formula, 423;
+ manus, 504;
+ pelvis, 513;
+ pectoral girdle, 494;
+ pes, 521;
+ ribs, 491;
+ sacrum, 452;
+ skull, 457;
+ sternum, 489;
+ thigh and shin, 517
+
+ Notoryctidae, _43_, _350_
+
+ Nuchal plate, turtle, 215;
+ -- shield, crocodile, 238;
+ turtle, 214
+
+ _Nyrania_, _35_;
+ palatines, 177
+
+
+ Occipital condyle, crocodile, 246;
+ dog, 386;
+ duck, 315;
+ frog, 154;
+ turtle, 224;
+ -- crest, dog, 386;
+ -- segment, crocodile, 246;
+ dog, 384;
+ turtle, 224
+
+ _Odontaspis_, _32_;
+ succession of teeth, =107=
+
+ Odontoblast, 7
+
+ Odontoceti, _44_;
+ general characters, 357;
+ manus, 505;
+ pectoral girdle, 495;
+ skull, 462;
+ sternum, 489;
+ teeth, 426
+
+ Odontolcae, _40_;
+ general characters, 299
+
+ _Odontopteryx_, _40_;
+ jaws, 334
+
+ _Ogmorhinus_, _48_;
+ mandibular ramus, =439=
+
+ Olecranon process, dog, 406;
+ duck, 323;
+ frog, 164
+
+ Olfactory capsule, see nasal capsule;
+ -- cavity, dog, 388;
+ -- chamber, dog, 395;
+ -- fossa, dog, 390
+
+ Olm, _35_
+
+ _Omosaurus_, exoskeleton, 272
+
+ Omosternum, frog, 163
+
+ _Onychodactylus_, _35_;
+ nails, 168
+
+ Operculum, cod, 101;
+ salmon, 95
+
+ Ophidia, _38_;
+ general characters, 202;
+ jaw bones, 280;
+ scales, 270;
+ skull, 277 f.;
+ vertebral column, 275
+
+ _Ophisaurus_, _38_;
+ limbs, 289;
+ pectoral girdle, 289
+
+ Opisthocoelous, defined, 14
+
+ _Opisthocomus_, 41;
+ skull, 334
+
+ Opisthotic, 20;
+ cod, 96;
+ crocodile, 250;
+ salmon, 89 f.;
+ turtle, 227
+
+ Opossum, _43_;
+ caudal vertebrae, 453;
+ teeth, 423
+
+ Optic capsule, 20;
+ crocodile, 251;
+ dog, 394;
+ turtle, 228
+
+ Orang, _49_;
+ carpus, 512;
+ ribs, 493;
+ thoraco-lumbar vertebrae, 450
+
+ Orbit, crocodile, 257;
+ dogfish, 74;
+ duck, 317
+
+ Orbital ring, cod, 97;
+ salmon, 93
+
+ Orbitosphenoid, 19;
+ dog, 388;
+ duck, 317;
+ newt, 141
+
+ _Orca_, _45_;
+ teeth, 427
+
+ _Oreodon_, _45_;
+ see _Cotylops_
+
+ Ornithodelphia, _42_;
+ general characters, 346
+
+ Ornithosauria, _212_
+
+ Ornithorhynchidae, _43_
+
+ _Ornithorhynchus_, _43_;
+ beak, 3, 418;
+ caudal vertebrae, 453;
+ manus, 504;
+ pelvis, 513;
+ sacral vertebrae, 451;
+ shoulder girdle, =347=;
+ skull, 455;
+ spur, 418;
+ tarsus, 27 n.;
+ teeth, 4, 346, 422;
+ thoraco-lumbar vertebrae, 447
+
+ Ornithopoda, _39_;
+ general characters, 209
+
+ Orthopoda, _39_;
+ general characters, 208;
+ pubes, 292
+
+ Orycteropodidae, _44_;
+ teeth 425
+
+ _Orycteropus_, _44_;
+ hair, 416;
+ manus, 505;
+ pectoral girdle, 495;
+ pelvis, 513;
+ skull, 459;
+ see Aard Vark
+
+ Osborn, H.F., on Mesozoic Mammals, 348
+
+ Os entoglossum, duck, 320
+
+ Osteoblast, 11
+
+ Osteoclast, 11
+
+ Osteodentine, 108
+
+ Osteostraci, _31_;
+ general characters, 54
+
+ _Ostracion_, _33_, 69;
+ plates, 105
+
+ Ostracionidae, _33_
+
+ Ostracodermi, _31_;
+ general characters, 54
+
+ Ostrich, _40_, _299_;
+ aftershaft, 329;
+ cervical vertebrae, =331=;
+ claws, 330;
+ foot, 342;
+ manus, 338;
+ pelvic girdle and sacrum, =340=;
+ pubis, 341;
+ tibio-tarsus, 341;
+ wing, =339=
+
+ _Otaria_, _48_;
+ dentition, 439;
+ tympanic bulla, 480
+
+ Otariidae, _48_, _369_;
+ auditory ossicles, 488;
+ scapula, 498;
+ skull, 480
+
+ Owen's apteryx, pelvic girdle and sacrum, =340=
+
+ Owen's chameleon, epidermal horns, 271
+
+ Owls, _42_, 335;
+ aftershaft, 329;
+ foot, 342
+
+ Owl-parrot, _42_;
+ see _Stringops_
+
+ Ox, _46_, 359;
+ atlas and axis, =445=;
+ three cervical vertebrae, =15=;
+ femur, =518=;
+ manus, 507;
+ teeth, 345;
+ two thoracic vertebrae, =449=
+
+
+ Paca, _48_
+
+ Paired fins, 127
+
+ Palaeoniscidae, _32_
+
+ _Palaeoniscus_, _32_;
+ scales, 67
+
+ Palaeospondylidae, _31_
+
+ _Palaeospondylus_, _31_, 58
+
+ _Palaeosyops_, _46_;
+ teeth, 432
+
+ Palaeotheriidae, _46_
+
+ _Palaeotherium_, 46;
+ skull, 471;
+ teeth, 430
+
+ _Palamedea_, _41_;
+ spur, 330, 338
+
+ Palamedeae, _41_
+
+ Palate, reptiles, 280 f.
+
+ Palatine, cod, 98;
+ crocodile, 254;
+ dog, 397;
+ duck, 318;
+ frog, 160;
+ salmon, 93;
+ turtle, 230
+
+ Palato-pterygo-quadrate bar, 22;
+ dogfish, 77;
+ fish, 120 f.;
+ salmon, 93
+
+ Palm civet, _48_
+
+ Pangolin, _44_;
+ pectoral girdle, 495;
+ pelvis, 513;
+ caudal vertebrae, 453;
+ see _Manis_
+
+ Parachordals, 17
+
+ _Paradoxurus_, _48_;
+ tail, 454
+
+ Parasphenoid, 21;
+ cod, 97;
+ frog, 156;
+ newt, 141;
+ reptiles, 278;
+ salmon, 93
+
+ Parasuchia, _39_;
+ general characters, 211
+
+ Parethmoid, 21 n.
+
+ Pariasauria, _36_
+
+ _Pariasaurus_, _36_, _192_;
+ pectoral girdle, 289;
+ pelvis, 292;
+ supratemporal fossa, 283;
+ teeth, 273
+
+ Parietal, 19;
+ cod, 96;
+ crocodile, 247;
+ dog, 386;
+ duck, 314;
+ newt, 141;
+ salmon, 91;
+ turtle, 225;
+ -- segment, crocodile, 247;
+ dog, 386;
+ turtle, 225
+
+ Paroccipital process, dog, 386
+
+ _Parra_, _42_;
+ spur, 330
+
+ Parrots, 335;
+ aftershaft, 328;
+ beak, 330;
+ epiphyses of centra, 332;
+ foot, 342;
+ powder-down feathers, 329;
+ skull, 334
+
+ Parrot fish, _33_;
+ see _Scarus_
+
+ Passeres, aftershaft, 328
+
+ Passeriformes, _42_
+
+ Patella, dog, 412;
+ duck, 327
+
+ _Pavo_, _41_;
+ _P. cristatus_, shoulder girdle and sternum, =337=
+
+ Peacock, _41_;
+ see _Pavo_
+
+ Peccary, pes, 523
+
+ Pecora, _46_, _359_;
+ teeth, 429
+
+ Pectinated incisors, _Galeopithecus_, 370, 440;
+ _Procavia_, 362
+
+ Pectineal process, duck, 326
+
+ Pectoral fins, cod, 103;
+ dogfish, 79;
+ -- girdle, 24;
+ Amphibia, 184;
+ birds, 336;
+ cod, 101;
+ crocodile, 262;
+ dog, 404;
+ dogfish, 79;
+ duck, 321;
+ fish, 126;
+ frog, 162;
+ mammalia, 493;
+ newt, 145;
+ reptiles, 288;
+ turtle, 231;
+ -- shield, turtle, 215
+
+ _Pedetes_, _47_;
+ manus, 511;
+ tail, 454
+
+ Pelican, _41_, 335;
+ clavicles, 338
+
+ _Pelicanus_, _41_;
+ _P. conspicillatus_ shoulder girdle and sternum, =337=
+
+ _Pelobates_, _36_;
+ vertebrae, 172;
+ _P. cultripes_ teeth, 169
+
+ Pelobatidae, _36_
+
+ Pelvic fins, cod, 103;
+ dogfish, 82;
+ fish, 131;
+ -- girdle, 25;
+ Amphibia, 187;
+ birds, 339;
+ crocodile, 266;
+ dog, 409;
+ dogfish, 81;
+ duck, 324;
+ fish, 127;
+ frog, 165;
+ mammals, 512;
+ newt, 149;
+ Ratitae, =340=;
+ Reptilia, 291;
+ turtle, 235
+
+ Penguin, _40_;
+ distribution of feathers, 328;
+ fibula, 341;
+ foot, 342;
+ manus, 338;
+ metatarsus, 342;
+ pneumaticity of skeleton, 331;
+ skull, 333;
+ sternum, 336;
+ thoracic vertebrae, 332;
+ wing, 329, =339=
+
+ Penna, duck, 303
+
+ Pentedactylate, defined, 26
+
+ _Perameles_, _43_;
+ atlas, 443;
+ pectoral girdle, 494
+
+ Peramelidae, _43_, _350_;
+ auditory ossicles, 486;
+ pes, 522
+
+ _Perca_, _34_
+
+ Perch, _34_;
+ pelvic fin, 132;
+ urostyle, 117
+
+ Percidae, _34_
+
+ Perennibranchiata, _35_;
+ characters, 135
+
+ Perichondrium, 10
+
+ Perichordal sheath, 16
+
+ Periosteal ossification, 10
+
+ Periosteum, 10
+
+ Periotic, dog, 390;
+ -- capsule, see Auditory capsule
+
+ Perissodactyla, _46_;
+ cervical vertebrae, 445;
+ general characters, 359;
+ manus, 507;
+ pes, 523;
+ ribs, 491;
+ scapula, 496;
+ skull, 470;
+ teeth, 429;
+ thoraco-lumbar vertebrae, 448
+
+ Persistent pulps, 5
+
+ Pes, 26;
+ crocodile, 268;
+ dog, =413=, 414;
+ duck, 327;
+ frog, 166;
+ mammals, 521;
+ reptiles, 293;
+ turtle, 236;
+ of Tapir, Rhinoceros, _Hipparion_ and Horse, =524=
+
+ _Petromyzon_, _31_, 55 f.;
+ notochordal sheath, 9
+
+ Petromyzontidae, _31_, 55
+
+ Petrous portion of periotic, dog, 391
+
+ _Pezophaps_, _42_;
+ see Solitaire
+
+ _Phacochaerus_, _45_;
+ teeth, 428
+
+ _Phaëthon_, _41_;
+ metatarsals, 342
+
+ _Phalacrocorax_, _41_
+
+ Phalangeridae, _43_, 350
+
+ Phalanges, 26;
+ see Manus and Pes
+
+ Phaneroglossa, _36_
+
+ Pharyngo-branchial, cod, 101;
+ dogfish, 78;
+ salmon, 95
+
+ Pharyngognathi, _33_
+
+ _Phascolarctus_, _43_;
+ see Koala
+
+ Phascolomyidae, _43_, _350_
+
+ _Phascolomys_, _43_, 349;
+ see Wombat
+
+ _Phascolotherium_, _43_, 348
+
+ Phenacodontidae, _47_
+
+ _Phenacodus_, _47_, _362_;
+ caudal vertebrae, 454;
+ manus, =510=;
+ pes, 525;
+ scapula, 497;
+ skull, 472;
+ thoraco-lumbar vertebrae, 449
+
+ _Phocaena_, _45_, _357_;
+ skull, 462;
+ thoraco-lumbar vertebrae, 448;
+ _P. phocaenoides_, ossicles, 420
+
+ Phocidae, _48_, _369_;
+ scapula, 497;
+ tympanic bulla, 480
+
+ _Phoronis_, _30_, 50 f.
+
+ _Phororhacos_, _41_;
+ anterior nares, 333;
+ ischia, 341
+
+ _Physeter_, _44_;
+ cervical vertebrae, 444;
+ manus, 505;
+ skull, 464;
+ teeth, 426
+
+ Physeteridae, _44_;
+ ribs, 491;
+ thoraco-lumbar vertebrae, 448
+
+ _Physodon_, _44_;
+ teeth, 426
+
+ Physodontidae, _44_
+
+ Physostomi, _33_
+
+ Phytosauridae, _39_
+
+ _Phytosaurus_, _39_;
+ see _Belodon_
+
+ Pici, _42_
+
+ _Picus_, _42_;
+ see Woodpecker
+
+ Pig, _45_, _359_;
+ skull, 465 f., =466=;
+ teeth, 345, 427
+
+ Pigeons, _42_, 334 f.;
+ aftershaft, 329;
+ pneumaticity of skeleton, 331
+
+ Pike, _33_;
+ pelvic fin, 132;
+ teeth, 107, 110
+
+ Pinnipedia, _48_;
+ arm bones, 502;
+ auditory ossicles, 488;
+ general characters, 369;
+ manus, 511;
+ pelvis, 515;
+ pes, 526;
+ skull, 480;
+ teeth, 439;
+ thigh and shin, 520;
+ thoraco-lumbar vertebrae, 450
+
+ _Pipa_, _36_;
+ hyoid apparatus, 182;
+ jaws, 169;
+ skull, 180;
+ sternum, 184;
+ vertebrae, 172
+
+ Pipidae, _36_
+
+ Pisces, _31_;
+ general characters, 60
+
+ Piscivorous dentition, 426, 440
+
+ Pisiform, 345, 504;
+ crocodile, 265;
+ dog, 408;
+ turtle, 233
+
+ Pituitary fossa, crocodile, 247;
+ -- space, 17
+
+ Placodontia, _36_
+
+ _Placodus_, _36_, _192_;
+ teeth 273
+
+ Placoid scale, 4, 60, 104
+
+ Plantigrade, defined, 358 n.
+
+ Plastron, _Dermochelys_, 272;
+ _Chelone midas_, 217, =218=, 271
+
+ Platanistidae, _45_
+
+ _Platanista_, _45_;
+ cervical vertebrae, 444;
+ skull, 464
+
+ Plectognathi, _33_;
+ vertebrae, 115
+
+ _Plectropterus_, _41_;
+ _P. gambensis_, spur, 330
+
+ Plesiosauridae, _37_;
+ limbs, 193;
+ parasphenoid, 192;
+ skull, 278
+
+ _Plesiosaurus_, _37_, _193_;
+ position of limbs, 28
+
+ Pleuracanthidae, _63_;
+ fins, 115
+
+ Pleurodira, _37_;
+ general characters, 195
+
+ Pleurodont, 159, 199, 273
+
+ Pleuronectidae, _33_
+
+ Pleuropterygii, _31_, 63
+
+ _Pliosaurus_, _37_, _193_
+
+ Plovers, _42_, 334;
+ thoracic vertebrae, 332
+
+ Pneumaticity of bird's skeleton, 331
+
+ _Polacanthus_, _39_;
+ exoskeleton, 272
+
+ Pollex, 26;
+ see Manus
+
+ _Polyodon_, _32_, 104;
+ distribution, 66;
+ pectoral fins, 131;
+ skull, 122;
+ spinal column, 112;
+ teeth, 110
+
+ Polyodontidae, _32_
+
+ _Polyonax_, _39_, _209_;
+ beak, 271;
+ frontals, 277;
+ jaw, 274;
+ predentary, 284
+
+ Polyprotodont, 423
+
+ Polyprotodontia, _43_;
+ general characters, 350
+
+ Polypteridae, _33_
+
+ _Polypterus_, _33_, _68_;
+ distribution, 66;
+ exoskeleton, 67;
+ pectoral fins, 131;
+ pelvic fins, 132;
+ pelvis, 127;
+ scales, 104;
+ skull, 122;
+ tail, 116
+
+ _Pontoporia_, _45_;
+ cervical vertebrae, 444;
+ teeth, 426
+
+ Porcupine, _47_;
+ pes, 526;
+ skull, 476, =477=;
+ spines, 417
+
+ Porpoise, _45_, _357_;
+ thoraco-lumbar vertebrae, 448
+
+ Postaxial, 28
+
+ Posterior cornu, duck, 320;
+ turtle, 231;
+ -- limb, 26;
+ Amphibia, 188;
+ birds, 341;
+ dog, 412;
+ duck, 326;
+ frog, 166;
+ newt, 149, =148=;
+ reptiles, 293;
+ turtle, 235, =234=
+
+ Postfrontal, 21;
+ crocodile, 250;
+ turtle, 225
+
+ Postorbital bar, crocodile, 250, 255 f.;
+ _Hatteria_, 283;
+ -- groove, dogfish, 76
+
+ Post-temporal, cod, 102;
+ reptiles, 283;
+ -- bar, crocodile, 256;
+ _Hatteria_, 283
+
+ _Potamogale_, _49_, 367, 370;
+ shoulder girdle, 499;
+ teeth, 440
+
+ Potamogalidae, _49_
+
+ Powder-down feathers, 329
+
+ Pre-axial, 28
+
+ Precoracoid, 25;
+ frog, 163;
+ newt, 147;
+ reptiles, 288;
+ turtle, 232
+
+ Predentary, reptiles, 284
+
+ Predigital quill, duck, 303
+
+ Prefrontal, 21;
+ crocodile, 249;
+ reptiles, 278;
+ turtle, 225
+
+ Prefronto-lachrymal, newt, 141
+
+ Prehallux, frog, 167 f.
+
+ Premaxilla, 22;
+ cod, 98;
+ crocodile, 252;
+ dog, 398;
+ duck, 314, 318;
+ frog, 158;
+ newt, 143;
+ salmon, 94;
+ turtle, 230
+
+ Premolar, dog, 370, 377;
+ mammals, 344
+
+ Prenasal process, frog, 158
+
+ Pre-orbital vacuity, reptiles, 283
+
+ Presphenoid, 19;
+ dog, 388
+
+ Prespiracular ligament, dogfish, 77
+
+ Presternum, dog, 404
+
+ Primaries, duck, 303
+
+ Primates, _49_;
+ arm bones, 503;
+ auditory ossicles, 488;
+ cervical vertebrae, 446;
+ general characters, 372;
+ manus, 512;
+ pelvis, 515;
+ pes, 527;
+ ribs, 493;
+ sacrum, 452;
+ shoulder girdle, 499;
+ skull, 482 f.;
+ sternum, 490;
+ tail, 454;
+ teeth, 441;
+ thigh and shin, 520;
+ thoraco-lumbar vertebrae, 450
+
+ _Priodon_, _44_;
+ caudal vertebrae, 453;
+ manus, 505;
+ stapes, 487;
+ sternum, 489;
+ teeth, 424
+
+ Pristidae, _32_
+
+ _Pristis_, _32_;
+ snout or rostrum, 109, 119
+
+ Proboscidea, _47_;
+ arm bones, 502;
+ cervical vertebrae, 445;
+ general characters, 364;
+ femur, 519;
+ manus, 511;
+ pelvis, 514;
+ pes, 526;
+ scapula, 497;
+ skull, 473;
+ teeth, 433;
+ thoraco-lumbar vertebrae, 449
+
+ _Procavia_, _47_, _363_;
+ auditory ossicles, 487;
+ caudal vertebrae, 453;
+ dental formula, 432;
+ humerus, 502;
+ manus, 510;
+ pelvis, 514;
+ pes, 525;
+ ribs, 491;
+ scapula, 497;
+ skull, 433, 472;
+ tarsus, 27;
+ thoraco-lumbar vertebrae, 449
+
+ Process, alinasal --, frog, 158;
+ basi-pterygoid --, birds, 334;
+ coracoid --, dog, 405;
+ coronoid -- (of mandible), dog, 398;
+ duck, 319;
+ coronoid -- (of ulna), dog, 408;
+ pectineal --, duck, 326;
+ postfrontal --, duck, 316;
+ postglenoid --, dog, 394;
+ postorbital -- (of frontal), dog, 388;
+ postorbital -- (of jugal), dog, 398;
+ posterior articular --, duck, 319;
+ zygomatic --, dog, 394
+
+ Processus brevis, 486;
+ -- gracilis, 486;
+ -- longus, 486;
+ dog, 393
+
+ Procoelous, defined, 14
+
+ _Prodelphinus_, _45_;
+ skull, 462
+
+ Proganosauria, _37_
+
+ Prone position, 29
+
+ Prongbuck, _46_;
+ horns, 417
+
+ Pro-otic, 20;
+ frog, 157;
+ turtle, 227
+
+ Pro-pterygium, dogfish, 79
+
+ Proteidae, _35_
+
+ _Proteles_, _48_;
+ teeth, 437
+
+ Protelidae, _48_
+
+ Proterosauridae, _37_
+
+ _Proterosaurus_, _37_;
+ teeth, 198, 274;
+ vertebrae, 197
+
+ _Proteus_, _35_, 135, 182;
+ branchial arches, 180;
+ digits, 187;
+ pes, 188;
+ skull, 174
+
+ _Protopterus_, _34_, _70_, 117;
+ branchial arches, 121, 124;
+ fins, 130;
+ skull, 124;
+ vestigial gill on pectoral girdle, 129
+
+ Prototheria, _42_;
+ general characters, 346
+
+ Proximal, defined, 23 n.
+
+ _Psephurus_, distribution, 66
+
+ _Pseudopus_, _38_;
+ limbs, 289
+
+ Psittaci, _42_;
+ see Parrots
+
+ _Pteranodon_, _39_, _274_;
+ pectoral girdle, 289
+
+ Pteranodontidae, _39_
+
+ _Pteraspis_, _31_, _54_
+
+ _Pterichthys_, _31_, _55_
+
+ _Pterocles_, _42_;
+ see Sandgrouse
+
+ Pteroclidae, _42_
+
+ Pterodactylidae, _39_
+
+ _Pterodactylus_, _39_, 213
+
+ Pteropidae, _49_;
+ skull, 481
+
+ _Pteropus_, _49_;
+ dental formula, 441;
+ tail, 454
+
+ Pterosauria, _39_;
+ general characters, 212;
+ ischia, 292;
+ limbs, 291;
+ pre-orbital vacuity, 284;
+ ribs, 285;
+ sternum, 287;
+ vertebrae, 275 f.
+
+ Pterotic, 20;
+ cod, 96;
+ salmon, 90 f.
+
+ Pterygoid, cod, 98;
+ crocodile, 255;
+ dog, 397;
+ duck, 318;
+ frog, 160;
+ newt, 144;
+ salmon, 93;
+ turtle, 230;
+ -- fossa, crocodile, 255;
+ -- plate, dog, 388
+
+ Pterylae, 328
+
+ Pubis, 25;
+ crocodile, 266 f.;
+ duck, 325;
+ dog, 411;
+ frog, 165;
+ newt, 149;
+ reptiles, 292;
+ turtle, 235
+
+ Pygal plate, turtle, 217;
+ -- shield, turtle, 214
+
+ Pygopodidae, _37_
+
+ Pygostyle, duck, 307, 312
+
+ _Python_, _38_;
+ ischio-pubis, 292;
+ jaws, 280;
+ vestiges of limbs, 289, 293
+
+ Pythonomorpha, _38_;
+ general characters, 204;
+ limbs, 290;
+ teeth, 273
+
+
+ Quadrate, 22;
+ cod, 98;
+ crocodile, 255;
+ duck, 319;
+ frog, 160;
+ newt, 144;
+ salmon, 93;
+ turtle, 229
+
+ Quadratojugal, 22;
+ crocodile, 255;
+ duck, 318;
+ frog, 160;
+ turtle, 229
+
+ Quill, duck, 302
+
+
+ Rabbit, _48_, 366;
+ pollex, 511
+
+ Raccoon, 369
+
+ Rachis, duck, 302
+
+ Rachitomous, defined, 171
+
+ Radiale, 27;
+ see Carpus
+
+ Radialia, 115;
+ dogfish, 79 f.
+
+ Radio-ulna, frog, 164
+
+ Radius, 26;
+ crocodile, 265;
+ dog, 406;
+ duck, 323;
+ newt, 147;
+ turtle, 233
+
+ _Raia_, _32_;
+ calcification of vertebrae, 114
+
+ Raiidae, _32_
+
+ _Rana_, _36_;
+ see Frog
+
+ Ranidae, _36_;
+ shoulder girdle, 185
+
+ Rat, pes, 526
+
+ Ratitae, 40;
+ caudal vertebrae, 333;
+ clavicles, 338;
+ foot, 342;
+ general characters, 298;
+ skull, 333;
+ sternum, 336;
+ vomers, 334;
+ wing, 338
+
+ Rattlesnake, _38_;
+ rattle, 3, 270
+
+ Ray, pectoral fin, 130;
+ Eagle --, Electric -- and Sting --, _32_
+
+ Rectrices, 303, 329
+
+ Reed-fish, _33_
+
+ Reindeer, antlers, 469
+
+ Remicle, duck, 304
+
+ Remiges, 303, 329
+
+ Reptiles, anterior limb, 290;
+ exoskeleton, 270;
+ fossae in skull, 281;
+ pectoral girdle, 288;
+ pelvic girdle, 291;
+ posterior limb, 293;
+ ribs, 285;
+ skull, 276;
+ sternum, 287;
+ teeth, 272;
+ vertebral column, 275
+
+ Reptilia, _36_;
+ general characters, 190;
+ see Reptiles
+
+ _Rhabdopleura_, _30_, 50
+
+ _Rhamphastos_, _42_;
+ see Toucan
+
+ Rhamphorhynchidae, _39_
+
+ _Rhamphorhynchus_, _39_, 213, 274
+
+ _Rhea_, _40_;
+ aftershaft, 329;
+ claws, 330;
+ ischia, 341;
+ manus, 338;
+ _R. macrorhyncha_, pelvic girdle and sacrum, =340=
+
+ Rheornithes, _40_
+
+ _Rhina_, _32_;
+ see _Squatina_
+
+ Rhinal process, frog, 158
+
+ _Rhinoceros_, _46_, 360, 419;
+ femur, =518=;
+ fibula, 519;
+ malleus, 487;
+ manus, 508;
+ nasal horns, 3, 417;
+ pes, 525;
+ skull, =421=, 470;
+ teeth, 430;
+ ulna, 501;
+ _R. antiquitatis_, 470
+
+ Rhinocerotidae, _46_
+
+ Rhinolophidae, _49_
+
+ Rhiptoglossa, _38_
+
+ Rhizodontidae, _33_
+
+ _Rhizodus_, _33_;
+ teeth, 110
+
+ Rhynchocephalia, _37_;
+ general characters, 197;
+ humerus, 290;
+ teeth, 273 f.;
+ vertebrae, 275
+
+ Rhynchosauridae, _37_;
+ maxillae, 198
+
+ _Rhytina_, _44_, 352, 425;
+ humerus, 501;
+ skull, =460=
+
+ Rhytinidae, _44_
+
+ Ribs, 23;
+ Amphibia, 182;
+ birds, 336;
+ cod, 86;
+ crocodile, 259;
+ dog, 402;
+ dogfish, 73;
+ duck, 320;
+ fish, 125;
+ frog, 153;
+ mammalia, 490;
+ newt, 145;
+ reptiles, 285
+
+ Ridge, supra-orbital and suborbital, dogfish, 74
+
+ Rodentia, _47_;
+ auditory ossicles, 488;
+ cervical vertebrae, 446;
+ dental formula, 435;
+ general characters, 365;
+ pelvis, 515;
+ pes, 526;
+ humerus, 502;
+ manus, 511;
+ ribs, 493;
+ sacrum, 452;
+ shoulder girdle, 497;
+ skull, 476;
+ sternum, 489;
+ tail, 454;
+ teeth, 421;
+ thigh and shin, 520;
+ thoraco-lumbar vertebrae, 449
+
+ Roller, _42_, 335
+
+ Rooted teeth, defined, 5
+
+ Rorqual, _44_, 357;
+ cervical vertebrae, 444
+
+ Rostrum, crocodile, 247;
+ dogfish, 74;
+ duck, 316;
+ _Pristis_, 119;
+ -- of sternum, duck, 321
+
+ Röse, C., on teeth of Marsupials, 422
+
+ Ruminantia, _46_, _359_;
+ auditory ossicles, 487;
+ fibula, 519;
+ horny plates on palate, 418;
+ hyoid, 470;
+ manus, 507;
+ odontoid process, 445;
+ pes, 523;
+ scapula, 495;
+ teeth, 420, 429
+
+
+ Sabre-toothed lion, _48_;
+ see _Machaerodus_
+
+ Sacral ribs, crocodile, 243;
+ -- surface of ilium, dog, 409;
+ -- vertebrae, 16;
+ crocodile, 243;
+ dog, 383;
+ duck, 312;
+ frog, 153;
+ newt, 140;
+ turtle, 222
+
+ Sacrum, duck, 310;
+ see Sacral vertebrae
+
+ Sagittal crest, dog, 386
+
+ _Saiga_, skull, 468
+
+ Salamander, _35_
+
+ _Salamandra_, _35_, _135_;
+ antibrachium and manus of larva, =186=;
+ manus of larva, 185;
+ tarsus, 27
+
+ Salamandrina, _35_, _135_;
+ skull, 175;
+ sternum, 182
+
+ _Salmo_, _33_
+
+ Salmon, _33_;
+ branchial arches, 95;
+ chondrocranium, 87;
+ opercular bones, 95;
+ pectoral fins, 131;
+ skull, 87
+
+ Salmonidae, _33_
+
+ Sandgrouse, _42_, 335
+
+ _Sarcophilus_, _43_;
+ teeth, 423
+
+ Sauropoda, _38_;
+ general characters, 205;
+ teeth, 273;
+ vertebrae, 276
+
+ Sauropsida, _36_;
+ general characters, 189
+
+ Sauropterygia, _37_;
+ general characters, 192;
+ limbs, 290;
+ palate, 281;
+ pectoral girdle, 288;
+ vertebrae, 276
+
+ Saw-fish, _32_;
+ see _Pristis_
+
+ Scales, cod, 83;
+ crocodile, 237;
+ ctenoid, 8;
+ cycloid, 8;
+ duck, 302;
+ ganoid, 8;
+ Gymnophiona, 168;
+ mammals, 417
+
+ Scale-foot, 37
+
+ Scalpriform, 366
+
+ _Scaphirhynchus_, _32_, _104_;
+ distribution, 66;
+ exoskeleton, 67;
+ spinal column, 112
+
+ Scaphoid, 27;
+ mammals, 504 f.
+
+ Scapho-lunar, dog, 408
+
+ Scapula, 25;
+ cod, 103;
+ crocodile, 263;
+ dog, 404;
+ duck, 322;
+ frog, 162;
+ newt, 146;
+ turtle, 232
+
+ Scapular shield, armadillo, 419
+
+ Scapus, duck, 302
+
+ _Scarus_, _33_;
+ beaks, 111
+
+ Scelidosauridae, _39_
+
+ Schizognathous, defined, 335
+
+ Scincidae, _38_
+
+ _Scincus_, _38_;
+ scutes, 271
+
+ Sciuromorpha, _47_
+
+ Sclerotic, turtle, 228
+
+ Screamer, _41_;
+ spurs, 330
+
+ Scutes, armadillos, 419;
+ crocodile, 237;
+ reptiles, 271
+
+ Scylliidae, _32_
+
+ _Scyllium_, _32_;
+ calcification of vertebrae, 114;
+ pectoral fins, 130;
+ suspensorium, 119;
+ see Dogfish
+
+ _Scymnus_, _32_, _118_;
+ calcification of vertebrae, 114;
+ mandibular arch, 120;
+ pectoral fins, 130
+
+ _Scythrops_, _42_;
+ interorbital septum, 333
+
+ Sea leopard, _48_;
+ see _Ogmorhinus_;
+ -- lion, _48_;
+ manus, 511;
+ pes, 526;
+ position of limbs, 29;
+ -- otter, _48_;
+ pes, 526
+
+ Seal, _369_;
+ manus, 511;
+ pes, 526;
+ sacral vertebrae, 452;
+ scapula, 497
+
+ Secondaries, duck, 303 f.
+
+ Secretary-bird, _41_;
+ claws, 330
+
+ Selachii, _31_;
+ general characters, 63;
+ teeth, 108
+
+ Selenodont, defined, 345, 428
+
+ Sella turcica, crocodile, 247;
+ dog, 386
+
+ Semionotidae, _33_
+
+ Semiplumae, 328
+
+ Sense capsules, see Auditory, Nasal and Optic capsule
+
+ _Seps_, 38;
+ limbs, 289
+
+ Shagreen, 61
+
+ Shaft of feather, 302
+
+ Shark, 64;
+ Frill-gilled --, _31_;
+ see _Chlamydoselache_;
+ Port Jackson --, _32_;
+ see _Cestracion_
+
+ Sheep, _359_;
+ manus, 507;
+ teeth, 345
+
+ Shields of turtle, 214
+
+ Shin, 26;
+ see Crus
+
+ Shoulder girdle, see Pectoral girdle
+
+ Shrew, _49_, _370_;
+ auditory ossicles, 488;
+ cervical vertebrae, 446;
+ presternum, 490;
+ skull, 481; teeth, 440
+
+ Sigmoid notch, dog, 406
+
+ Siluridae, _33_;
+ plates, 105
+
+ _Simia_, _49_;
+ ribs, 493;
+ skull, 484;
+ thoraco-lumbar vertebrae, 450
+
+ Simiidae, _49_, _373_
+
+ Simplicidentata, _47_, 366
+
+ _Siphonops_, _35_;
+ _S. annulatus_, skull, =178=
+
+ _Siredon_, _35_;
+ skull, 175;
+ teeth, 169;
+ visceral arches, =181=
+
+ _Siren_, _35_, _135_, 188;
+ beaks, 168;
+ branchial arches, 180;
+ digits, 187;
+ skull, 174;
+ teeth, 169
+
+ Sirenia, _44_, 522;
+ arm bones, 501;
+ caudal vertebrae, 453;
+ cervical vertebrae, 443;
+ general characters, 352;
+ hair, 416;
+ horny plates, 418;
+ manus, 505;
+ pectoral girdle, 495;
+ pelvis, 514;
+ ribs, 491;
+ skull, 459;
+ sternum, 489;
+ teeth, 425;
+ thoraco-lumbar vertebrae, 448
+
+ Sirenidae, _35_
+
+ Sirenoidei, _34_;
+ general characters, 70
+
+ _Sivatherium_, _46_;
+ skull, 469
+
+ Skate, _32_
+
+ Skeletogenous layer, 14, 16;
+ _Amphioxus_, 52, 112
+
+ Skeleton, defined, 1;
+ Cape Buffalo, =492=;
+ _Ceratodus_, =128=;
+ cod, 83 f.;
+ crocodile, 237 f.;
+ dog, 374 f.;
+ duck, 302 f.;
+ frog, 151 f.;
+ llama, =496=;
+ newt, 138 f.;
+ turtle, 214 f.
+
+ Skink, _38_;
+ see _Tiliqua_
+
+ Skull, 16 f.;
+ Amphibia, 173 f.;
+ Anura, 179 f.;
+ birds, 333 f.;
+ cod, 96 f.;
+ crocodile, 243 f.;
+ diagram of Mammalian, 385;
+ Dipnoi, 124;
+ dog, 383 f.;
+ dogfish, 73 f.;
+ donkey, =431=;
+ duck, 312 f.;
+ fish, 117;
+ frog, 154 f., =159=;
+ _Globicephalus_, =463=;
+ Gymnophiona, 177;
+ Indian elephant, =474=;
+ Mammalia, 455;
+ Marsipobranchii, 57;
+ pig, =466=;
+ _Procavia_, =433=;
+ reptiles, 276 f.;
+ Rhinoceros, =421=;
+ _Rhytina_, =460=;
+ sloth, =458=;
+ Tasmanian wolf, =456=;
+ Teleostei, 124;
+ turtle, 222 f.;
+ wombat, =456=
+
+ Sloth, _43_, _352_;
+ auditory ossicles, 487;
+ arm bones, 500;
+ claws, 418;
+ leg bones, 517;
+ manus, 505;
+ pectoral girdle, 495;
+ pelvis, 513;
+ pes, 522;
+ ribs, 491;
+ sacrum, 452;
+ skull, 457;
+ sternum, 489;
+ teeth, 424;
+ thoraco-lumbar vertebrae, 447
+
+ Snake, _38_;
+ see Ophidia
+
+ Sole, _33_
+
+ _Solea_, _33_
+
+ _Solenodon_, _49_;
+ teeth, 440
+
+ Solenodontidae, _49_
+
+ Solitaire, _42_, _330_;
+ wing, 338;
+ wrist, 330
+
+ _Sorex_, _49_;
+ pelvis, 515;
+ see Shrew
+
+ Soricidae, _49_;
+ skull, 481
+
+ Spalacidae, _47_
+
+ _Spatularia_, _32_;
+ distribution, 66
+
+ _Spelerpes_, _35_;
+ branchial arches, 180;
+ ribs, 182;
+ _S. belli_ teeth, 169
+
+ Sperm whale, _44_, _357_;
+ see _Physeter_
+
+ _Sphargis_, _37_;
+ see _Dermochelys_
+
+ Sphenethmoid, frog, 156
+
+ Sphenisci, _40_;
+ see Penguins
+
+ Sphenisciformes, _40_
+
+ _Sphenodon_, _37_, 197 f.;
+ carpus, 291;
+ cervical vertebrae, 275;
+ fossae in skull, 281;
+ humerus, 290;
+ interparietal foramen, 277;
+ ribs, 286;
+ skull, =282=;
+ tarsus, 293;
+ teeth, 274
+
+ Sphenodontidae, _37_
+
+ Sphenoidal fissure, dog, 388
+
+ Sphenotic, 20;
+ cod, 97;
+ salmon, 89
+
+ Spider monkey, _49_;
+ see _Ateles_
+
+ Spinacidae, _32_
+
+ Spinal column, 13;
+ Dipnoi, 113;
+ fish, 112;
+ Holocephali, 113;
+ Marsipobranchii, 56
+
+ Spines, Elasmobranchs, 61;
+ mammals, 417
+
+ Spiny ant-eater, _43_;
+ see _Echidna_;
+ -- mouse, 47;
+ see _Acanthomys_
+
+ Splenial, 22;
+ crocodile, 258;
+ duck, 320;
+ turtle, 231
+
+ Spurs, birds, 330;
+ Monotremata, 418
+
+ Spur-winged goose, 330;
+ -- plover, 330
+
+ Squalidae _31_, _64_
+
+ _Squalodon_, _45_, _357_;
+ dental formula, 427
+
+ Squalodontidae, _45_
+
+ Squamata, _37_;
+ general characters, 198;
+ position of teeth, 272;
+ skull, 278
+
+ Squamosal, 21;
+ crocodile, 256;
+ dog, 394;
+ duck, 316;
+ frog, 160;
+ newt, 144;
+ turtle, 229
+
+ _Squatina_, _32_;
+ calcification of vertebrae, 114;
+ labial cartilages, 119;
+ tail, 63;
+ vertebral column, 114
+
+ Squatinidae, _32_
+
+ Squirrels, frontals, 476;
+ pes, 526
+
+ Stapes, dog, 393;
+ frog, 157;
+ man, dog, rabbit, =485=;
+ newt, 141
+
+ Steganopodes, _41_
+
+ Stegosauria, _39_;
+ general characters, 209
+
+ Stegosauridae, _39_
+
+ _Stegosaurus_, _39_, _208_ f.;
+ exoskeleton, 272
+
+ Steller's sea-cow, _44_;
+ see _Rhytina_
+
+ Stereornithes, _41_
+
+ Stereospondyli, _35_
+
+ Sternal ribs, crocodile, 259;
+ dog, 402;
+ duck, 320;
+ mammals, 490 f.
+
+ Sternebra, dog, 404
+
+ Sternum, 24;
+ Amphibia, 182;
+ birds, 336;
+ crocodile, 260, =261=;
+ dog, =403=;
+ duck, 321;
+ frog, 163;
+ Mammalia, 489;
+ newt, 145;
+ reptiles, 287
+
+ Stork, 335;
+ White --, _41_
+
+ Striges, _42_;
+ see Owls
+
+ _Stringops_, _42_;
+ sternum, 336
+
+ _Struthio_, _40_, _299_;
+ see Ostrich
+
+ Struthiornithes, _40_, _299_
+
+ Sturgeon, _32_;
+ see _Acipenser_
+
+ Stylo-hyal, dog, 399
+
+ Suborbital bar, duck, 318;
+ -- ridge, dogfish, 76
+
+ Subplantigrade, defined, 358 n.
+
+ Subungulata, _46_;
+ arm bones, 502;
+ general characters, 360;
+ manus, 509;
+ pelvis, 514;
+ pes, 525;
+ shoulder girdle, 497;
+ skull, 471;
+ teeth, 432;
+ thigh and shin, 519
+
+ Suidae, _45_
+
+ Suina, _45_, 358 f.;
+ fibula, 519;
+ manus, 507;
+ odontoid process, 445;
+ pelvis, 514;
+ pes, 523;
+ ulna, 501
+
+ Sula, _41_;
+ see Gannet
+
+ Supinator ridge, dog, 406
+
+ Supine position, defined, 29
+
+ Supra-angular, 22;
+ crocodile, 258;
+ duck, 319;
+ turtle, 230 f.
+
+ Supracaudal shield, turtle, 214
+
+ Supraclavicle, cod, 102
+
+ Supra-occipital, 19;
+ crocodile, 247;
+ dog, 386;
+ duck, 315;
+ turtle, 224
+
+ Supra-orbital, 20;
+ crocodile, 251
+
+ Suprapharyngeal bone, cod, 101
+
+ Suprascapula, crocodile, 263;
+ frog, 162
+
+ Supratemporal arcade, crocodile, 256;
+ reptiles, 281
+
+ Surinam toad, _36_;
+ see _Pipa_
+
+ Sus, _45_;
+ dental formula, 428;
+ see Pig
+
+ Suspensorium, Amphibia, 173;
+ dogfish, 78;
+ duck, 319;
+ frog, 160;
+ newt, 144;
+ Pisces, 61
+
+ Sutures, 12
+
+ Swan, _41_;
+ cervical and thoracic vertebrae, 332
+
+ Swift, _42_, 335;
+ foot, 342
+
+ Symplectic, cod, 100;
+ salmon, 94
+
+
+ Tails, fish, 60
+
+ Talpa, _49_;
+ pelvis, 515;
+ see Mole
+
+ Talpidae, _49_
+
+ Tapir, _46_, 360;
+ malleus, 487;
+ pes, =524=, 525;
+ teeth, 345;
+ see _Tapirus_
+
+ Tapiridae, _46_;
+ dental formula, 429
+
+ _Tapirus_, _46_;
+ fibula, 519;
+ manus, =508=;
+ skull, 471;
+ see Tapir
+
+ Tarsier, _49_
+
+ Tarsiidae, _49_
+
+ _Tarsipes_, _43_, 349;
+ mandible, 457
+
+ _Tarsius_, _49_, 372;
+ pes, 527
+
+ Tarso-metatarsus, duck, 327
+
+ Tarsus, 26 f.;
+ crocodile, 268;
+ dog, 414;
+ frog, 166;
+ newt, 150;
+ turtle, 236
+
+ Tasmanian devil, _43_;
+ see _Sarcophilus_;
+ -- wolf, _43_;
+ see _Thylacinus_
+
+ _Tatusia_, _44_;
+ stapes, 487;
+ teeth, 424
+
+ Tectospondyli, 114
+
+ Tectrices, duck, 306
+
+ Teeth, =6=;
+ Amphibia, 169;
+ birds, 330;
+ cod, 83;
+ crocodile, 238;
+ development, 7;
+ dog, 374 f.;
+ fish, 106 f.;
+ frog, 158 f.;
+ horses, 5;
+ mammals, 344, 420 f.;
+ pharyngeal, 8;
+ reptiles, 272 f.;
+ structure, 4;
+ succession, 7
+
+ Teleosauridae, _39_
+
+ _Teleosaurus_, _39_;
+ palate, 281;
+ scutes, 271;
+ vertebrae, 275
+
+ Teleostei, _33_;
+ general characters, 69;
+ ribs, 126;
+ skull, 124;
+ tail, 117;
+ teeth, 110;
+ vertebral column, 115
+
+ Temnospondyli, _35_
+
+ Tenrec, _49_;
+ see _Centetes_
+
+ Tentorium, dog, 392
+
+ Terrapin, _37_
+
+ _Testudo_, _37_, _194_
+
+ _Tetraceros_, _46_;
+ horns, 417
+
+ _Thalassochelys_, carapace, =216=
+
+ Thecodont, defined, 273
+
+ Theriodontia, _36_
+
+ Theromorpha, _36_;
+ general characters, 191;
+ humerus, 290;
+ pectoral girdle, 288;
+ ribs, 285;
+ skull, 278;
+ teeth, 273;
+ vertebral column, 275 f.
+
+ Theropoda, _38_;
+ general characters, 207;
+ teeth, 273
+
+ Thoracic ribs, crocodile, 259;
+ see Ribs;
+ -- vertebrae, 16;
+ crocodile, 241, =242=;
+ dog, 381, =382=;
+ duck, 310;
+ turtle, 221
+
+ Thoraco-lumbar vertebrae, mammals, 447 f.
+
+ Thornback skate, 104
+
+ _Thylacinus_, _43_;
+ atlas, 443;
+ dental formula, 423;
+ pelvis, 513;
+ pes, 521;
+ skull, =456=
+
+ _Thylacoleo_, _43_;
+ skull, 457
+
+ Thyro-hyal, dog, 399
+
+ Tibia, 26;
+ crocodile, 268;
+ dog, 412;
+ newt, 149;
+ turtle, 235
+
+ Tibiale 27;
+ see Tarsus
+
+ Tibio-fibula, frog, 166
+
+ Tibio-tarsus, duck, 326
+
+ Tichorhine Rhinoceros, 470
+
+ Tiger, _48_
+
+ _Tiliqua_, _38_;
+ scutes, 200, 271
+
+ Tillodontia _47_, 365;
+ femur, 520;
+ manus, 511;
+ teeth, 435
+
+ Tinamidae 300;
+ caudal vertebrae, 333;
+ vomers, 334
+
+ Tinamiformes, _41_
+
+ _Tinamus_, _41_;
+ ischia, 341
+
+ Titanotheriidae, _46_;
+ skull, 470;
+ teeth, 432
+
+ _Titanotherium_, _46_;
+ humerus, 501;
+ manus, =508=;
+ pes, 525
+
+ Toad, _36_;
+ shoulder girdle, 185
+
+ Tope, _32_
+
+ Torpedinidae, _32_
+
+ _Torpedo_, _32_, 104
+
+ Tortoise, _37_;
+ position of limbs, 28
+
+ _Tortrix_, ischio-pubis, 292;
+ traces of posterior limb, 293
+
+ Toucan, _42_;
+ foot, 342
+
+ _Toxodon_, _46_, 361;
+ femur, 519;
+ pes, 525;
+ teeth, 432
+
+ Toxodontia, _46_;
+ general characters, 361;
+ manus, 509;
+ skull, 472;
+ teeth, 432
+
+ Toxodontidae, _46_
+
+ Trabeculae 11;
+ -- cranii, 17
+
+ Tragulidae, _45_
+
+ Tragulina, _45_, 359;
+ fibula, 519;
+ manus, 507;
+ odontoid process, 445;
+ pes, 523;
+ skull, 468;
+ teeth, 429;
+ ulna, 501
+
+ Transpalatine, crocodile, 255;
+ reptiles, 278
+
+ Trapezium, 27;
+ dog, 408
+
+ Trapezoid, 27;
+ dog, 408
+
+ Trichechidae, _48_, 369
+
+ _Trichechus_, _48_;
+ see Walrus
+
+ Trionychia, _37_;
+ general characters, 194
+
+ Trionychidae, _37_
+
+ _Trionyx_, _37_, 193 f.;
+ exoskeleton, 214, 270;
+ skull, 283;
+ vestiges of teeth, 274
+
+ _Trissolepis_, _32_;
+ scales, 104
+
+ _Tritylodon_, _43_;
+ teeth, 348
+
+ Trochanter, dog 412;
+ duck, 326
+
+ Trochilidae, _42_;
+ see Humming-birds
+
+ Trochlea, crocodile, 263;
+ dog, 405 f.;
+ duck, 323;
+ turtle, 232
+
+ Trogon, _42_;
+ foot, 342
+
+ Trogonidae, _42_
+
+ _Tropidonotus_, _38_;
+ jaws, 280;
+ skull, =279=
+
+ Trunk vertebrae, cod, 84;
+ see thoracic and lumbar vertebrae
+
+ _Trygon_, _32_;
+ calcification of vertebrae, 114;
+ caudal spine, 106
+
+ Trygonidae, _32_
+
+ Tuberosities of humerus, dog, 405;
+ of ischium, dog, 411
+
+ Tunicata, _30_, 51
+
+ _Tupaia_, skull, 480;
+ thoraco-lumbar vertebrae, 450
+
+ Turbinals, dog, 395
+
+ _Tursiops_, _45_;
+ skull, 462
+
+ Turtle, _37_;
+ anterior limb, 232, =234=;
+ cranium, 222 f., =226=;
+ hyoid, 231, =285=;
+ mandible, 230;
+ pectoral girdle, 231;
+ pelvic girdle, 235;
+ pes, 236;
+ plastron, 217, =218=;
+ posterior limb, =234=, 235;
+ sense capsules, 227;
+ skull, 222;
+ vertebral column, 219;
+ Leathery --, see _Dermochelys_;
+ Snapping --, see _Trionyx_
+
+ Tylopoda, _45_, 359;
+ fibula, 519;
+ manus, 507;
+ odontoid process, 445;
+ pelvis, 514;
+ pes, 523;
+ skull, 468;
+ teeth, 428;
+ ulna, 502
+
+ Tympanic, dog, 392;
+ -- cavity, crocodile, 250;
+ diagram of mammalian, =391=;
+ dog, 393;
+ duck, 315 f.;
+ turtle, 228;
+ -- recess, duck, 315
+
+ Tympano-hyal, dog, 399
+
+ Typhlopidae, _38_;
+ scales, 270;
+ skull, 278
+
+ _Typhlops_, _38_;
+ ischio-pubis, 292;
+ traces of posterior limb, 293
+
+ Typotheriidae, _46_
+
+ _Typotherium_, _46_, 358, 361;
+ clavicle, 495, 497;
+ femur, 519;
+ pes, 525;
+ skull, 472;
+ teeth, 432
+
+
+ _Udenodon_, _36_, _192_;
+ beak, 271
+
+ Uintatheriidae, _47_;
+ skull, 364
+
+ _Uintatherium_, _47_;
+ dental formula, 433;
+ leg, 519;
+ limbs and limb girdles, =516=;
+ manus, 510;
+ pelvis, 514;
+ skull, 473
+
+ Ulna, 26;
+ crocodile, 265;
+ dog, 406;
+ duck, 323;
+ frog, 164;
+ newt, 147;
+ turtle, 233
+
+ Ulnare, 27;
+ see Carpus
+
+ Umbilicus, inferior and superior, duck, 303
+
+ Uncinate process, 190;
+ crocodile, 259;
+ duck, 320
+
+ Unciform, 27, 345, 504;
+ dog, 408
+
+ Ungulata, _45_;
+ auditory ossicles, 487;
+ caudal vertebrae, 453;
+ cervical vertebrae, 445;
+ general characters, 357;
+ manus, 506;
+ pectoral girdle, 495;
+ pes, 522;
+ ribs, 491;
+ sacrum, 452;
+ skull, 464 f.;
+ sternum, 489;
+ teeth, 427 f.;
+ thoraco-lumbar vertebrae, 448
+
+ Ungulata vera, _45_;
+ arm bones, 501;
+ general characters, 358;
+ manus, 506;
+ pelvis, 514;
+ thigh and shin, 519
+
+ Upper arm, 26;
+ crocodile, 263;
+ dog, 405;
+ duck, 323;
+ frog, 164;
+ newt, 147;
+ turtle, 232
+
+ _Upupa_, _42_;
+ see Hoopoe
+
+ Urochordata, _30_, 51
+
+ Urodela, _35_;
+ general characters, 134;
+ pelvis, 187;
+ ribs, 182;
+ skull, 174
+
+ Urohyal, cod, 101;
+ duck, 320
+
+ Urostyle, Anura 172;
+ cod, 85;
+ frog, 153;
+ Teleostei, 117
+
+ Ursidae, _48_;
+ humerus, 502
+
+ _Ursus_, _48_;
+ dental formula, 439;
+ see Bears
+
+
+ Vacuities, anterior palatine --, crocodile, 252, 258;
+ -- in reptilian skull, 281;
+ posterior palatine --, crocodile, 254, 257;
+ pre-orbital --, reptiles, 283 f.
+
+ Vampire, _49_;
+ teeth, 441
+
+ Vane, of feather, 303
+
+ Varanidae, _38_
+
+ _Varanus_, _38_;
+ shoulder girdle, =202=;
+ skull, =201=
+
+ Vasodentine, 108, 272
+
+ Vertebral column, 14;
+ Amphibia, 170;
+ birds, 332;
+ cod, 83;
+ crocodile, 239;
+ dog, 378;
+ duck, 307;
+ Elasmobranchs, 113;
+ frog, 152;
+ mammals, 442;
+ newt, 138;
+ turtle, 219;
+ -- ribs, crocodile, 259;
+ dog, 402;
+ duck, 320;
+ -- shield, turtle, 214
+
+ Vertebrata, general characters, 53
+
+ Vexillum, of feather, 303
+
+ Vibrissae, dog, 374
+
+ Viscacha, _47_
+
+ Visceral skeleton, 21;
+ dogfish, 77;
+ Elasmobranchs, 119 f.
+
+ _Viverra_, _48_;
+ acetabular bone, 515
+
+ Viverridae, _48_
+
+ Vomer, 21;
+ cod, 98;
+ crocodile, 252;
+ dog, 395;
+ duck, 317;
+ frog, 158;
+ salmon, 93;
+ turtle, 229
+
+ Vomero-palatine, newt, 143
+
+ _Vultur_, _41_
+
+ Vulture, _41_;
+ Black --, shoulder girdle and sternum, =337=
+
+
+ Waders, 335
+
+ Walrus, _48_, 367, 369;
+ canines, 420;
+ manus, 511;
+ pes, 526;
+ position of limbs, 29;
+ skull, 480;
+ teeth, 440
+
+ Warblers, _42_
+
+ Wart hog, _45_;
+ teeth, 428
+
+ Weasel, _369_
+
+ Whale, baleen, 3, 418;
+ Ca'ing --, _45_,
+ see _Globicephalus_;
+ Humpbacked --, _44_, 357;
+ Right --, _44_, 357;
+ Sperm --, _44_, 357,
+ see _Physeter_;
+ True or Whalebone --, 356
+
+ Whiting, _33_
+
+ Wild duck, _41_;
+ see Duck
+
+ Wing, duck, 322;
+ Gannet, Ostrich, and Penguin, =339=
+
+ Wolf, _48_
+
+ Wombat, _43_;
+ atlas, 443;
+ pes, 521;
+ sacrum, 451;
+ skull, =456=;
+ tail, 453;
+ teeth, 423
+
+ Woodpecker, _42_, 335;
+ foot, 342;
+ hyoid, 336
+
+ Wrasse, _33_;
+ teeth, 111
+
+
+ _Xenacanthus_, _31_;
+ pectoral fins, 130
+
+ Xenopidae, _36_
+
+ _Xenopus_, _36_;
+ branchial arches, 182;
+ nails, 168;
+ pelvis, 188;
+ ribs, 182
+
+ Xiphiplastron, turtle, 217
+
+ Xiphisternal horn, crocodile, 260
+
+ Xiphisternum, dog, 404;
+ frog, 163
+
+ Xiphoid process, duck, 321
+
+
+ _Zeuglodon_, _44_, 353, 356;
+ dental formula, 426;
+ dermal plates, 420
+
+ Zeuglodontidae, _44_
+
+ Zygantra, defined, 199 n.;
+ reptiles, 276
+
+ Zygapophyses, cod, 84;
+ crocodile, 240 f.;
+ dog, 379 f.;
+ duck, 308 f.;
+ frog, 152 f.;
+ newt, 139;
+ turtle, 219 f.
+
+ Zygosphene, defined, 199 n.;
+ reptiles, 276
+
+
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+ literature.
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+E. HAMILTON ACTON, M.A. Crown 8vo. With 45 Illustrations. _Second
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+ His portraits live, and live in a live world; they are not mere
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+
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+ trace the gradual growth of man's knowledge of the physical
+ basis of his life.
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+
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+ Cambridge University Press on having added to their admirable
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+8vo. With numerous illustrations. 10_s._ 6_d._ _net_.
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+=The Vertebrate Skeleton.= By SIDNEY H. REYNOLDS, M.A. Crown 8vo. With
+110 Illustrations. 12_s._ 6_d._
+
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+Post-mortem Room. By H.D. ROLLESTON, M.A., M.D., F.R.C.P., and A.A.
+KANTHACK, M.D., M.R.C.P. Crown 8vo. 6_s._
+
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+
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+ confiance. Au total, son oeuvre est appuyée sur des bases
+ solides, et elle restera sans doute longtemps le bréviaire, le
+ manuel de ceux qui veulent, non pas seulement s'initier à la
+ paléobotanique, mais retrouver les renseignements qui sont
+ épars dans des centaines de monographies qu'on a souvent peine
+ à se procurer. Le livre de M. Seward fait partie des _Cambridge
+ Natural Science Manuals_, et il est digne de cette collection,
+ qui est elle-même digne du foyer scientifique universellement
+ réputé, où il a vu le jour.
+
+ _Guardian._ We have already alluded to the spirit of caution
+ which characterises the book, and we may add that it promises
+ to be most helpful to the botanist who would extend his
+ researches into the past.
+
+ _Natural Science._ A most successful treatment of a difficult
+ subject. All of importance is brought forward and impartially
+ discussed.... Such a book has long been a desideratum. Mr
+ Seward's style is clear and concise, and the many pitfalls into
+ which beginners are apt to stumble are clearly pointed out.
+
+
+=Zoology. An Elementary Text-Book=. By A.E. SHIPLEY, M.A., F.R.S., and
+E.W. MACBRIDE, M.A. (Cantab.), D.Sc. (London), Professor of Zoology in
+McGill University, Montreal. Demy 8vo. With numerous Illustrations.
+10_s._ 6_d._ _Net_.
+
+ _Pilot._ A very business-like and convenient manual of modern
+ Zoology.
+
+ _School World._ As a thoroughly trustworthy and instructive
+ text-book for serious students, the work can be strongly
+ recommended. Its value is enhanced by the large number of
+ excellent illustrations, many of which are delightfully fresh.
+
+ _Oxford Magazine._ It is readable, well arranged, well printed,
+ copiously and admirably illustrated, and it covers the whole
+ field of zoology.
+
+ _Nature._ There pervades the pages of the work a freshness of
+ style and unconventionality which render them pleasant reading
+ and attractive; while, in the frequent allusion to the
+ commonest occurrences of daily life and human affairs, the
+ interest of the reader is assured.
+
+ _Pall Mall Gazette._ Precisely the sort of book which, if it
+ came into a thoughtful boy's hands, would turn him from a
+ smatterer into a student.... One of the most instructive and
+ attractive books that could be put into the hands of a young
+ naturalist.
+
+=Grasses=: a Handbook for use in the Field and Laboratory. By H.
+MARSHALL WARD, Sc.D., F.R.S., Fellow of Sidney Sussex College,
+Professor of Botany in the University of Cambridge. With 81 figures.
+Crown 8vo. 6_s._
+
+ _Pilot._ Brimful of matter.
+
+ _Field._ The work is essentially suited to the requirements of
+ those desirous of studying the grasses commonly grown in this
+ country, and it can fairly be said that it furnishes an amount
+ of information seldom obtained in more pretentious volumes.
+
+ _Athenaeum._ Botanists and Agriculturists alike have reason to
+ thank Prof. Ward for this very serviceable addition to the
+ literature of grasses.
+
+=Trees=: A Handbook of Forest Botany for the Woodlands and the
+Laboratory. By H. MARSHALL WARD, Sc.D., F.R.S., Fellow of Sidney
+Sussex College, Honorary Fellow of Christ's College and Professor of
+Botany in the University of Cambridge. In six volumes. 1. Buds and
+Twigs, 2. Leaves, 3. Inflorescences and Flowers, 4. Fruits and Seeds,
+5. Seedlings, 6. General Characters. Vol. I. Buds and Twigs. Crown
+8vo. Illustrated. 4_s._ 6_d._ _net_.
+
+=A Treatise on the British Freshwater Algae.= By G.S. WEST, M.A.,
+A.R.C.S., F.L.S., Professor of Natural History at the Royal
+Agricultural College, Cirencester. Demy 8vo. 10_s._ 6_d._ _net_.
+
+=A Manual and Dictionary of the Flowering Plants and Ferns=. By J.C.
+WILLIS, M.A., Director of the Royal Botanic Gardens, Ceylon. _Second
+Edition._ Complete in one volume. Crown 8vo. 10_s._ 6_d._
+
+=Elementary Palaeontology--Invertebrate.= By HENRY WOODS, M.A., F.G.S.,
+University Lecturer in Palaeozoology. Crown 8vo. _Third Edition._
+Revised and enlarged, with 112 Illustrations. 6_s._
+
+=Outlines of Vertebrate Palaeontology for students of Zoology.= By
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+Geology in the British Museum. Demy 8vo. With numerous Illustrations.
+14_s._
+
+ _Athenaeum._ The author is to be congratulated on having produced
+ a work of exceptional value, dealing with a difficult subject in a
+ thoroughly sound manner.
+
+
+_In preparation._
+
+=Morphology and Anthropology.= By W.L.H. DUCKWORTH, M.A., Fellow and
+Lecturer of Jesus College, University Lecturer in Physical Anthropology.
+
+=The Origin and Influence of the Thorough-bred Horse.= By W. RIDGEWAY,
+M.A., Disney Professor of Archaeology and Fellow of Gonville and Caius
+College. With numerous Illustrations. Demy 8vo.
+
+=The Morphology of Plants.= By J.C. WILLIS, M.A.
+
+ =London=: C.J. CLAY AND SONS,
+ CAMBRIDGE UNIVERSITY PRESS WAREHOUSE,
+ AVE MARIA LANE,
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+ H.K. LEWIS, 136, GOWER STREET, W.C.
+
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+ * * * * *
+
+
+ Transcriber Notes
+
+ Italic text is denoted by _underscores_ and bold text by =equal
+ signs=. Subscripts are represented using braces, e.g. V{1}, and
+ superscripts are introduced with a caret, e.g. 2^e
+
+ Obvious punctuation and spelling errors, and inconsistent hyphenation
+ have been corrected.
+
+ The oe ligature in the text has been replaced with the characters oe.
+
+
+
+
+
+End of Project Gutenberg's The Vertebrate Skeleton, by Sidney H. Reynolds
+
+*** END OF THE PROJECT GUTENBERG EBOOK 43431 ***
generated by cgit v1.2.3 (git 2.25.1) at 2026-01-07 01:57:17 +0000