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diff --git a/43431-8.txt b/43431-8.txt deleted file mode 100644 index 047f089..0000000 --- a/43431-8.txt +++ /dev/null @@ -1,26206 +0,0 @@ -The Project Gutenberg EBook of The Vertebrate Skeleton, by Sidney H. Reynolds - -This eBook is for the use of anyone anywhere at no cost and with -almost no restrictions whatsoever. You may copy it, give it away or -re-use it under the terms of the Project Gutenberg License included -with this eBook or online at www.gutenberg.org - - -Title: The Vertebrate Skeleton - -Author: Sidney H. Reynolds - -Release Date: August 9, 2013 [EBook #43431] - -Language: English - -Character set encoding: ISO-8859-1 - -*** START OF THIS PROJECT GUTENBERG EBOOK THE VERTEBRATE SKELETON *** - - - - -Produced by Chris Curnow, Mark Young and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - - - - - - - - - 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.; 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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 -ARTHUR SMITH WOODWARD, M.A., F.R.S., Keeper of the Department of -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, - AND - H.K. LEWIS, 136, GOWER STREET, W.C. - - =Glasgow=: 50, WELLINGTON STREET - - * * * * * - - - 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. 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