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+*** START OF THE PROJECT GUTENBERG EBOOK 41695 ***
+
+Transcriber's Notes
+
+ Text emphasis is denoted as _Text_ for italic and =Text= for bold.
+ Whole and fractional parts are shown as 4-2/3.
+ OE and oe ligature converted to Oe and oe respectively.
+
+
+ * * * * *
+
+
+
+
+MEMOIRS OF
+
+THE CONNECTICUT ACADEMY
+
+OF ARTS AND SCIENCES
+
+VOLUME VII DECEMBER, 1920
+
+
+
+The Appendages, Anatomy, and Relationships of Trilobites
+
+
+
+BY
+
+
+
+PERCY E. RAYMOND, Ph.D.
+
+ASSOCIATE PROFESSOR OF PALAEONTOLOGY, AND CURATOR OF INVERTEBRATE
+
+PALAEONTOLOGY IN THE MUSEUM OF COMPARATIVE ZOOLOGY,
+
+HARVARD UNIVERSITY
+
+
+[Illustration: (logo)]
+
+
+NEW HAVEN, CONNECTICUT
+
+PUBLISHED BY THE
+
+CONNECTICUT ACADEMY OF ARTS AND SCIENCES
+
+AND TO BE OBTAINED ALSO FROM THE
+
+YALE UNIVERSITY PRESS
+
+
+
+
+
+
+[Illustration (photo)]
+
+[Illustration (signature)]
+
+
+
+
+
+MEMOIRS OF
+
+THE CONNECTICUT ACADEMY
+OF ARTS AND SCIENCES
+
+VOLUME VII DECEMBER, 1920
+
+
+
+The Appendages, Anatomy, and Relationships
+of Trilobites
+
+
+
+BY
+
+
+
+PERCY E. RAYMOND, Ph.D.
+
+ASSOCIATE PROFESSOR OF PALAEONTOLOGY, AND CURATOR OF INVERTEBRATE
+
+PALAEONTOLOGY IN THE MUSEUM OF COMPARATIVE ZOOLOGY,
+
+HARVARD UNIVERSITY
+
+[Illustration: (logo)]
+
+
+NEW HAVEN, CONNECTICUT
+
+PUBLISHED BY THE
+
+CONNECTICUT ACADEMY OF ARTS AND SCIENCES
+
+AND TO BE OBTAINED ALSO FROM THE
+
+YALE UNIVERSITY PRESS
+
+
+
+
+
+THE TUTTLE, MOREHOUSE & TAYLOR COMPANY
+
+
+
+
+TO THE MEMORY OF
+
+
+CHARLES EMERSON BEECHER
+
+
+SKILLFUL WITH HAND, BRAIN, AND PEN; REVEALER OF THE MYSTERIES
+
+OF TRILOBITES;
+
+THIS MEMOIR IS DEDICATED
+
+
+
+
+
+
+FOREWORD.
+
+
+By CHARLES SCHUCHERT.
+
+
+Trilobites are among the most interesting of invertebrate fossils and
+have long attracted the attention of amateur collectors and men of
+science. These "three-lobed minerals" have been mentioned or described
+in books at least since 1698 and now several thousand species are
+known to palæontologists. To this group of students they are the most
+characteristic animals of the seas of Palæozoic time, and even though
+they are usually preserved as dismembered parts, thousands upon
+thousands of "whole ones" are stored in the museums of the world. By
+"whole ones" perfect individuals are not meant, for before they became
+fossils the wear and tear of their time and the process of
+decomposition had taken away all the softer parts and even most of the
+harder exterior covering. What is usually preserved and revealed to us
+when the trilobites weather out of the embrace of their entombing
+rocks is the test, the hard shell of the upper or dorsal side. From
+time to time fragments of the under or limb-bearing side had been
+discovered, first by Elkanah Billings, but before 1876 there was no
+known place to which one could go to dig out of the ground trilobites
+retaining the parts of the ventral side.
+
+Students of trilobites have always wanted specimens to be delivered to
+them weathered out of the rock by nature and revealing the ventral
+anatomy without further work than the collecting, but the wish has
+never been fulfilled. In the Utica black shales, near Rome, New York,
+there was finally discovered in 1892 a layer less than ten millimeters
+thick, bearing hundreds of _Triarthrus becki_ with most of the ventral
+anatomy intact. The collector's first inkling that such were present
+in the Utica formation came to him in a chance find in 1884, and for
+eight years he sought off and on for the stratum whence this specimen
+came. His long search was finally rewarded by the discovery of the
+bed, and lo! here were to be had, in golden color, prostrate specimens
+with the breathing and crawling legs and the long and beautifully
+curved feeling organs all replaced by iron pyrites. Fool's gold in
+this case helped to make a palæontologic paradise. The bed contained
+not only such specimens of _Triarthrus becki_, but also, though more
+rarely, of _Cryptolithus tessellatus_ and exceptionally of _Acidaspis
+trentonensis_. This important discovery, which has figured so largely
+in unraveling the evolution of the Crustacea and even has a bearing on
+that of most of the Arthropoda, was made by Mr. W. S. Valiant, then
+curator of the Museum of Rutgers College.
+
+There were, however, great material difficulties to overcome before
+the specimens revealed themselves with all of their information
+exposed for study. No surgeon was needed, but a worker knowing the
+great scientific value of what was hidden, and with endless patience
+and marked skill in preparation of fossils. Much could be revealed
+with the hammer, because specimens were fairly abundant. A chance
+fracture at times showed considerable portions, often both antennæ
+entire, and more rarely the limbs protruding beyond the test, but the
+entire detail of any one limb or the variation between the limbs of
+the head, thorax, and tail was the problem to be solved. No man ever
+loved a knotty problem more than Charles E. Beecher. Any new puzzle
+tempted him, and this one of _Triarthrus becki_ interested him most of
+all and kept him busy for years. From the summer of 1893. when he
+quarried out two tons of the pay stratum at Rome, until his death in
+1904, his time was devoted in the main to its solution by preparing
+these trilobites and learning their anatomical significance.
+
+The specimens of _Triarthrus becki_ from Rome are pseudomorphs
+composed of iron pyrites, as has been said, and are buried in a
+gray-black carbonaceous shale. A little rubbing of the specimens soon
+makes of them bronze images of the former trilobite and while under
+preparation they are therefore easily seen. However, as the average
+individual is under an inch in length and as all the limbs other than
+the antennæ are double or biramous, one lying over the other, and the
+outer one fringed with a filamentous beard, the parts to be revealed
+by the preparator are so small and delicate that the final touch often
+obliterates them. These inherent difficulties in the material were
+finally overcome by endless trials on several thousand specimens, each
+one of which revealed something of the ventral anatomy. Finally some
+500 specimens worthy of detailed preparation were left, and on about
+50 of these Beecher's descriptions of _Triarthrus_ and _Cryptolithus_
+were based.
+
+The black shale in which the specimens are buried is softer than the
+pseudomorphous trilobites, a condition that is of the greatest value
+in preparation. With chisel and mallet the trilobites are sought in
+the slabs of shale and then with sharp chisels of the dental type they
+are revealed in the rough. At first Beecher sought to clean them
+further by chemical methods, and together with his friends, the
+chemist Horace L. Wells, and the petrologist Louis V. Pirsson, several
+solutions were tried, but in all cases the fossils were so much
+decomposed as to make them useless in study. Therefore Beecher had to
+depend wholly oh abrasives applied to the specimens with pieces of
+rubber. Much of this delicate work was done on a dental lathe, but in
+the final cleaning most of it was done with patient work by hand.
+Rubber has the great advantage of being tough and yet much softer than
+either specimen or shale. As the shale is softer than the iron
+pyrites, the abrasives (carborundum, emery, or pumice) took away the
+matrix more quickly than the trilobite itself. When a part was fully
+developed, the rubbers were cut to smaller and smaller dimensions and
+the abrading reduced to minute areas. So the work went on and on,
+helped along from time to time by the dental chisels. Finally Beecher
+became so expert with these fossils that after one side was developed
+he would embed the specimen in Canada balsam and fix it on a glass
+slide, thus enabling him to cut down from the opposite side. This was
+done especially with _Cryptolithus_ because of the great scarcity of
+material preserving the limbs, and two of these revealed both sides of
+the individuals, though they were then hardly thicker than writing
+paper.
+
+Then came illustrations, which at first were camera-lucida drawings in
+pencil smoothed out with pen and ink. "In some quarters," however, it
+has been said, "his methods unknown, their results were not accepted;
+they were regarded as startling, as iconoclastic, and even
+unreliable." He therefore decided to rework his material and to
+illustrate his publications with enlarged photographs. The specimens
+were black, there was little relief between fossil and matrix, and the
+ammonium chloride process of coating them white and photographing
+under artificial light was unsuitable. Nevertheless, after many
+trials, he finally succeeded in making fine enlarged photographs of
+the trilobites immersed in liquid Canada balsam, with a contact cover
+of glass through which the picture was taken, the camera standing
+vertically over the horizontal specimen. Beecher had completed this
+work in 1903 and in the winter of 1903-1904 was making the drawings,
+nearly all of which are here reproduced. On Sunday morning, February
+14, 1904, as he was working at home on a large wash drawing of
+_Cryptolithus_, death came to him suddenly, leaving the trilobite
+problem but partially solved.
+
+When the writer, in the autumn of 1904, succeeded Professor Beecher in
+the chair of Palæontology at Yale, he expected to find considerable
+manuscript relating to the ventral anatomy of the trilobites, but
+there was only one page. It was Beecher's method first to prepare and
+thoroughly study the material in hand, then to make the necessary
+illustrations, and between times to read what others had written.
+There was no written output until everything had been investigated and
+read, certain passages being marked for later reference. Then when all
+was assimilated, he would write the headings of topics as they came to
+him, later cutting them apart and arranging them in a logical
+sequence. When the writer visited him in his home in January 1904, he
+was primed for his final trilobite memoir, but the writing of it had
+not been begun.
+
+The writer has never made the trilobites his special subjects for
+study as he has the brachiopods, and therefore felt that he should not
+try to bring to light merely the material things that Beecher had so
+well wrought out. It seemed at first an impossible task to find the
+specialist and friend to do Beecher justice, but as the years have
+passed, one of Beecher's students, always especially interested in
+trilobites, has grown into a full appreciation of their structures and
+significance, and to him has fallen the continuation of his master's
+work. If in the following pages he departs here and there from the
+accepted interpretation and the results of others, it is because his
+scientific training, in desiring to see with his own eyes the
+structures as they are, has led him to accept only those
+interpretations that are based on tangible evidence as he understands
+such. Furthermore, in seeking the relationship of the trilobites to
+the rest of the Arthropoda, his wide study of material and literature,
+checked up by the ontogeny of fossil and recent forms, has led him in
+places from the beaten path of supposedly ascertained phylogenies. His
+results, however, have been won through a detailed study of the
+interrelations of the Arthropoda, starting from the fact that the
+Trilobita are chronogenetically the oldest and most primitive. The
+trilobites are held by him to be the most simple, generalized, ancient
+Crustacea known, and the progenitors, directly and indirectly, of all
+Arthropoda.
+
+It is now twenty-six years since Professor Beecher began his
+publications on the class Trilobita, and in commemoration of him and
+his work, Professor Percy E. Raymond of Harvard University presents
+this memoir, to bring to fruition the studies and teachings of his
+honored guide. It has been with Professor Raymond a labor of love, and
+it is for the writer of this foreword a long-desired memorial to the
+man to whose position in the Museum and University he had the
+privilege of succeeding.
+
+ Yale University, New Haven, Connecticut.
+
+
+
+
+
+
+
+PREFACE.
+
+
+The primary object of this memoir is, as has been stated by Professor
+Schuchert, to rescue from oblivion the results of the last few years
+of Professor Beecher's investigations on the ventral anatomy of
+trilobites. Since he left his data in the form of drawings and
+photographs, without even rough notes, it became necessary, in order
+to write a text to accompany the plates, to restudy the entire
+subject. Under these circumstances, it seemed best to include all that
+is known about the appendages of trilobites, thus bringing together a
+summary of present information on the subject.
+
+The growth of the memoir to its present size has been a gradual one.
+As first completed in 1917, it contained an account of the appendages
+only. Thoughts upon the probable use of the appendages led to the
+discussion of possible habits, and that in turn to a consideration of
+all that is known or could be inferred of the structure and anatomy of
+the trilobite. Then followed an inquiry into the relationships to
+other Arthropoda, which ultimately upset firmly established
+preconceptions of the isolated position of the group, and led to a
+modification of Bernard's view of its ancestry.
+
+During the progress of the work, I have had the opportunity of
+examining most of the known specimens retaining appendages. From the
+Marsh collection in the Yale University Museum were selected the
+forty-six specimens showing best the appendages of _Triarthrus_,
+_Cryptolithus_, and _Acidaspis_. Dr. Charles D. Walcott very kindly
+returned to the Museum of Comparative Zoology the slices of
+_Ceraurus_, _Calymene_, and _Isotelus_ which were the basis of his
+paper of 1881, and which had been loaned him for further study. He
+loaned also eight of the more important specimens of _Neolenus
+serratus_, and two of _Triarthrus becki_. At the United States
+National Museum I saw the specimens of _Isotelus_ described by
+Mickleborough and the isolated limbs of _Calymene_ from near
+Cincinnati. The _Isotelus_ at Ottawa I had already studied with some
+care while an officer of the Geological Survey of Canada.
+
+This memoir consists, as shown in the table of contents, of four
+parts. The appendages of _Neolenus_, _Isotelus_, _Ptychoparia_,
+_Kootenia_, _Ceraurus_, _Calymene_, and _Acidaspis_ are discussed, as
+fully as circumstances warrant, in the first part, and new
+restorations of the ventral surfaces of _Neolenus_, _Isotelus_,
+_Triarthrus_, _Ceraurus_ and _Cryptolithus_ are included It is not
+supposed that these restorations will be of permanent value in all of
+their detail, but they are put forward as the best approximations to
+the real structure that the writer is able to present from the
+materials so far discovered. I am greatly indebted to Doctor Elvira
+Wood for the care and skill with which she has worked up these
+restorations from my rather sketchy suggestions. She has put into them
+not only a great amount of patient work, but also the results of
+considerable study of the specimens.
+
+Part II is a discussion of the internal anatomy of the trilobite and a
+brief statement of some of the possible habits and methods of life of
+these animals. Part III, which begins with a survey of the
+relationships of the trilobites to other Arthropoda, is largely taken
+up with an attempt to demonstrate the primitive characteristics of the
+former, and their probable ancestral position. The form of the
+ancestor of the trilobite is deduced from a study of the morphology,
+ontogeny, and phylogeny of the group, and evidence adduced to indicate
+that it was a depressed, flattened, free-swimming animal of few
+segments.
+
+In Part IV are included somewhat detailed descriptions of a few of the
+best specimens of _Triarthrus_ and _Cryptolithus_. Professor Beecher,
+while an observer of the minutest details, believed in publishing only
+the broader, more general results of his investigations. This method
+made his papers brief, readable, and striking, but it also resulted in
+leaving in some minds a certain amount of doubt about the correctness
+of the observations. In a matter so important as this, it has seemed
+that palæontologists are entitled to the fullest possible knowledge of
+the specimens on which the conclusions are based. The last part is,
+therefore, a record of the data for the restorations of _Triarthrus_
+and _Cryptolithus_.
+
+The illustrations in the plates were nearly all made by or under the
+supervision of Professor Beecher, as were also text figures 45 and 46.
+
+In conclusion, I wish to express my thanks to Mrs. Charles E. Beecher
+for the use of drawings which were the personal property of Professor
+Beecher; to Doctor Charles D. Walcott for photographs of the limbs of
+_Calymene_, and for his kindness in sending me the slices of
+trilobites from Trenton Falls and specimens of _Neolenus_ and
+_Triarthrus_; to Doctor R. V. Chamberlin for suggestions and
+criticisms in regard to the relationship of trilobites to Insecta,
+Arachnida, Chilopoda, and Diplopoda; to Mr. Samuel Henshaw, Director
+of the Museum of Comparative Zoology, for permission to use the time
+which has been devoted to this work; and to Miss Clara M. Le Vene, for
+assistance in the preparation of the manuscript. My greatest debt is
+to Professor Charles Schuchert, to whom the work owed its inception,
+who has assisted in many ways during its prosecution, and who read the
+manuscript, and arranged for its publication. To him I can only
+express my warmest thanks for the favors which I have received and for
+the efforts which he has put forth to make this a worthy memorial to
+our friend and my teacher, Professor Charles Emerson Beecher.
+
+ Harvard University, Cambridge, Mass.
+ November, 1919.
+
+
+
+
+
+
+TABLE OF CONTENTS.
+
+
+ Historical review 17
+
+ Part I. The appendages of trilobites 20
+
+ Terminology 20
+
+ The appendages of _Neolenus_ 21
+ Historical 21
+ _Neolenus serratus_ (Rominger) 21
+ Cephalon 21
+ Thorax 22
+ Pygidium 23
+ Epipodites and exites 23
+ Description of individual specimens 23
+ Restoration of _Neolenus_ 30
+ _Nathorstia transitans_ Walcott 31
+
+ The appendages of _Isotelus_ 32
+ Historical 32
+ _Isotelus latus_ Raymond 34
+ _Isotelus maximus_ Locke 35
+ Restoration of _Isotelus_ 37
+ _Isotelus gigas_ Dekay 37
+ _Isotelus arenicola_ Raymond 39
+
+ The appendages of _Triarthrus_ (see also Part IV) 39
+ _Triarthrus becki_ Green 39
+ Historical 40
+ Restoration of _Triarthrus_ 42
+ Relation of cephalic appendages to marking on
+ dorsal surface of glabella 43
+ Anal plate 44
+
+ The appendages of _Ptychoparia_ 45
+ _Ptychoparia striata_ (Emmrich) 45
+ _Ptychoparia cordilleræ_ (Rominger) 45
+ _Ptychoparia permulta_ Walcott 45
+
+ The appendages of _Kootenia_ 46
+ _Kootenia dawsoni_ Walcott 46
+
+ The appendages of _Calymene_ and _Ceraurus_ 46
+ Historical 46
+ Comparison of the appendages of _Calymene_ and
+ _Ceraurus_ with those of _Triarthrus_ 47
+ Spiral branchiæ 48
+ Ventral membrane 50
+ Appendifers 51
+
+ _Calymene senaria_ Conrad 52
+ Cephalic appendages 52
+ Thoracic appendages 53
+ Pygidial appendages 54
+ Relation of hypostoma to cephalon in _Calymene_ 55
+ Restoration of _Calymene_ 56
+
+ _Calymene_ sp. ind. 56
+
+ _Ceraurus pleurexanthemus_ Green 57
+ Cephalic appendages 58
+ Thoracic appendages 59
+ Pygidial appendages 59
+ Relation of hypostoma to cephalon 59
+ Restoration of _Ceraurus pleurexanthemus_ 60
+ The appendages of _Acidaspis trentonensis_ Walcott 61
+
+ The appendages of _Cryptolithus_ (see also Part IV) 61
+ _Cryptolithus tessellatus_ Green 61
+ Restoration of _Cryptolithus_ 62
+
+ Summary on the ventral anatomy of trilobites 64
+ Comparison of appendages of different genera 64
+ Coxopodite 64
+ Cephalon 64
+ Thorax 66
+ Pygidium 67
+ Caudal rami 68
+ Homology of cephalic appendages with those of
+ other Crustacea 69
+ Functions of the appendages 70
+ Antennules 70
+ Exopodites 70
+ Endopodites 71
+ Use of the pygidium in swimming 72
+ Coxopodites 74
+ Position of the appendages in life 74
+
+
+ Part II. Structure and habits of trilobites 77
+
+ Internal organs and muscles 77
+ Alimentary canal 77
+ _Ceraurus pleurexanthemus_ 79
+ _Calymene senaria_ 80
+ _Cryptolithus goldfussi_ 80
+ Summary 81
+ Gastric glands 82
+ Summary 84
+ Heart 85
+ _Illænus_ 85
+ _Ceraurus_ and _Calymene_ 85
+ The median "ocellus" or "dorsal organ" 86
+ Nervous system 89
+ Various glands 89
+ Dermal glands 89
+ Renal excretory organs 90
+ Reproductive organs 90
+ Panderian organs 90
+ Musculature 91
+ Flexor muscles 92
+ Extensor muscles 92
+ Hypostomial muscles 94
+
+ Eyes 96
+ Summary 97
+
+ Sex 98
+
+ Eggs 98
+
+ Methods of life (See also under "Functions of
+ the Appendages") 98
+ Habits of locomotion 99
+ Food and feeding methods 103
+ Tracks and trails 104
+
+
+ Part III. Relationship of the trilobites to other
+ Arthropoda 106
+
+ Crustacea 106
+ Branchiopoda 106
+ _Burgessia bella_ Walcott 108
+ _Waptia fieldensis_ Walcott 108
+ _Yohoia tenuis_ Walcott 109
+ _Opabina regalis_ Walcott 109
+ Summary 109
+ Copepoda 110
+ Archicopepoda 111
+ Ostracoda 112
+ Cirripedia 113
+ Malacostraca 113
+ Phyllocarida 113
+ Syncarida 114
+ Isopoda 114
+ _Marrella splendens_ Walcott 115
+ Restoration of _Marrella_ 116
+
+ Arachnida 117
+ Trilobites not Arachnida 117
+ Merostomata 119
+ _Sidneyia inexpectans_ Walcott 119
+ _Emeraldella brocki_ Walcott 119
+ _Molaria_ and _Habelia_ 120
+ Araneæ 121
+
+ Insecta 122
+
+ Chilopoda 123
+
+ Diplopoda 124
+
+ Primitive characteristics of trilobites 125
+ Trilobites the most primitive arthropods 125
+ Limbs of trilobites primitive 125
+ Summary 128
+ Number of segments in the trunk 128
+ Form of the simplest protaspis 132
+ Origin of the pygidium 134
+ Width of the axial lobe 137
+ Presence or absence of a "brim" 137
+ Segmentation of the glabella 137
+ Summary 138
+
+ The simplest trilobite 138
+ _Naraoia compacta_ Walcott 139
+
+ The ancestor of the trilobites, and the descent
+ of the Arthropoda 140
+ Evolution within the Crustacea 142
+ Summary 144
+ Evolution of the Merostomata 146
+ Evolution of the "Tracheata" 147
+ Summary on lines of descent 147
+
+ Final summary 151
+
+
+ Part IV. Description of the appendages of
+ individual specimens 152
+
+ _Triarthrus becki_ Green 152
+ _Cryptolithus tessellatus_ Green 158
+
+
+ Bibliography 163
+
+
+
+
+LIST OF ILLUSTRATIONS.
+
+
+ 1. _Triarthrus becki_ Green. Diagram of limb to show
+ nomenclature employed 20
+
+ 2. _Neolenus serratus_ (Rominger). Two thoracic appendages 24
+
+ 3. The same. An exopodite 26
+
+ 4. The same. A so-called "epipodite" 26
+
+ 5. The same. The so-called "exites" 29
+
+ 6. The same. A cephalic limb 29
+
+ 7. The same. Restoration of a transverse section 30
+
+ 8. The same. Restoration of the ventral surface 31
+
+ 9. _Isotelus_. Restoration of the ventral surface 38
+
+ 10. _Triarthrus becki_ Green. Restoration of the ventral surface 41
+
+ 11. The same. Median appendage 44
+
+ 12. _Ceraurus pleurexanthemus_ Green. Slice showing an exopodite 49
+
+ 13. _Calymene senaria_ Conrad. Slice showing cephalic coxopodites 53
+
+ 14. The same. Another similar slice 53
+
+ 15. The same. Slice showing method of articulation of
+ the appendages 53
+
+ 16. The same. Restoration of the ventral surface 55
+
+ 17. _Ceraurus pleurexanthemus_ Green. Slice showing the method
+ of articulation of the appendages 58
+
+ 18. The same. Slice showing an exopodite above an endopodite 58
+
+ 19. The same. Restoration of a transverse section 60
+
+ 20. _Cryptolithus tessellatus_ Green. Restoration of the
+ ventral surface 63
+
+ 21. _Ceraurus pleurexanthemus_ Green. Slice showing the
+ abdominal sheath 79
+
+ 22. The same. Slice showing the large alimentary canal 79
+
+ 23. _Calymene senaria_ Green. Slice showing the large
+ alimentary canal 79
+
+ 24. _Ceraurus pleurexanthemus_ Green. Restoration of a
+ longitudinal section 81
+
+ 25. _Cryptolithus tessellatus_ Green. Cheek showing the
+ genal cæca 84
+
+ 26. _Illænus._ Volborth's figure of the heart 85
+
+ 27. Heart of _Apus_ 85
+
+ 28. _Isotelus gigas_ Dekay. The Panderian organs 91
+
+ 29. _Ceraurus pleurexanthemus_ Green. Restoration, showing
+ heart, alimentary canal, and extensor muscles 93
+
+ 30. The same. Longitudinal section of cephalon 95
+
+ 31. _Nileus armadillo_ Dalman. Moberg's figure of the
+ muscle-scars 95
+
+ 32. _Marrella splendens_ Walcott. Restoration of the
+ ventral surface 116
+
+ 33. _Triarthrus becki_ Green. Appendage of the anterior part
+ of the thorax 126
+
+ 34. _Apus._ Appendage from the anterior part of the trunk 127
+
+ 35. _Weymouthia nobilis_ (Ford) 138
+
+ 36. _Naraoia compacta_ Walcott 145
+
+ 37. _Pagetia clytia_ Walcott 145
+
+ 38. _Asaphiscus wheeleri_ Meek 145
+
+ 39. _Pædeumias robsonensis_ Burling 145
+
+ 40. _Robergia_ sp. 145
+
+ 41. Diagram showing possible lines of descent of the Arthropoda 150
+
+ 42. _Triarthrus becki_ Green. Thoracic appendages 155
+
+ 43. The same. Pygidial appendages 157
+
+ 44. The same. Pygidial appendages 158
+
+ 45. _Cryptolithus tessellatus_ Green. Drawing of the best
+ single specimen 159
+
+ 46. The same. Part of the thorax and pygidium, with appendages 162
+
+ _Frontispiece._ Charles Emerson Beecher, 1896.
+
+ Plates 1-5. Photographs of _Triarthrus becki_, made by C. E. Beecher.
+
+ Plate 6. Photographs of _Triarthrus becki_ (figs. 1-3), _Acidaspis
+ trentonensis_ (fig. 6), and _Cryptolithus tessellatus_ (fig. 7),
+ made by C. E. Beecher. Photographs of the endopodites of a probable
+ species of _Calymene_ (figs. 4, 5)
+
+ Plates 7-8. Photographs of _Cryptolithus tessellatus_, made by C. E.
+ Beecher.
+
+ Plate 9. Drawings of _Cryptolithus tessellatus_, made by C. E.
+ Beecher or under his direction.
+
+ Plate 10. Photographs of _Isotelus latus_ and _I. maximus_, made by
+ C. E. Beecher.
+
+ Plate 11. Drawing of a restoration of _Ceraurus pleurexanthemus_,
+ made by Elvira Wood.
+
+
+
+
+
+
+HISTORICAL REVIEW.
+
+
+The beginning of the search for the limbs of trilobites was coeval
+with the beginning of scientific study of the group, knowledge of the
+appendages being essential to the proper systematic allocation of the
+animals.
+
+The early search was so barren of results that negative evidence came
+to be accepted as of positive value, and it was for many years
+generally believed that such organs as may have been present beneath
+the dorsal test were so soft as to be incapable of preservation. This
+view is best expressed by Burmeister (1846, p. 43):
+
+ There is good proof that the feet of trilobites must have been soft
+ membranous organs, for the absence of the slightest remains of
+ these organs in the numerous specimens observed is of itself
+ evidence of the fact, and it can indeed scarcely be supposed that
+ hard horny extremities should be affixed to a soft membranous
+ abdominal surface; since they would not have possessed that firm
+ basis, which all solid organs of locomotion require, in order that
+ they may be properly available.
+
+Very well reasoned, and were it not for the discovery of new material
+in American localities, Burmeister's views would probably never have
+been proved incorrect. One can not escape the suspicion that some of
+the accepted hypotheses of today, founded on similar "proof," may
+yield in time to the weight of bits of positive evidence.
+
+The history of the study of appendages of trilobites may be divided
+into two periods. The first, in which there was a general belief that
+the appendages were soft organs, but during which numerous "finds" of
+limbs were reported, extended from the time of Linné to the year
+(1876) in which Walcott demonstrated the fact that the animals
+possessed jointed ambulatory and breathing organs.
+
+The second, much more fruitful period, began with Walcott's
+publication of 1881, descriptive of the appendages of _Ceraurus_ and
+_Calymene_, and for the purposes of this memoir, closes with his great
+contribution on the anatomy of _Neolenus_ (1918). Beecher's brilliant
+productions came in the middle of the second period.
+
+In the first period, there were at least two authentic discoveries of
+appendages, those of Eichwald (1825) and Billings (1870), but since
+neither of these men convinced his confreres of the value of his
+finds, the work of neither can be considered as having marked an
+especial epoch in the history.
+
+As all the authentic finds will be treated in detail on later pages,
+only a brief résumé of the first period will be given here. This has
+already been done by Burmeister (1843, 1846) and Barrande (1852,
+1872), whose works have been my primary sources of information, but I
+have looked up the original papers, copies of nearly all of which are
+to be seen in the libraries in Cambridge and Boston. Brig.-Gen. A. W.
+Vogdes, U. S. A. (retired), has very kindly placed at my disposal a
+number of references and notes.
+
+Linné (1759) was the first to report the discovery of appendages of
+trilobites. Törnquist (1896) has pressed for a recognition of the
+contribution of the great Swedish naturalist to this problem, but
+Beecher (1896 B) doubted the validity of the find. Linné figured a
+specimen of _Parabolina spinulosa_ (Wahlenberg), with what he
+interpreted as a pair of antennæ attached. He states (translation
+quoted from Törnquist): "Most remarkable in this specimen are the
+antennæ in the front, which I never saw in any other sample, and which
+clearly prove this fossil to belong to the insects." Beecher has shown
+as conclusively as can be shown without access to the original
+specimen that the supposed antennæ were really only portions of the
+thickened anterior border, the appearance being due to imperfect
+preservation. Brünnich as early as 1781 called attention to the
+imperfection of this specimen, and it is also referred to by
+Wahlenberg (1821, p. 39), Brongniart (1822, p. 42), Dalman (1828, p.
+73), and Angelin (1854, p. 46).
+
+Audouin (1821) seems to have been the first naturalist with sufficient
+knowledge of the Arthropoda to be competent to undertake the study of
+the trilobites. He concluded that the absence of ventral appendages
+was probably a necessary consequence of the skeletal conformation, and
+thought if any were discovered, they would prove to be of a branchial
+nature.
+
+Wahlenberg (1821) in the same year expressed his belief that the
+trilobites were nearly allied to _Limulus_ and in particular tried to
+show that the trilobites could have had masticatory appendages
+attached about the mouth as in that modern "insect" (p. 20).
+Wahlenberg was also the first to describe an hypostoma of a trilobite
+(p. 37, pl. 1, fig. 6), but did not understand the nature of his
+specimen, which he described as a distinct species.
+
+Brongniart (1822, p. 40) devoted five pages of his monograph to a
+discussion of the affinities of trilobites, concluding that it was
+very probable that the animals lacked antennæ and feet, unless it
+might be that they had short soft feet which would allow them to creep
+about and fix themselves to other bodies.
+
+Schlotheim (1823) thought that the spines on _Agnostus pisiformis_
+were segmented and compared them with the antennæ of _Acarus_.
+
+Stokes (1823) was the first who, with understanding, published an
+illustration of the ventral side of a trilobite, having figured the
+hypostoma of an _Isotelus_. He was followed in the next year (1824) by
+Dekay, who also figured the hypostoma of an _Isotelus_, and added some
+observations on the structure of trilobites. The researches of
+Barrande, Novak, Broegger, Lindstroem, and others have dealt so fully
+with the hypostoma that further references to that organ need not be
+included here.
+
+Dalman (1826, 1828) reviewed the opinions of his predecessors, and
+thought it not impossible that organs of mastication may have been
+present under the head shield of the trilobite as in _Limulus_ (1828,
+p. 18). In this he of course followed Wahlenberg.
+
+Goldfuss (1828) figured sections of _Dalmanites hausmanni_, _Phacops
+macrophthalma_, and _Calymene tristani_, which remind one of some of
+Doctor Walcott's translucent slices. So far as one can judge from the
+illustrations, it is probable that what he took for limbs were really
+fragments of other trilobites. Such is certainly the case in his
+figures 9 and 10, where a number of more or less broken thoracic
+segments are present. The section of _Encrinurus punctatus_ shown in
+figure 7 may possibly exhibit the position and folds of the ventral
+membrane beneath the axial lobe, and also, perhaps, the appendages.
+His figures 4, 5 and 8 show the hypostoma in section.
+
+Pander (1830) described the hypostoma in greater detail than had been
+done by previous authors, but otherwise added nothing to the subject.
+
+Sternberg (1830) thought he had individuals showing appendages, but
+judging from his poor figures, he was deceived by fragmentary
+specimens.
+
+Green (1839 A, B, C) described specimens of _Phacops_ from Berkeley
+Springs, West Virginia, which had the hypostoma in position, and
+appear to have had a tubular opening under the axial lobe. While
+appendages were not actually present, these specimens suggested fairly
+correct ideas about the swimming and breathing organs of trilobites.
+They were similar to the ones which Castelnau obtained, and all were
+perhaps from the same locality.
+
+It is not worth while to do more than enumerate the other authors of
+this period: Hisinger 1837, Emmrich 1839, Milne-Edwards 1841, for they
+all shared the same views, and added nothing to what was already
+known.
+
+Castelnau (1843) described and figured a _Phacops_ said to come from
+Cacapon Springs, West Virginia, which he thought possessed remains of
+appendages. There is nothing in the description or figures to indicate
+exactly what was present, but it is very unlikely that any limbs were
+preserved. The broad thin "appendage" figured may have been a fragment
+of a thoracic segment. This specimen was evidently described by
+Castelnau before 1843, as is inferred from a reference in the Neues
+Jahrbuch, 1843, P. 504, but I have not seen the earlier publication.
+
+Burmeister (1843-1846), in his "Organization of the Trilobites,"
+reviewed in _extenso_ the history of the search for appendages, and
+concluded that they must have been so soft as to preclude the
+possibility of their being preserved as fossils. "Their very absence
+in fossils most distinctly proves their former real structure" (p.
+10). In figures 7 and 8 on plate 6 he gave a restoration of the
+ventral surface of an _Asaphus_, the first restoration of the ventral
+anatomy to be attempted. Since he chose modern branchiopods as his
+model, he did not go so far wrong as he might have done. Still, there
+is little in the figure that would now be accepted as correct. The
+following quotation will serve to give the opinion of this zoologist,
+who from his knowledge of the Crustacea, was the most competent of the
+men of his time to undertake a restoration of the appendages of the
+trilobites:
+
+ ... in giving a certain form to the feet in the restored figure, I
+ have done so rather intending to indicate what they might have
+ resembled, than with any idea of assuming their actual form. I
+ merely assert that these organs were soft, membranous, and fringed,
+ adapted for locomotion in water, placed on the abdominal portion of
+ the body, and extending sidewise beneath the lateral lobes of the
+ rings, as shown in the ideal transverse section. These feet were
+ also indented, and thus divided into several lobes at the open
+ lower side, and each separate lobe was furnished at the margin with
+ small bristles serving as fins. The last and external lobe was
+ probably longer, smaller, and more movable, and reached to the
+ termination of the projecting shell lobe, bearing a bladder-shaped
+ gill on the inner side (1846, p. 45).
+
+McCoy (1846) observed in several trilobites a pair of pores situated
+in the dorsal furrows near the anterior end of the glabella. He showed
+that the pits occupy precisely the position of the antennæ of insects
+and suggested that they indicated the former presence of antennæ in
+these trilobites (chiefly _Anipyx_ and "_Trinucleus_"). The evidence
+from _Cryptolithus_, set forth on a later page, indicates the
+correctness of McCoy's view.
+
+Richter (1848, p. 20, pl. 2, fig. 32) described and figured what he
+took to be a phyllopod-like appendage found in a section through a
+_Phacops_. Without the specimen it is impossible to say just what the
+structure really was. The outline figure is so obviously modeled on an
+appendage of _Apus_ that one is inclined to think it somewhat
+diagrammatic. In calling attention to this neglected "find," Clarke
+(1888, p. 254, fig.) interprets the appendage as similar to the spiral
+branchiæ of _Calymene senaria_, and adds that he himself has seen
+evidence of spiral branchiæ in the American Phacops rana.
+
+Beyrich (1846) described a cast of the intestine of "_Trinucleus_,"
+and Barrande (1852) further elaborated on this discovery.
+
+Corda (1847) made a number of claims for appendages, but all were
+shown by Barrande (1852) to be erroneous.
+
+Barrande (1852, 1872) gave a somewhat incomplete summary of the
+various attempts to describe the appendages of trilobites, concluding
+that none showed any evidence of other than soft appendages, until
+Billings' discovery of 1870.
+
+Volborth (1863) described a long chambered tubular organ in _Illænus_
+which he believed to represent a cast of the heart of a trilobite, but
+which has since been likened by writers to the intestinal tract in
+"_Trinucleus_."
+
+
+
+
+PART I.
+
+THE APPENDAGES OF TRILOBITES.
+
+
+
+
+Terminology.
+
+
+The terminology employed in the succeeding pages is essentially the
+same as that used by Beecher, with two new terms added. Beecher
+assigned to the various segments of the limbs the names suggested by
+Huxley, but sometimes used the name protopodite instead of coxopodite
+for the proximal one. It is obvious that he did not use protopodite in
+the correct sense, as indicating a segment formed by the fusion of the
+coxopodite and basipodite. The usage employed here is shown in figure
+1.
+
+[Illustration: Fig. 1.--_Triarthrus becki_ Green. Diagram of one of
+the limbs of the thorax, viewed from above, with the endopodite in
+advance of the exopodite. 1, coxopodite, the inner extension being
+the endobase (gnathobase on cephalon); 2, basipodite, springing from
+the coxopodite, and supporting the exopodite, which also rests upon
+the coxopodite; 3, ischiopodite; 4, meropodite; 5, carpopodite; 6,
+propodite; 7, dactylopodite, with terminal spines.]
+
+The investigation of _Ceraurus_ showed that the appendages were
+supported by processes extending downward from the dorsal test,
+and on comparison with other trilobites it appeared that the same was
+true in _Calymene_, _Cryptolithus_, _Neolenus_, and other genera. Thin
+sections showed that these processes were formed by invagination of
+the test beneath the dorsal and glabellar furrows. While these
+processes are entirely homologous with the entopophyses of _Limulus_,
+I have chosen to apply the name _appendifer_ to them in the
+trilobites.
+
+The only other new term employed is the substitution of _endobase_ for
+gnathobase in speaking of the inner prolongation of a coxopodite of
+the trunk region. The term gnathobase implies a function which can not
+in all cases be proved.
+
+The individual portions of which the limbs are made up are called
+_segments_, and the articulations between them, _joints_. Such a
+procedure is unusual, but promotes clearness.
+
+
+
+
+The Appendages of Neolenus.
+
+
+HISTORICAL.
+
+The first mention of _Neolenus_ with appendages preserved was in
+Doctor Walcott's paper of 1911, in which two figures were given to
+show the form of the exopodites in comparison with the branchiæ of the
+eurypterid-like _Sidneyia_. In 1912, two more figures were presented,
+showing the antennules, exopodites, and cerci. The specimens were
+found in the Burgess shale (Middle Cambrian) near Field, in British
+Columbia. This shale is exceedingly fine-grained, and has yielded a
+very large fauna of beautifully preserved fossils, either unknown or
+extraordinarily rare elsewhere. It was stated in this paper (1912 A)
+that trilobites, with the exception of _Agnostus_ and _Microdiscus_,
+were not abundant in the shale.
+
+In discussing the origin of the tracks known as _Protichnites_,
+Walcott presented four figures of _Neolenus_ with appendages, and
+described the three claw-like spines at the tip of each endopodite.
+
+Three new figures of the appendages were also contributed to the
+second edition of the Eastman-Zittel "Text-book of Paleontology"
+(1913, p. 701). Later (1916, pl. 9) there was published a photograph
+of a wonderful slab, bearing on its surface numerous Middle Cambrian
+Crustacea. Several of the specimens of _Neolenus_ showed appendages.
+
+Finally, in 1918, appeared the "Appendages of Trilobites," in which
+the limbs of _Neolenus_ were fully described and figured (p. 126),
+and a restoration presented. Organs previously unknown in trilobites,
+epipodites and exites, attached to the coxopodites, were found.
+
+
+=Neolenus serratus= (Rominger).
+
+(Text fig. 2-8.)
+
+ Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1911, p. 20,
+ pl. 6, figs. 1, 2 (exopodites of thorax and cephalon);--Ibid., vol.
+ 57, 1912, p. 191, pl. 24, figs. 1, la (antennules, caudal rami, and
+ endopodites of thorax);--Ibid., vol. 57, 1912, p. 277, pl. 45,
+ figs. 1-4 (antennules, endopodites of cephalon and thorax, caudal
+ rami);--Text-book of Paleontology, edited by C. R. Eastman, 2d ed.,
+ vol. 1. 1913, p. 701, fig. 1343 (exopodites), p. 716, fig. 1376
+ (abdominal appendages), fig. 1377 (appendages of thorax and
+ pygidium);--Ann. Rept. Smithson. Inst. for 1915, 1916, pl.
+ 9;--Smithson. Misc. Coll., vol. 67, 1918, pp. 126-131 et al., pl.
+ 14, fig. 1; pls. 15-20; pl. 21, fig. 6; pls. 22, 23; pl. 31
+ (restoration); pl. 34, fig. 3 (restored section); pl. 35, fig. 4;
+ pl. 36, fig. 3 (hypostoma).
+
+The following description of the appendages of _Neolenus_ is
+summarized from Walcott's paper of 1918, and from a study of the eight
+specimens mentioned below.
+
+_Cephalon._
+
+The antennules are long, slender, and flexible, and lack the formal
+double curvature so characteristic of those of _Triarthrus_. There are
+short fine spines on the distal rims of the segments of the proximal
+half of each, thus giving great sensitiveness to these organs. In the
+proximal portion of each, the individual segments are short and wider
+than long, and in the distal region they are narrow and longer than
+wide.
+
+There are four pairs of biramous cephalic appendages, which differ
+only very slightly from the appendages of the thorax. All are of
+course excessively flattened, and they are here described as they
+appear.
+
+The coxopodites, shown for the first time in Walcott's paper of 1918,
+are broad, longer than wide, and truncated on the inner ends, where
+they bear short, stout, unequal spines similar to those along the
+anterior margin. The gnathobases are but slightly modified to serve as
+mouth parts, much less so than in _Triarthrus_, but the coxopodites
+of the cephalon are shorter and wider than those of the thorax.
+
+At the distal end of the coxopodite arise the endopodite and
+exopodite. The endopodite consists of six segments, the distal ones,
+propodite and dactylopodite, more slender than the others, the last
+bearing three terminal spines. The first endopodite is shorter than
+the others and slightly more slender (pl. 16, fig. 1)[1] and the
+anterior appendages turn forward more or less parallel to the sides of
+the hypostoma (pl. 22). The basipodite, ischiopodite, meropodite, and
+carpopodite are, in their flattened condition, roughly rectangular,
+only a little longer than wide, taper gradually distally, each bears
+small spines on the outer rim, and some of the proximal ones usually
+have a row along the margin.
+
+[Footnote 1: _Nota bene!_ All references in this section are to the
+plates of Doctor Walcott's paper in 1918.]
+
+The exopodites of the cephalon, as of the body of Neolenus, are very
+different from those of any other trilobite whose appendages were
+previously known. As shown in the photographs (pl. 20, fig. 2; pl.
+22), each exopodite consists of a single long, broad, leaf-like blade,
+not with many segments as in _Triarthrus_, but consisting of a large
+basal and small terminal lobe. It bears on its outer margin numerous
+relatively short, slender, flat setæ. The long axes of the exopodites
+point forward, and the setæ are directed forward and outward. They
+stand more nearly at right angles to the shaft on the cephalic
+exopodites than on those of the thorax. This same type of broad-bladed
+exopodite is also found on the thorax and pygidium.
+
+The number of functional gnathobases on the cephalon is unknown. That
+four endopodites were present on one side is shown pretty clearly
+by specimen 58591 (pl. 16, fig. 3) and while no more than two well
+preserved exopodites have been seen on a side, there probably were
+four. Specimen 65513 (pl. 16, fig. 1) shows gnathobases on the second
+and third appendages of that individual as preserved, but there is
+no positive evidence that these are really the second and third
+appendages, for they are obviously displaced. The hypostoma of
+Neolenus is narrow but long, several specimens showing that it
+extended back to the horizon of the outer ends of the last pair of
+glabellar furrows. It is not as wide as the axial lobe, so that, while
+gnathobases attached beneath the first pair of furrows would probably
+not reach back to the posterior end of the hypostoma, they might lie
+parallel to it and not extend beneath. It seems possible, then, that
+there were four pairs of endobases but that the second rather than the
+first pair served as mandibles, as seems to be the case in Ceraurus.
+
+_Thorax._
+
+The thorax of _Neolenus_ consists of seven segments, and the
+appendages are well shown (pl. 17, fig. 1; pl. 18, figs. 1, 2; pl. 20,
+fig. 1.), The endopodites of successive segments vary but little,
+all are slender but compact, and consist of a long coxopodite with
+six short, rather broad segments beyond it. In the figures, the
+endopodites extend some distance in a horizontal direction beyond the
+edges of the dorsal test, as many as four segments being in some cases
+visible, but measurements show that the appendages tended to fall
+outward on decay of the animal. The dactylopodites are provided
+with terminal spines as in _Triarthrus_. The coxopodites are long,
+straight, and slender. They are well shown on only one specimen (pl.
+18), where they are seen to be as wide as the basipodite, and the
+endobases are set with spines on the posterior and inner margins. They
+are so long that those on opposite sides must have almost met on the
+median line. The segments of the endopodites are mostly but little,
+if any, longer than broad, and at the distal end each shows two or
+more spines. The propodite and dactylopodite are notably more slender
+than the others. The exopodites of the thorax are broad and flat, and
+each shaft has two distinct parts with different kinds of setæ. The
+posterior edge of the proximal lobe is fringed with a slender, flat,
+overlapping hairs which are a little longer than the width of the
+lobe, and stand at an angle of about 60 degrees with the direction of
+the axis of the appendage. The outer lobe is at an angle with the main
+one, and has short, very fine setæ oh the margin. One or two specimens
+show some evidence of a joint between the inner and outer lobes,
+but in the great majority of cases they seem to be continuous; if
+originally in two segments, they have become firmly united. The
+exopodites of the thorax, like those of the cephalon, are directed
+diagonally forward and outward. (pl. 21, fig. 6; pl. 22.)
+
+_Pygidium._
+
+The pygidium of _Neolenus serratus_ is large, and usually shows five
+rings on the axial lobe and four pairs of ribs on the sides. There are
+five pairs of biramous appendages belonging to this shield, and behind
+these a pair of jointed cerci. That the number of abdominal appendages
+should correspond to the number of divisions of the axial lobe rather
+than to the number of ribs on the pleural lobes is of interest, and in
+accord with other trilobites, as first shown by Beecher.
+
+The endopodites of the pygidium have the same form as those of the
+thorax, are long, and very much less modified than those of any other
+trilobite whose appendages are known. On some specimens, they extend
+out far beyond the dorsal test, so that nearly all the segments are
+visible (pl. 17, fig. 3; pl. 18; pl. 19; pl. 20, fig. 1), but in these
+cases are probably displaced. The segments are short and wide, the
+whole endopodite tapering gradually outward. The dactylopodite bears
+terminal spines, and the individual segments also have outward-directed
+spines.
+
+The cerci appear to have been long, slender, very spinose organs much
+like the antennules, but stiff rather than flexible. They are a little
+longer than the pygidium (pl. 17, figs. 1, 2), and seem to be attached
+to a plate on the under surface of the posterior end and in front of
+the very narrow doublure. The precise form of this attachment can not
+be determined from the published figures. They bear numerous fine
+spines (pl. 17, fig. 3).
+
+_Epipodites and Exiles._
+
+Doctor Walcott has found on several specimens of _Neolenus_ remains of
+organs which he interprets as epipodites and exites attached to the
+coxopodites. A study of the specimens has, however, convinced me that
+both the large and small epipodites are really exopodites, and that
+the exites are badly preserved and displaced coxopodites. Detailed
+explanation of this interpretation is given below in the description
+of the several specimens involved.
+
+_Description of Individual Specimens._
+
+Doctor Walcott was kind enough to send me eight of the more important
+specimens of _Neolenus_ figured by him, and since my interpretation
+of them does not agree in all respects with his, I have thought it
+fairer to the reader to present here rather full notes explaining
+the position I have taken. I understand that since I communicated my
+interpretation of the epipodites and exites to him, Doctor Walcott has
+submitted the specimens to several palæontologists, who consider that
+epipodites are really present. Since I am not able to convince myself
+that their conclusion is based upon sound evidence, I give here my own
+interpretation. There is of course, no a priori reason why trilobites
+should not have had epipodites.
+
+Specimen No. 58589.
+
+ Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, pl. 45,
+ fig. 2;--Zittel-Eastman Text-book of Paleontology, vol. 1, 1913,
+ fig. 1377;--Smithson. Misc. Coll., vol. 67, 1918, pl. 18, fig. 1;
+ pl. 20, fig. 1.
+
+This is one of the most important of the specimens, as it shows the
+coxopodites of three thoracic limbs and the well preserved endopodites
+of six thoracic and five pairs of pygidial appendages.
+
+The appendages are all shifted to the left till the articular socket
+of the coxopodite is about 8 mm. outside of its proper position. The
+endopodites extend a corresponding amount beyond the edge of the
+dorsal test and are there so flattened that they are revealed as a
+mere impression. The coxopodites, which are beneath the test, seem to
+have been somewhat protected by it, and while hopelessly crushed, are
+not flattened, but rather conformed to the ridges and grooves of the
+thorax.
+
+[Illustration: Fig. 2. _Neolenus serratus_ (Rominger). A sketch of the
+coxopodites and endopodites of two thoracic segments. Note notch for
+the reception of the lower end of the appendifer. × 3.]
+
+The coxopodite of the appendage of the last thoracic segment is best
+preserved. It is rectangular, about one third as wide as long, with a
+slight notch in the posterior margin near the outer end. The inner end
+is obliquely truncated and shows about ten sharp spines which do not
+appear to be articulated to the segment, but rather to be direct
+outgrowths from it. There are similar spines along the posterior
+margin, but only two or three of what was probably once a continuous
+series are now preserved. On the opposite margin of the coxopodite
+from the slight depression mentioned above, there is a slight
+convexity in the outline, which is better shown and explained by the
+coxopodite just in front of this. That basal segment has the same form
+as the one just described, but as its posterior margin is for the
+greater part of its length pushed under the one behind it, the spines
+are not shown. On the posterior margin, two-thirds of the length from
+the proximal end, there is a shallow notch, and corresponding to it, a
+bulge on the anterior side. From analogy with Ceraurus and _Calymene_
+it becomes plain that the notch and bulge represent the position of
+the socket where the coxopodite articulated with the appendifer. Since
+these structures have not been shown in previous illustrations, a
+drawing giving my interpretation of them is here inserted (fig. 2).
+It is evident from the position of the notch that the row of spines
+was on the dorsal (inner) side of the coxopodite and that the
+truncation was obliquely downward and outward.
+
+The endopodite of the last thoracic appendage is well preserved and
+may be described as typical of such a leg in this part. The basipodite
+is as wide as the coxopodite, and it and the three succeeding
+segments, ischiopodite, meropodite, and carpopodite, are all
+parallel-sided, not expanded at the joints, and decrease regularly in
+width. The propodite and dactylopodite are also parallel-sided, but
+more slender than the inner segments, and on the end of the
+dactylopodite there are four little spines, three of them--one large
+and two small--articulated at the distal end, and the fourth
+projecting from the posterior outer angle. Each segment has one or
+more spines on the outer articular end, and the ischiopodite has
+several directed obliquely outward on the posterior margin. All of the
+four proximal segments show a low ridge parallel to and near the
+anterior margin, and several endopodites of the pygidium have a
+similar ridge and a row of spines along the posterior margin of some
+of the segments. These features indicate that the segments in question
+were not cylindrical in life, but compressed. From the almost
+universal location of the spines on the posterior side of the limbs as
+preserved, it seems probable that in the natural position the segments
+were held in a plane at a high angle with the horizontal, the ridge
+was dorsal and anterior and the row of spines ventral and posterior.
+Because the spines on the endobases are dorsal it does not follow that
+those on the endopodites were, for the position of the coxopodite in a
+crushed specimen does not indicate the position of the endopodite of
+even the same appendage.
+
+The endopodites of the pygidium are similar to the one just described,
+except that some of them have spines on the posterior margin of the
+segments, and a few on the right side have extremely fine, faintly
+visible spines on the anterior side. The specimen shows fragments
+of a few exopodites, but nothing worth describing. In the middle
+of the right pleural lobe there is a small organ which Walcott has
+interpreted as a small epipodite. It is oval in form, broken at the
+end toward the axial lobe, and has exceedingly minute short setæ on
+the posterior margin. From analogy with other specimens, it appears
+to me to be the outer end of an exopodite.
+
+_Measurements:_ The entire specimen is about 64 mm. long and
+52 mm. wide at the genal angles. The thorax is about 41 mm. wide
+(disregarding the spines) at the seventh segment, and the axial lobe
+about 13 mm. wide at the same horizon. The measurements of the
+individual segments of the seventh left thoracic limb are:
+
+ Coxopodite, 9 mm. long, 3 mm. wide, the middle of the notch 8 mm.
+ from the inner end, measured along the bottom, and 6 mm. measured
+ along the top.
+ Basipodite, 5 mm. long, 3 mm. wide
+ Ischiopodite, 4 " " 3 " "
+ Meropodite, 3.5 " " 2.5 " "
+ Carpopodite, 3.5 " " 2 " "
+ Propodite, 3 " " 1.25 " "
+ Dactylopodite, 2 " " 1.25 " "
+
+The five distal segments of the last pygidial endopodite are together
+10.5 mm. long. The whole six segments of the endopodite of the third
+thoracic segments are together 21 mm. long. The distance from the
+appendifer of the third segment to the outer end of the spine is 17
+mm. From the center of the notch in the coxopodite to the outer end
+is 1.5 mm., which, added to the length of the endopodite, 21 mm.,
+makes a distance of 22.5 mm. from the appendifer to the tip of
+the dactylopodite, showing that if projected straight outward, the
+endopodites of the thorax would project 5.5 mm. beyond the test,
+including spines.
+
+The distance across the axial lobe from appendifer to appendifer on
+the seventh thoracic segment is 12.5 mm. Measured along the top of
+the coxopodite, it is 6 mm. from the middle of the notch to the inner
+end, and measured along the bottom it is 8 mm. From the truncated form
+of the ends it is evident that the coxopodites extended inward and
+downward from the appendifers, and with the dimensions given above,
+the inner toothed ends would practically meet on the median line.
+
+Measurements on the appendages of the pygidia show that on this
+specimen they extend back about twice as far beyond the edge of the
+pygidium as they should, all being displaced.
+
+Specimen No. 65514.
+
+ Illustrated: Walcott, Smithson. Misc. Coll., vol. 67, 1918,
+ pl. 19, figs. 1-3.
+
+This specimen is so twisted apart that it is not possible to determine
+to what segments the appendages belong, but it exhibits the best
+preserved exopodites I have seen. The best one is just in front of the
+pygidium on the matrix, and shows a form more easily seen than
+described (our fig. 3). There is a broad, flat, leaf-like shaft, the
+anterior side of which follows a smooth curve, while in the curve on
+the posterior side, which is convex backward, there is a re-entrant,
+setting off a small outer lobe whose length is about one third the
+length of the whole. This lobe seems to be a continuation of the
+shaft, and the test of the whole is wrinkled and evidently very thin.
+The main and distal lobes of the shaft both bear numerous delicate
+setæ, but those of the outer lobe are much shorter and finer than
+those on the main portion. The latter are flattened and blade-like.
+
+[Illustration: Fig. 3. Exopodite of _Neolenus serratus_ (Rominger), to
+show form of the lobes of the shaft, and the setæ. × 4.]
+
+[Illustration: Fig. 4. _Neolenus serratus_ (Rominger). One of the
+so-called epipodites of specimen 65515, showing that it has the same
+outline as an exopodite (compare figure 3) and fragments of setæ on
+the margin. × 3.]
+
+The anterior edge of the shaft shows a narrow stiffening ridge and the
+setæ are but little longer than its greatest width. The second segment
+of the pygidium has another exopodite like this one, but shows faintly
+the line between the two lobes, as though there were two segments.
+
+This specimen also shows some very well preserved endopodites, but
+they differ in no way from those described from specimen No. 58589.
+Walcott mentions two large epipodites projecting from beneath the
+exopodites. I judge that he has reference to the distal lobes of the
+exopodites, but as these are continuous with the main shaft, there can
+be no other interpretation of them than that which I have given above.
+
+
+_Measurements:_ The pygidium is 19 mm. long (without the spines) and
+about 34 mm. wide at the front. The exopodites show faintly beneath
+the pygidial shield, but their proximal ends are too indistinct to
+allow accurate measurement. Apparently they were just about long
+enough to reach to the margin of the shield. The best preserved one,
+that of the second segment in the pygidium, is about 11 mm. long, 2.5
+mm. wide at the widest; the distal lobe is 2.5 mm. long, and the
+longest setæ of the main lobe 3.5 mm. long. The pleural lobe of the
+pygidium is just 11 mm. wide at this point.
+
+The endopodites project from 8 to 12 mm. beyond the pygidium, showing
+about four segments.
+
+The thoracic exopodite described above is 11 mm. long and 2.75 mm.
+wide at the widest part. The distal lobe is 3.5 mm. long and 2.25 mm.
+wide, and the longest setæ on the main lobe 3 mm. long.
+
+Specimen No. 65519.
+
+ Illustrated: Walcott, Zittel-Eastman Text-book of Paleontology,
+ vol. 1, 1913, fig. 1343;--Smithson. Misc. Coll., vol. 67, 1918,
+ pl. 21, fig. 6.
+
+This specimen is somewhat difficult to study but is very valuable as
+showing the natural position of the exopodites of the anterior part of
+the thorax. Walcott's figures are excellent and show the broad
+leaf-like shafts, the distal lobes with the re-entrant angles in the
+posterior margin, and the long fine setæ of the main lobes. None of
+the distal lobes retains its setæ. All extend back to the dorsal
+furrows, but the proximal ends are not actually shown.
+
+The specimen is especially important because it shows the same distal
+lobes as specimen No. 65514, and demonstrates that they are a part of
+the exopodite and not of any other structure.
+
+_Measurements:_ The exopodite belonging to the fourth thoracic segment
+is 23 mm. long and 4 mm. wide at the widest part. The longest setæ are
+7 mm. in length.
+
+Specimen No. 65520.
+
+ Illustrated: Walcott, Smithson. Misc. Coll., vol. 67, 1918, pl. 20,
+ fig. 2; pl. 22, fig. 1.
+
+This is a practically entire specimen, on two blocks, one showing the
+interior of the shell, and the other the one figured by Walcott, a
+cast of the interior. The first shows the low rounded appendifers at
+the anterior angle of each axial tergite. They are almost entirely
+beneath the dorsal furrows and do not project so far into the axial
+lobe as those of Ceraurus and _Calymene_. In fact, only those at the
+anterior end of the thorax project inward at all. As expected, there
+are five pairs on the pygidium. The cephalon is unfortunately so
+exfoliated that the appendifers there are not preserved. The doublure
+of the pygidium is extremely narrow.
+
+The cast of the interior shows, rather faintly, the exopodites of the
+right side of the thorax and of the left side of the cephalon, and,
+still more faintly, the caudal rami and a few pygidial endopodites.
+The exopodites on the right side are in what seems to be the customary
+position, directed obliquely forward and outward, and the tips of
+their distal lobes project slightly beyond the edge of the test. These
+lobes were interpreted by Walcott as epipodites, but after comparing
+them with the terminal lobes of the exopodites of specimens No. 65519
+and 65514 I think there can be no doubt that they represent the same
+structure. The pleura of the individual thoracic segments on this side
+of the specimen have an unusual appearance, for they are bluntly
+rounded or obtusely pointed, instead of being spinose.
+
+The interpretation of the appendages of the cephalon is somewhat
+difficult. At the left of the glabella there are two large exopodites,
+the anterior of which lies over and partially conceals the other.
+These show by their position that they belong to the fourth and fifth
+cephalic appendages. In front of these lie two appendages which may be
+either endopodites or exopodites, but which I am inclined to refer to
+the latter. Both are narrow and shaped like endopodites, but bear on
+their outer edges close-set fine setæ. They also show what might be
+considered as faint traces of segmentation. If the first of these ran
+under the end of the exopodite behind it, as shown in Walcott's figure
+(pl. 22), then it would be necessary to interpret it as an endopodite,
+but it really continues down between the exopodite and the glabella,
+and seems to be attached opposite the middle of the eye. The specimen
+does not indicate clearly whether this appendage is above or below
+the exopodite behind it, but one's impression is that it is above, in
+which case it also must be an exopodite. The appendage in front, being
+similar, is similarly interpreted. If this be correct, then the
+exopodites of the second and third cephalic appendages are much
+shorter and narrower than those of the fourth and fifth. All of these
+appendages are obviously out of position, for the cheek has been
+pushed forward away from the thorax, though still pivoting on its
+inner angle at the neck-ring, till the eye has been brought up to the
+dorsal furrow. In this way the anterior exopodites have been thrust
+under the glabella and all the appendages have been moved to the right
+of their original position. The anterior exopodite is very poorly
+shown, but seems to be articulated in front of the eye. The posterior
+exopodites are very similar to those on the thorax. The distal lobe is
+shown only by the second from the last. It has the same form as the
+distal lobes on the thoracic exopodites, and like them has much finer
+setæ than the main lobe, but it does not stand at so great an angle
+with the axis of the main lobe, nor yet is it so straight as shown in
+Walcott's figure.
+
+_Measurements:_ The specimen is about 72 mm. long and 54 mm. wide at
+the genal angles. The pygidium is 22 mm. long and 37 mm. wide. The
+doublure is 1.5 mm. wide. The exopodite of the third thoracic segment
+is 19.5 mm. long. The pleural lobe at this point is 13 mm. wide
+without the spines and 18.5 mm. wide with them. The third exopodite of
+the cephalon was apparently about 15 mm. long when complete.
+
+Specimen No. 65515.
+
+ Illustrated: Walcott, Smithson. Misc. Coll., vol. 67, 1918, pl. 20,
+ figs. 3, 4.
+
+This is a small piece of the axial portion of a badly crushed
+Neolenus, showing appendages on the left side as viewed from above. On
+the posterior half there are three large appendages which have the
+exact form of the exopodites of other specimens. There is a broad,
+oval, proximal lobe and a distal one at an angle with it. The proximal
+part of the shaft has fine setæ or the bases of them, and the distal
+lobe faint traces of much finer ones. The form, and the setæ so far as
+they are preserved, are exactly like those of the exopodites on the
+specimens previously described. (See fig. 4, page 26.) Beneath them
+there are slender, poorly preserved endopodites.
+
+In front of the exopodites and endopodites lie a series of structures
+which Walcott has called exites, but for which I can see another
+explanation. Walcott has shown them as four broad rounded lobes, but
+his figure must be looked upon as a drawing and not as a photograph,
+for it has been very much retouched.
+
+For convenience of discussion, these lobes may be called Nos. 1, 2, 3,
+and 4, the last being the posterior one (fig. 5). This lobe is best
+shown on the matrix, where the anterior end is seen to be margined by
+stout spines, while the posterior end lies over the endopodite and
+under the exopodite behind it. No. 3 is sunk below the level of the
+others, and only a part of it has been uncovered. Its margin bears
+strong spines of different sizes. Its full shape can not be made out,
+but it has neither the shape nor the form of spines shown in figure 3,
+plate 20 (1918). Lobes 2 and 1 and another lobe in front of 1 seem to
+form a continuous series and to be part of a single appendage. They
+are all in one plane, arc so continuous that the joints between them
+can be made out with difficulty and if they do belong together, can
+easily be explained.
+
+[Illustration: Fig. 5.--A sketch of the so-called exites of _Neolenus
+serratus_ (Rominger), to show the form and the character of the
+spines. × 2.]
+
+[Illustration: Fig. 6.--Endopodite of a cephalic appendage of
+_Neolenus serratus_ (Rominger), showing the very broad coxopodite.
+× 2.]
+
+Before calling these structures new organs not previously seen on
+trilobites, it is of course necessary to inquire if they can be
+interpreted as representing any known structures. That they can not be
+exopodites is obvious, since they are bordered by short stout spines
+instead of setæ. The same stout spines that negate the above possible
+explanation at once suggest that they are coxopodites (compare fig 6).
+At first sight, the so-called exites seem too wide and too rounded to
+be so interpreted, but if reference be had to the specimens rather
+than the figures, it will be noted that the only well preserved
+structure (No. 2) is longer than wide, has spines only on one side and
+one end, and does not differ greatly from the coxopodite of specimen
+No. 58589 (pl. 18, 1918). If structures 2, 1, and the segment ahead of
+1 are really parts of one appendage, it can only be an endopodite, of
+which No. 2 is the coxopodite, No. 1 the basipodite, and the next
+segment the ischiopodite. If one looks carefully, there are no traces
+of spines on either end of No. 1, but only on the margin. The extreme
+width of No. 2 is against this interpretation as a coxopodite (see,
+however, fig. 6), but it may be rolled out very flat, as this is an
+unusually crushed specimen. No. 2 is 10 mm. long and 6 mm. wide at
+the widest point. No. 1 is 5 mm. long and 3.5 mm. wide.
+
+The crucial point in this determination is whether 2 and 1 are parts
+of the same appendage. I believe they are, but others may differ.
+
+Specimen No. 65513.
+
+ Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, pl. 45,
+ fig. 3;--Ibid., vol. 67, 1918, pl. 16, figs. 1, 2.
+
+This is nearly all of the right half of an entire specimen, but the
+only appendages of any interest are those of the cephalon. Five
+endopodites emerge from beneath that shield, but as all are displaced
+it is not possible to say how many belong to the head. When held at
+the proper angle to the light, the second and third from the front
+show faintly the partial outlines of the coxopodites. The anterior
+side and end of the best preserved one shows irregular stout spines of
+unequal sizes, and the inner end is truncated obliquely (fig. 6).
+These coxopodites are like those on the thorax of specimen No. 58589,
+but shorter and wider. This of course suggests that the "exite" No. 2
+of specimen No. 65515 may be a cephalic coxopodite. The endopodite of
+this appendage, like the others on this cephalon, is shorter and
+stouter than the thoracic or pygidial endopodites of the others
+described.
+
+[Illustration: Fig. 7.--A restored section across the thorax of
+_Neolenus serratus_, showing the probable form of attachment of the
+appendages, their relation to the ventral membrane, and the jaw-like
+endobases of the coxopodites.]
+
+_Measurements:_ The cephalon is 24 mm. long and about 60 mm. wide. The
+coxopodite of the third appendage is about 10 mm. long and 5.5 mm.
+wide at the widest point. The corresponding endopodite is 19 mm. long
+and projects 11 mm. beyond the margin, which is about 5 mm. further
+than it would project were the appendage restored to its proper
+position.
+
+
+RESTORATION OF NEOLENUS.
+
+(Text figs. 7, 8.)
+
+This restoration is based upon the information obtained from the
+studies which have been detailed in the preceding pages, and differs
+materially from that presented by Doctor Walcott. The appendages are
+not shown in their natural positions, but as if flattened nearly into
+a horizontal plane. The metastoma is added without any evidence for
+its former presence.
+
+The striking features of the appendages are the broad unsegmented
+exopodites which point forward all along the body, and the strong
+endopodites, which show practically no regional modification. Although
+the exopodites have a form which is especially adapted for use in
+swimming, their position is such as to indicate that they were not so
+used. The stout endopodites, on the other hand, probably performed the
+double function of natatory and ambulatory legs.
+
+[Illustration: Fig. 8.--_Neolenus serratus_ (Rominger). A restoration
+of the ventral surface, with the endopodites omitted from one side, to
+permit a better exposition of the exopodites. The position and number
+of the appendages about the mouth are in considerable doubt. Restored
+by Doctor Elvira Wood under the supervision of the writer. About
+one-half larger than the average specimen.]
+
+
+=Nathorstia transitans= Walcott.
+
+Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, pl. 28,
+fig. 2.
+
+The badly preserved specimen on which this genus and species was
+based is undoubtedly a trilobite, but for some reason it does not
+find a place in Walcott's recent article on "Appendages" (1918). The
+preservation is different from that of the associated trilobites,
+being merely a shadowy impression, indicating a very soft test. The
+general outline of the body, the position of the eye, and even a
+trace of spines about the pygidium (in the figure) are similar
+to those of _Neolenus_, and I would venture the suggestion that
+_Nathorstia transitans_ is a recently moulted _Neolenus serratus_,
+still in the "soft-shelled" condition. Even if not a Neolenus, it is
+probable, from the state of preservation, that it is an animal which
+had recently cast its shell.
+
+Walcott describes such fragments of appendages as remain, as follows:
+
+ Head. A portion of what may be an antenna projects from beneath the
+ right anterior margin; from near the left posterolateral angle a
+ large four-jointed appendage extends backward. I assume that this
+ may be the outer portion of the large posterior appendage (maxilla)
+ of the head.
+
+ Thorax. Traces of several slender-jointed thoracic legs project
+ from beneath the anterior segments and back of these on the right
+ side more or less of six legs have been pushed out from beneath the
+ dorsal shield; these are composed of three or four long slender
+ joints; fragments of the three proximal joints indicate that they
+ are shorter and larger and that they have a fringe of fine setæ.
+ Indications of a branchial lobe (gill) are seen in two specimens
+ where the legs are not preserved. This is often the case both among
+ the Merostomata (pl. 29, fig. 3, _Molaria_) and Trilobita (pl. 24,
+ fig. 2, _Ptychoparia_).
+
+ Two caudal rami project a little distance beneath the posterior
+ margin of the dorsal shield.
+
+This latter feature of course suggests _Neolenus_. The other
+appendages are too poorly preserved to allow comparison without seeing
+the specimen.
+
+The specific name was given "on account of its suggesting a transition
+between a Merostome-like form, such as _Molaria spinifera_, and the
+trilobites." In what respect it is transitional does not appear.
+
+Formation and locality: Same as that of _Neolenus serratus_. One
+nearly complete specimen and a few fragments were found.
+
+
+
+
+The Appendages of Isotelus.
+
+
+HISTORICAL.
+
+The first specimen of _Isotelus_ with appendages was described orally
+by Billings before the Natural History Society of Montreal in 1864,
+and in print six years later (1870, p. 479, pls. 31, 32). The specimen
+is described in detail on a later page. Billings recognized the
+remains of eight pairs of legs on the thorax, a pair for each segment,
+and he inferred from the fact that the appendages projected forward
+that they were ambulatory rather than natatory organs. He was unable
+to make out the exact number of the segments in the appendages, but
+thought each showed at least four or five.
+
+Having examined the individual sent to London by Billings, Woodward
+(1870, p. 486, fig, 1) reviewed the collection from the American
+Trenton in the British Museum and found a specimen in the "Black
+Trenton limestone," from Ottawa, Ontario, in which, alongside the
+hypostoma, was a jointed appendage, which he described as the "jointed
+palpus of one of the maxillæ." This has always been considered an
+authentic "find," but I am informed by Doctor Bather that the specimen
+does not show any real appendage. For further discussion, see under
+_Isotelus gigas_.
+
+In 1871, Billings' specimen was examined by Professors James D. Dana
+(1871, p. 320), A. E. Verrill, and Sydney I. Smith, who agreed
+that the structures identified by Billings as legs were merely
+semicalcified arches of the membrane of the ventral surface, which
+opinion seems to have been adopted by zoologists generally in spite of
+the fact that the most elementary consideration of the structure of
+the thorax of a trilobite should have shown its falsity. While the
+curvature of the thoracic segments was convex forward, that of the
+supposed ventral arches was convex backward, and the supposed arches
+extended across so many segments as to have absolutely prevented any
+great amount of motion of the segments of the thorax on each other.
+Enrollment, a common occurrence in _Isotelus_, would have been
+absolutely impossible had any such calcified arches been present.
+
+Walcott, in his study of trilobites in thin section (1881, pp. 192,
+206, pl. 2, fig. 9), obtained eleven slices of _Isotelus gigas_ which
+showed remains of appendages. He figured one of the sections, stating
+that it "shows the basal joint of a leg and another specimen not
+illustrated gives evidence that the legs extended out beneath the
+pygidium, as indicated by their basal joints."
+
+The second important specimen of an _Isotelus_ with appendages was
+found by Mr. James Pugh in strata of Richmond age 2 miles north of
+Oxford, Ohio, and is now in the U. S. National Museum. It was first
+described by Mickleborough (1883, p. 200, fig. 1-3). In two successive
+finds, a year apart, the specimen itself and its impression were
+recovered. Since I am redescribing the specimen in this memoir (see
+p. 35), it only remains to state here that Mickleborough interpreted
+the structures essentially correctly, though not using the same
+terminology as that at present adopted. His view that the anterior
+appendages were chelate can not, however, be supported, nor can his
+idea that the sole appendages of the pygidium were foliaceous
+branchial organs.
+
+Walcott (1884, p. 279, fig. 1) studied the original specimens and
+presented a figure which is much more detailed and clear than those of
+Mickleborough. By further cleaning the specimen he made out altogether
+twenty-six pairs of appendages. He stated that one of these belonged
+to the cephalon, nine to the thorax,[1] and the remaining sixteen to
+the pygidium. He showed that the endopodites of the pygidium were of
+practically the same form as those on the thorax, and stated that the
+"leg beneath the thorax of the Ohio trilobite shows seven joints in
+two instances; the character of the terminal joint is unknown." His
+figure shows, and he mentions, markings which are interpreted as
+traces of the fringes of the exopodites.
+
+[Footnote 1: The posterior one of these he believed to have been
+crowded forward from beneath the pygidium.]
+
+In the same year Woodward (1884, p. 162, fig. 1-3) reproduced all of
+Mickleborough's figures, and suggested that the last seven pairs of
+appendages on the pygidium of _Calymene_ and _Isotelus_ were probably
+"lamelliform branchiferous appendages, as in _Limulus_ and in living
+Isopoda."
+
+Professor Beecher published, in 1902, an outline taken from
+Mickleborough's figure of this specimen, to call attention to certain
+discontinuous ridges along the axial cavity of the anterior part of
+the pygidium and posterior end of the thorax. These ridges are well
+shown in Mickleborough's figure, though not in that of Walcott, and
+their presence on the specimen was confirmed by a study by Schuchert,
+who contributed a diagrammatic cross-section to Beecher's paper (1902,
+p. 169, pl. 5, figs. 5, 6). Beecher summarized in a paragraph his
+interpretation of this specimen:
+
+ The club-shaped bodies lying within the axis are the gnathobases
+ attached at the sides of the axis; the curved members extending
+ outward from the gnathobases are the endopodites; the longitudinal
+ ridges in the ventral membrane between the inner ends of the
+ gnathobases are the buttresses and apodemes of the mesosternites;
+ the slender oblique rod-like bodies shown in the right pleural
+ region in Walcott's figure are portions of the fringes of the
+ exopodites.
+
+In 1910, Mr. W. C. King of Ottawa, Ontario, found at Britannia, a few
+miles west of Ottawa, the impression in sandstone of the under surface
+of a large specimen of _Isotelus arenicola_, described on a later page
+(p. 39).
+
+Finally (1918, p. 133, pl. 24, figs. 3, 3a; pl. 25), Walcott has
+redescribed the specimen from Ohio, presenting a new and partially
+restored figure. He refers also to the specimen from Ottawa under the
+name _Isotelus covingtonensis?_ Foerste (not Ulrich). He advances the
+view, which I am unable to share, that the cylindrical appearance of
+the segments of the appendages of _Isotelus_ is due to post-mortem
+changes.
+
+
+=Isotelus latus= Raymond.
+
+(pl. 10, fig. 1.)
+
+ Illustrated: _Asaphus platycephalus_ Billings, Quart. Jour. Geol.
+ Soc., London, vol. 26, 1870, pl. 31, figs. 1-3; pl. 32, figs. 1,
+ 2.--Woodward, Geol. Mag., vol. 8, 1871, pl. 8, figs. 1,
+ 1a.--Gerstäcker, in Bronn's "Klassen u. Ordnungen d. Thier-Reichs,"
+ 1879, pl. 49, fig. 1.--von Koenen, N. Jahrb. f. Min., etc., vol. 1,
+ 1880, pl. 8, fig. 8.--Milne-Edwards, Ann. Sci. Nat., Zoologie, ser.
+ 6, vol. 12, 1881, pl. 12, fig. 45.
+
+ _Isotelus latus_ Raymond, Bull. Victoria Mem. Mus., Geol. Survey
+ Canada, No. 1, 1913, p. 45 (species named).
+
+ _Isotelus covingtonensis?_ Walcott (not Foerste), Smithson. Misc.
+ Coll., vol. 67, 1918, p. 134.
+
+Knowledge of the appendages of this species is derived from the
+specimen which Billings described in 1870. It was found in the
+Trenton, probably the Middle Trenton, near Ottawa, Ontario, and is
+preserved in the Victoria Memorial Museum at Ottawa.
+
+Viewed from the upper surface, it shows a large part of the test,
+but is broken along the sides, so that parts of the free cheeks,
+considerable of the pleural lobes of the thorax, and one side of the
+pygidium are missing. Viewed from the lower surface, the appendages
+are practically confined to the cephalon and thorax.
+
+A short time before his death, Professor Beecher had this specimen and
+succeeded in cleaning away a part of the matrix so that the appendages
+show somewhat more clearly than in Billings' time, but they are not so
+well preserved as on the Mickleborough specimen, found in Ohio
+somewhat later.
+
+The hypostoma is in place and well preserved; the posterior points are
+but 3 mm. in advance of the posterior margin of the cephalon. Behind
+the hypostoma there are only two pairs of cephalic appendages, the
+first of which is represented by the coxopodite and a trace of the
+endopodite. The outer end of the coxopodite is close to the outer
+margin of one of the prongs of the hypostoma and about 3 mm. in front
+of its posterior end. The gnathobase curves backward and inward, and
+appears to pass under the tip of the hypostoma. There were probably
+two appendages in front of this, whose gnathobases projected under the
+hypostoma, but the specimen shows nothing of them unless it be that
+one small fragment about 2 mm. back of the center is really a part of
+a gnathobase.
+
+The specimen retains only the coxopodite and basipodite of the
+posterior cephalic appendage on the left side. The coxopodite is
+long and apparently cylindrical, the cross-section being of uniform
+diameter throughout the length. The inner portion is nearly straight,
+while the outer part is curved gently forward.
+
+It is possible to make out remains of eight pairs of appendages on the
+thorax, some of them represented by coxopodites only, but most with
+more or less poorly preserved endopodites as well. No exopodites are
+visible. The coxopodites of the thorax seem to be of the same form
+as the last one on the cephalon, but slightly less curved. All are
+long and heavy, and there seems to be no decrease in size toward the
+pygidium. The endopodites are very imperfectly shown. They seem to be
+longer than those of _Isotelus maximus_, and the segments, while of
+less diameter than the coxopodites, do not show so great a contrast to
+them as do those of that species. The direction of the endopodites is
+diagonally forward, and the outer portions do not appear to be curved
+backward as in _Isotelus maximus_. It would appear also that the
+endopodites were nearly or quite long enough to reach the outer margin
+of the dorsal test. On no endopodite can more than three segments be
+definitely distinguished, but the longest ones are the most obscurely
+segmented.
+
+No appendages are preserved on the pygidium, but at one side of the
+median groove there are two projections which may be processes to
+which the appendages were attached.
+
+_Measurements:_ Total length of specimen, 109 mm. Probable length when
+complete, 116 mm. Length of cephalon, 40 mm.; width at genal angles,
+restored, about 62 mm. (Billings' restoration). Width of doublure of
+front of cephalon on median line, 17 mm.; length of hypostoma, 20 mm.
+Length of coxopodite of last appendage on left side of cephalon,
+10.5 mm.; length of basipodite of the same appendage, 5 mm. Diameter
+of coxopodite, 2 mm.; diameter of basipodite, 1.5 mm. Length of
+coxopodite on left side of the second segment of the thorax, 11 mm.;
+diameter, about 2.5 mm. Length of basipodite of the same, 5 mm.;
+diameter, about 1.5 mm. Length of ischiopodite, 3.5 mm.; diameter,
+about 1.5 mm. Length of meropodite, 2.5 mm. (this may be less than
+the total length as the segment is not completely exposed.) Distance
+between proximal ends of gnathobases of the fifth thoracic segment,
+about 7 mm. Distance between outer ends of the coxopodites of the
+first thoracic segment (estimated from measurements on the left side),
+27 mm Distance apart of the dorsal furrows at the first thoracic
+segment, 27 mm. Length of the longest exopodite which can be traced,
+about 20 mm.
+
+
+=Isotelus maximus= Locke.
+
+(pl. 10, fig. 2.)
+
+ Illustrated: Mickleborough, Jour. Cincinnati Soc. Nat. Hist., vol.
+ 6, 1883, p. 200, figs. 1-3 (endopodites and coxopodites). Walcott,
+ Science, vol. 3, 1884, p. 279, fig. 1 (endopodites, coxopodites,
+ and traces of exopodites). Woodward, Geol. Mag., dec. 3, vol. 1,
+ 1884, p. 162, figs. 1-3 (copies of Mickleborough's figures).
+ Bernard, The Apodidæ, 1892, text fig. 49. Beecher, Amer. Jour.
+ Sci., vol. 13, 1902, p. 169, pl. 5. figs. 5, 6 (outline from one of
+ Mickleborough's figures and an original figure). Walcott, Smithson.
+ Misc. Coll., vol. 67, 1918, p. 133, pl. 24, figs. 3, 3a; pl. 25,
+ fig. 1.
+
+This specimen, which conies from the Richmond strata 2 miles north of
+Oxford, Ohio, is the best preserved of the specimens of _Isotelus_
+with appendages which has so far been found. The individual consists
+of two parts, the actual specimen, and the impression of the ventral
+side.
+
+To describe it I am using very skillfully made plaster reproductions
+of both parts, presented to the Museum of Comparative Zoology by
+Doctor Charles D. Walcott, and presumably made after he cleaned the
+specimen as described in Science (1884). I have also an enlarged
+photograph (pl. 10, fig. 2) which seems to have been made after some
+later period of cleaning, probably by Professor Beecher, and I have
+examined the original specimens in Washington.
+
+Viewed from the dorsal side, it is seen that the individual is very
+imperfect, the greater part of the cephalon being removed by a
+diagonal break which cuts off the anterior third of the left eye and
+extends to the front of the second thoracic segment on the right side.
+The ends of the pleura of both sides of the thorax are broken away, as
+are also the greater parts of the pleural lobes and the posterior end
+of the pygidium. On the ventral side, merely the posterior tips of the
+hypostoma remain, but the distal ends of the appendages were so far
+within the outer margin that the appendagiferous area is quite fully
+retained.
+
+The most conspicuous feature of this specimen is the presence of nine
+pairs of large coxopodites behind the hypostoma, and of the remains of
+ten pairs of endopodites, making in all ten pairs of appendages which
+are easily seen. The apportionment of these segments to cephalon,
+thorax, and pygidium is not agreed upon by the people who have
+examined the specimens, but if one remembers that it is the outer
+and not the inner end of the coxopodite which articulates with the
+appendifer, it at once becomes evident that the first two pairs of
+appendages on the specimen are the last two pairs belonging to the
+cephalon, and that the next eight pairs are those of the thorax.
+
+The impressions of fourteen pairs of coxopodites are readily counted
+on the pygidium, and as Doctor Walcott noted sixteen pairs on the
+actual specimens, his number was probably correct.
+
+_Cephalon._
+
+Projecting the line of the back of the cephalon through from the
+dorsal side, it is found that the posterior tips of the hypostoma are
+7 mm. in front of the posterior margin of the cephalon, and that the
+points of attachment of the posterior pair of cephalic appendages
+(the second pair shown on the specimen) are just within the posterior
+margin. The gnathobases of this pair of appendages extend back some
+distance beneath the thorax, and so give the impression that they
+belong to that part of the body. So far as can be determined, the
+cephalic appendages do not differ in any way from those of the thorax.
+On the mould of the ventral surface, just outside of the lateral edge
+of the right lobe of the hypostoma, is the somewhat imperfectly shown
+impression of the endopodite of the third cephalic appendage. The
+point of junction of the endopodite and coxopodite is about 2 mm. in
+front of the tip of the adjacent branch of the hypostoma, and the
+gnathobase is curved around just behind it. This accounts for three of
+the pairs of cephalic appendages. The second cephalic appendages must
+have thrust their gnathobases under the prongs of the hypostoma, and
+the endopodites were probably close to its edge. No trace of this pair
+appears on the specimen.
+
+_Thorax._
+
+The thoracic appendages are the best preserved of any, and show the
+large coxopodites and the more slender endopodites which do not extend
+to the outer margin of the test. The latter extend forward and outward
+for about one half their length, then turn backward in a graceful
+curve.
+
+Walcott's figure in Science shows hair-like markings on the under
+side of the right half of the thorax. These were interpreted by both
+Walcott and Beecher as fringes of the exopodites, but since the
+setæ of those organs on all other trilobites are always above the
+endopodites, while these are represented as below them, it would seem
+doubtful if this interpretation can be sustained. Furthermore, I find
+no trace of them on either cast or mould, and the actual specimen does
+not now show them.
+
+_Pygidium._
+
+The coxopodites and endopodites of the pygidium seem to be similar
+to those on the thorax, but both are shorter and more slender, and
+the former decrease in length rapidly toward the posterior end. As
+mentioned above, it is not perfectly plain how many appendages are
+present, but I have accepted Doctor Walcott's count of sixteen pairs.
+Of the endopodites only the barest traces are seen, and of exopodites
+nothing.
+
+One point of considerable interest in this specimen is the thickness,
+as it probably gives some measure of the space occupied by the animal.
+In _Triarthrus_ and other trilobites from Rome, New York, the
+appendages are pressed directly against the dorsal test, but in this
+specimen a considerable space intervenes between the plane of the
+appendages and the shell. Between the central furrow and the inner
+surface of the dorsal test at the anterior end of the thorax is a
+distance of 13 mm. and under the dorsal furrows the thickness is about
+7 or 8 mm., no accurate measurement being possible in the present
+state of the specimen.
+
+_Measurements:_ Length of specimen on median line, 121 mm.; probable
+original length, about 195 mm. (Walcott's restoration). Length of
+thorax, 58 mm.[1] Width of axial lobe at the first thoracic segment,
+45 mm.; total width as preserved, 92 mm.; width as estimated from the
+mould of the ventral surface, no mm.; Walcott's restoration, 105 mm.
+
+[Footnote 1: If this specimen had the same proportions as specimens of
+_Isotelus maximus_ from Toronto, the total length would be only 174
+mm. The cephalon would be about 52 mm. long, the thorax 58 mm., and
+the pygidium about 64 mm. long.]
+
+Length of coxopodite of fourth left cephalic appendage, about 18 mm.;
+diameter, about 2.5 mm. Length of coxopodite of last left cephalic
+appendage, about 18.5 mm. Distance apart of inner ends of gnathobases
+of fourth cephalic appendages, about 4 mm. Distance apart of inner
+ends of endobases of first thoracic segment, about 6 mm. Distance
+apart of outer ends of coxopodites of first thoracic segment, about 43
+mm.
+
+Length of coxopodite of seventh left thoracic appendage 16 mm.,
+diameter about 3.5 mm.; length of basipodite of the endopodite of the
+same appendage 6 mm.; diameter about 2 mm.; length of ischiopodite 5
+mm.; length of meropodite 4.5 mm.; length of carpopodite 4.5 mm.;
+length of propodite 3 mm.; length of dactylopodite 2.75 mm.; total
+length of endopodite 25.75 mm.
+
+Length of coxopodite of fourth left thoracic appendage 20 mm.,
+diameter 4 mm.; length of five proximal joints of the endopodite 25
+mm.; diameter of basipodite about 2 mm.
+
+
+RESTORATION OF ISOTELUS.
+
+(Text fig. 9.)
+
+The exopodites have been omitted from this restoration since nothing
+is known of their actual form. The chief reason for the figure is to
+contrast the greatly developed coxopodites of the posterior part of
+the cephalon and thorax with those of other trilobites. The antennules
+and first two pairs of biramous appendages of the cephalon are more or
+less hypothetical, and less is known of the appendages of the pygidium
+than is shown here. The restoration is based somewhat upon Walcott's
+figure in Science. The outline is that of a specimen of _Isotelus
+maximus_ from Toronto, Ontario.
+
+
+=Isotelus gigas= Dekay.
+
+ Illustrated: Woodward, Quart. Jour. Geol. Soc., London, vol. 26,
+ 1870, text fig. 1; Geol. Mag., dec. 3, vol. 1. 1884, p. 78, text
+ fig. Milne-Edwards, Ann. Sci. Nat, Zoologie, ser. 6, vol. 12, 1881,
+ pl. 12, fig. 46. Walcott, Bull. Mus. Comp. Zool., Harvard Coll.,
+ vol. 8, 1881, pl. 2, fig. 9; Geol. Mag., dec. 4, vol. 1, 1894, pl.
+ 8, fig. 9; Proc. Biol. Soc. Washington, vol. 9, 1894, pl. 1, fig.
+ 9.
+
+The specimen in the British Museum which Woodward called _Asaphus
+platycephalus_, is, in all probability, an _Isotelus gigas_. Woodward
+says of it:
+
+ I was at once attracted by a specimen of _Asaphus_, from the Black
+ Trenton Limestone (Lower Silurian), which has been much eroded on
+ its upper surface, leaving the hypostoma and what appear to be
+ the appendages belonging to the first, second, and third somites,
+ exposed to view, united along the median line by a longitudinal
+ ridge. The pseudo-appendages, however, have no evidence of any
+ articulations. But what appears to me to be of the highest
+ importance, as a piece of additional information afforded by
+ the Museum specimen, is the discovery of what I believe to be
+ the _jointed palpus_ of one of the maxillæ, which has left its
+ impression upon the side of the hypostoma--just, in fact, in that
+ position which it must have occupied in life, judging by other
+ Crustaceans which are furnished with an hypostoma, as _Apus_,
+ _Serolis_, etc.
+
+ The palpus is 9 lines in length, the basal joint measures 3 lines,
+ and is 2 lines broad, and somewhat triangular in form.
+
+ There appear to be about 7 articulations in the palpus itself,
+ above the basal joint, marked by swellings upon its tubular stem,
+ which is 1 line in diameter.
+
+
+[Illustration: Fig. 9.--A restored composite of _Isotelus maximus_ and
+_I. latus_. The exopodites are left out because entirely unknown.
+Drawn by Doctor Elvira Wood. Natural size.]
+
+Desiring to know more of this individual, I wrote to Doctor Bather
+and was surprised to learn that the specimen which was the basis of
+Woodward's observations is so badly preserved as to be of no real
+value. With his permission, I append a note made by Doctor Bather
+some years ago when selecting fossils to be placed on exhibition:
+
+ _Asaphus gigas_ Dekay. Ordovician, Trenton Limestone. N. America,
+ Canada. Descr. H. Woodward, 1870, Q. J. G. S., XXVI, pp. 486-488,
+ text fig. 1, as _Asaphus platycephalus_. Coll. and presd. J. J.
+ Bigsby, 1851. Regd. I 14431.
+
+ This specimen is in the Brit. Mus. Geol. Dept. I 14431. The
+ supposed hypostome is exceedingly doubtful; it lies dorsad of the
+ crushed glabellar skeleton. The "appendage" is merely the edge of
+ a part in the head-shield; the maxilla is some calcite filling,
+ between two such laminæ.
+
+ 13 Sept. 1911. (Signed) F. A. BATHER.
+
+Walcott figured a slice of _Isotelus gigas_ from Trenton Falls, New
+York, which shows a few fragments of appendages, but is of particular
+importance because it shows the presence of well developed appendifers
+beneath the axial lobe.
+
+
+=Isotelus arenicola= Raymond.
+
+ Illustrated: Ottawa Nat, vol. 24, 1910, p. 129, pl. 2, fig. 5.
+
+The following quotations from my paper are inserted here to complete
+the record of appendage-bearing specimens:
+
+ A rather remarkable specimen of this species was found by W. C.
+ King, Esq., on the shore of Lake Deschenes at Britannia [near
+ Ottawa, Ontario]. This specimen is an impression of the lower
+ surface of the trilobite, and shows a longitudinal ridge
+ corresponding to the central furrow along the axis of the ventral
+ side of the animal, ten pairs of transverse furrows, and the
+ impression of the hypostoma. The doublure of the pygidium has
+ also left a wide smooth impression, but in the cephalic region
+ the hypostoma is the only portion of which there are any traces
+ remaining. The specimen was found on a waterworn surface of the
+ beach, partially covered by shingle....
+
+ The transverse furrows are the impressions left by the gnathobases
+ of the basal joints of the legs. They were evidently long and very
+ heavy, but the specimen has been so abraded that all details are
+ obscured. The first six pairs of impressions are longer and deeper
+ than the four behind. The first eight pairs seem to pertain to the
+ thoracic appendages, while the last two belong to the pygidium.
+ From the posterior tips of the hypostoma to the first gnathobases
+ of which traces are present there is a distance of about 22 mm.
+ without impressions. In _Isotelus gigas_ the hypostoma normally
+ extends back to the posterior margin of the cephalon, so that it
+ seems that in this specimen the impressions of the first two pairs
+ of gnathobases under the thorax may not have been preserved. In
+ that case, the six pairs of strong impressions may represent the
+ last six pairs of thoracic segments, and the pygidium might begin
+ with the first of the fainter ones.
+
+_Horizon and locality:_ From the sandstone near the base of the Aylmer
+(Upper Chazy) formation at Britannia, west of Ottawa, Ontario.
+Specimen in the Victoria Memorial Museum, Geological Survey of Canada,
+Ottawa.
+
+
+
+
+The Appendages of Triarthrus.
+
+
+=Triarthrus becki= Green.
+
+(Pls. 1-5; pl. 6, figs. 1-3; text figs. 1, 10, 11, 33, 42.)
+
+(Also see Part IV.)
+
+ Illustrated: Matthew, Amer. Jour. Sci., vol. 46, 1893, pl. 1, figs.
+ 1-7;--Trans. N. Y. Acad. Sci., vol. 12, pl. 8, figs. 1-7.--Beecher,
+ Amer. Jour. Sci., vol. 46, 1893, text figs. 1-3;--Amer. Geol., vol.
+ 13, 1894, pl. 3;--Amer. Jour. Sci., vol. 47, pl. 7, text fig.
+ 1;--Amer. Geol., vol. 15, 1895, pls. 4, 5;--Ibid., vol. 16, 1895,
+ pl. 8, figs. 12-14; pl. 10. fig. 1;--Amer. Jour. Sci.,
+ vol. 1, 1896, pl. 8; Geol. Mag., dec. 4, vol. 3, 1896, pl.
+ 9;--Eastman-Zittel Text-book of Paleontology, vol. 1, 1900, text
+ figs. 1267-1269;--2d ed., 1913, fig. 1375; Studies in Evolution,
+ 1901, reprint of all previous figs.;--Amer. Jour. Sci., vol. 13,
+ 1902, pl. 2, figs. 1-5; pl. 3, fig. 1; pl. 4, fig. 1; pl. 5, figs.
+ 2-4;--Geol. Mag., dec. 10, vol. 9, 1902, pls. 9-11, text figs.
+ 1-3.--Walcott, Proc. Biol. Soc. Washington, vol. 9, 1894, pl. 1
+ figs. 1-6;--Geol. Mag., dec. 4, vol. 1, 1894, pl. 8;--Smithson.
+ Misc. Coll., vol. 67, 1918, pl. 29, figs. 1-11; pl. 30, figs.
+ 17-20; pl. 32; pl. 34, figs. 4-7; pl. 35, fig. 5.--Bernard, Quart.
+ Jour. Geol. Soc., London, vol. 50, 1894, text figs. 11,
+ 12.--Oehlert, Bull. Soc. Géol. France, ser. 3, vol. 24, 1896,
+ text figs. 1-17, 34.--Jaekel, Zeits. d. d. geol. Gesell., vol. 53,
+ 1901, text fig. 24. Moberg, Geol. Fören. Förhandl., vol. 29, pl. 5,
+ 1907, pl. 4, fig. 2; pl. 5, fig. 1.--Handlirsch, Foss. Insekten,
+ 1908, text fig. 6.--Tothill, Amer. Jour. Sci., vol. 42, 1916, p.
+ 380, text fig. 5.--Crampton, Jour. N. Y. Entomol. Soc., vol. 24,
+ 1917, pl. 2, fig. 20.
+
+
+
+
+Historical.
+
+
+Specimens of _Triarthrus_ retaining appendages were first obtained by
+Mr. W. S. Valiant from the dark carbonaceous Utica shale near Rome,
+New York, in 1884, but no considerable amount of material was found
+until 1892. The first specimens were sent to Columbia University, and
+were described by Doctor W. D. Matthew (1893). This article was
+accompanied by a plate of sketches, showing for the first time the
+presence of antennules in trilobites and indicating something of the
+endopodites and exopodites of the appendages of the cephalon, thorax,
+and pygidium. Specimens had not yet been cleaned from the lower side,
+so that no great amount could then be learned of the detailed
+structure. Matthew concluded that "The homology with _Limulus_ seems
+not to be as close in _Triarthrus_ as in the forms studied by Mr.
+Walcott; but the characters seem to be of a more comprehensive type,
+approaching the general structure of the other Crustacea rather than
+any special form."
+
+Professor Beecher's first paper, dated October 9, 1893, merely
+mentioned the fact that the Yale University Museum had obtained
+material from Valiant's locality, but was quickly followed by a paper
+read before the National Academy of Sciences on November 8, and
+published in December, 1893. This paper described particularly the
+thoracic appendages.
+
+This was followed in January (1894 A) by an article in which some
+information about the mode of occurrence of the specimens was added,
+and in April (1894 B), the limbs of the pygidium were described and
+figured. The determination of the structure of the appendages of the
+head evidently presented some difficulty, for the article describing
+this portion of the animal did not appear until the next February
+(1895 A). This cleared up the ventral anatomy of _Triarthrus_, and was
+followed by a short article (1896 A) accompanied by a restoration of
+the trilobite showing all the appendages.
+
+This ended Professor Beecher's publications on _Triarthrus_ until his
+final paper in 1902, although he contributed some of his results and
+figures to his chapter on the trilobites in the Eastman-Zittel
+Text-book of Paleontology in 1900.
+
+The discovery of these excellent specimens had of course excited very
+great interest. Doctor Walcott also studied a number of specimens from
+Valiant's locality, and published in 1894, with some original figures,
+the results of his comparison of the appendages of _Triarthrus_ with
+those of _Calymene_ and _Ceraurus_.
+
+In his article on the "Systematic Position of the Trilobites," Bernard
+(1894) used the results of Professor Beecher's studies of 1893, and
+also quoted the papers by Matthew (1893) and Walcott (1894), though
+the article by the latter appeared too late to be used except for a
+note added while Bernard's paper was in press. A final footnote quoted
+from Professor Beecher's paper of April, 1894 (1894 B).
+
+Oehlert (1896) gave an excellent summary in French of the work of
+Beecher and Walcott on _Triarthrus_, with reproductions of many of
+their figures.
+
+Valiant (1901) in a non-technical article described his long search
+for trilobites with antennas. The discovery of the wonderful pyritized
+trilobites at Cleveland's Glen near Rome was not the result of a lucky
+accident, but the culmination of eight years of labor in a locality
+especially selected on account of the fineness of grain of the shale.
+
+[Illustration: Fig. 10.--_Triarthrus becki_ Green. A new restoration,
+modified from Professor Beecher's, to incorporate the results of his
+later work. The inner ends of the endobases are probably too far
+apart, as it was not discovered until after the drawing had been made
+that the appendifers projected within the dorsal furrows. Drawn by
+Doctor Elvira Wood. × about 3.8.]
+
+After 1896, Professor Beecher turned his attention largely to the
+problem of the classification of trilobites, and while he continued
+the arduous task of cleaning the matrix from specimens of
+_Triarthrus_ and _Cryptolithus_ he did not again publish upon the
+subject of appendages until forced to do so by the doubts cast by
+Jaekel (1901) upon the validity of his earlier conclusions. Because of
+certain structures which he thought he had interpreted correctly from
+a poorly preserved specimen of _Ptychoparia_, Jaekel came to the
+conclusion that Beecher's material was not well preserved. Professor
+Beecher would have taken much more kindly to aspersions upon his
+opinions than to any slight upon his beloved trilobites, and his
+article on the "Ventral Integument of Trilobites" of 1902 was designed
+not only as an answer to Jaekel, but also to show by means of
+photographs the unusually perfect state of preservation of the
+specimens of _Triarthrus_. This article, like so many describing the
+appendages of trilobites, beginning with Matthew's, was published in
+two places (Beecher 1902).
+
+Most of Beecher's papers, except the last one, were reprinted in
+the volume entitled "Studies in Evolution," published by Charles
+Scribner's Sons at the time of the Yale Bicentennial in 1901. The
+part pertaining particularly to _Triarthrus_ is on pages 197 to 219.
+
+Moberg (1907), in connection with a specimen of _Eurycare angustatum_
+which he thought preserved some appendages, described and illustrated
+some of the appendages of _Triarthrus_.
+
+The most recent discussion of _Triarthrus_, with some new figures,
+is by Walcott (1918, p. 135, pls. 29, 30). He gives a summary of
+Beecher's work with numerous quotations. The principal original
+contribution is a discussion of the form and shape of the appendages
+before they were flattened out in the shale. He found also what
+he thought might possibly be the remains of epipodites on three
+specimens, one of which he illustrated with a photograph. I have seen
+nothing which could be interpreted as such an organ in the many
+specimens I have studied.
+
+A point in which Walcott differs from Beecher in the interpretation of
+specimens is in regard to the development of the endopodites of small
+pygidia. Beecher (1894 B, pl. 7, fig. 3) illustrated a series of
+endopodites which he likened to the endites of a thoracic limb of
+_Apus_. Doctor Walcott finds that specimens in the United States
+National Museum show slender endopodites all the way to the back of
+the pygidium, and thinks that Beecher mistook a mass of terminal
+segments of exopodites for a series of endopodites. On careful
+examination, however, the specimen shows, as Beecher indicated, a
+series of endopodites in undisturbed condition (No. 222, our pl. 4,
+fig. 5).
+
+_Restoration of Triarthrus._
+
+One of the more important points noted in the later studies of
+_Triarthrus_ is that the gnathites of the cephalic appendages are much
+less like the endobases under the thorax than Beecher earlier thought,
+and showed in his restored figures and in his model. The four
+gnathites of each side are curved, flattened, not club-shaped, and
+so wide and so close together that they overlap one another. The
+metastoma is somewhat larger and more nearly circular than Beecher's
+earlier preparations led him to suppose.
+
+The restoration here presented is modified only slightly from the
+one designed by Professor Beecher, and the modifications are taken
+principally from figures published by him. The gnathites are drawn in
+form more like that shown by the specimens and his figures in the
+American Geologist (1895 A), and the metastoma is taken from one of
+the specimens. On the thorax the chief modification is in the addition
+of a considerable number of spines to the endopodites. In spite of the
+trivial character of most of these changes, they emphasize one of the
+important characteristics of _Triarthrus_ the regional differentiation
+of the appendages.
+
+It should be pointed out that although _Triarthrus_ is usually
+considered to be a very primitive trilobite, its appendages are more
+specialized than those of any of the others known. This is shown in
+their great length, the double curvature of the antennules, the
+differentiation of four pairs of endobases on the cephalon as
+gnathites, and the flattening of the segments of the posterior
+endopodites. These departures from the uniformity existing among the
+appendages of the other genera lead one to question whether the genus
+is really so primitive as has been supposed.
+
+_Relation of the Cephalic Appendages to the Markings on the Dorsal
+Surface of the Glabella._
+
+_Triarthrus becki_ is usually represented as having four pairs of
+glabellar furrows, but the two pairs at the front are exceedingly
+faint and the first of them is hardly ever visible, though that it
+does exist is proved by a number of authentic specimens. The neck
+furrow is narrow and sharply impressed, continuing across the glabella
+with a slightly backward curvature. In front of it are two pairs of
+linear, deeply impressed furrows which in their inward and backward
+sweep are bowed slightly forward, the ends of the corresponding
+furrows on opposite sides nearly meeting along the crest of the
+glabella. In front of these, near the median line, is a pair of slight
+indentations, having the appearance and position of the inner ends of
+a pair of furrows similar to those situated just behind them.
+
+In front of and just outside this pair are the exceedingly faint
+impressions of the anterior pair of furrows, these, as said above,
+being but seldom seen. They are short, slightly indented linear
+furrows which have their axes perpendicular to the axis of the
+cephalon, and do not connect with each other or with the dorsal
+furrows. The latter are narrow, sharply impressed, and merge into a
+circumglabellar furrow at the front. In front of the circumglabellar
+furrow is a very narrow rounded ridge, but the anterior end of the
+glabella is very close to the margin of the cephalon.
+
+Specimen No. 214, which was cleaned from the dorsal side, shows the
+posterior tip of the hypostoma, apparently in its natural position,
+3.5 mm. back from the anterior margin. The entire length of the
+cephalon is 6 mm., so that the hypostoma reaches back slightly over
+one half the length (0.583). The greater part of it has been cleaned
+off, and one sees the proximal portions of the antennules, which are
+apparently attached just at the sides of the hypostoma, 2.5 mm. apart
+and 2.25 mm. back from the anterior edge of the cephalon. This
+position is distinctly within the outline of the glabella and
+corresponds approximately to the location of the second pair of
+glabellar furrows. Specimens 214, 215, 216, 217, and 219 all seem to
+show the same location for the bases of the antennules. Specimen 220
+is the one in which the basal shafts are best preserved and the points
+of attachment seem to be further apart in it than in any of the
+others. This specimen is 38 mm. long, and the bases of the antennules
+are 5.5 mm. apart and 4 mm. behind the anterior margin. As the
+specimen is cleaned from the ventral side, the dorsal furrows do not
+show distinctly, but another specimen of about the same size (No. 228,
+38.5 mm. long) has the dorsal furrows 8 mm. apart 4 mm. back of the
+anterior margin.
+
+On the same slab with specimens 209 and 210 there is an individual
+which, although retaining the test, has had the proximal ends of the
+antennules so pressed against it that the course of the one on the
+left side is readily visible. It originates in a small oval mound
+whose posterior margin impinges upon the third glabellar furrow near
+the middle of its course, and just outside the outer end of the second
+glabellar furrow. The cephalon of this specimen is 5 mm. long, and the
+point of origin of the left antennule is 2.75 mm. in front of the
+posterior margin and 0.75 mm. from the dorsal furrow.
+
+It is therefore evident that the antennules in this species are not
+attached beneath the dorsal furrows, but within them and opposite the
+second pair of glabellar furrows.
+
+All cephalic appendages behind the antennules are attached somewhat
+within the dorsal furrows, the first pair as far forward as the
+antennules and the last pair apparently under the anterior edge of
+the neck ring. They do not appear to correspond in position to the
+posterior glabellar furrows and neck ring, being more crowded. The
+last pair is attached to appendifers beneath the nuchal segment, and
+the first pair beneath the third glabellar furrows. There are no
+depressions on the dorsal surface corresponding to the points of
+attachment of the mandibles.
+
+Anal Plate.
+
+Professor Beecher, during his first studies of _Triarthrus_, found no
+appendages pertaining to the anal segment, but later evidently came
+upon a spinose anal plate which he caused to be figured. The specimen
+(No. 201) on which this appendage is preserved is cleaned from the
+dorsal side, and the anal plate is a small, bilaterally symmetrical,
+nearly semicircular structure margined with small spines. Specimen 202
+also shows the same plate (pl. 5, fig. 6), but it is imperfectly
+preserved. It has a large perforation in the anterior half. Both of
+these specimens are in the Yale University Museum.
+
+[Illustration: Fig. 11.--_Triarthrus becki_ Green. Anal plate of
+specimen 65525 in the U. S. National Museum. Drawn by Doctor Wood. ×
+20.]
+
+The anal plate is especially well shown by specimen 65525 in the
+United States National Museum (fig. 11). This specimen is from Rome,
+New York, and two photographs of it have been published by Walcott
+(1918, pl. 29, fig. 6; pl. 30, fig. 19). It is developed from the
+dorsal side, and the anal plate is displaced, so that it projects
+behind the end of the pygidium. It is semicircular in shape, with a
+hemispheric mound at the middle of the anterior half. Two furrows
+starting from the anterior edge on either side of the mound border its
+sides, and, uniting back of it, continue as an axial furrow to the
+posterior margin. The mound is perforated for the opening of the
+posterior end of the alimentary canal. The lateral borders of the
+plate bear five pairs of short, symmetrically placed spines. The plate
+is 1 mm. wide and 0.5 mm. long, and the entire trilobite is 11.5 mm.
+long.
+
+
+
+
+THE APPENDAGES OF PTYCHOPARIA.
+
+
+=Ptychoparia striata= (Emmrich).
+
+ Illustrated: Jaekel, Zeits. d. d. geol. Gesell., 1901, vol. 53,
+ part 1, pls. 4, 5.
+
+Jaekel has described a specimen of this species obtained from the
+Middle Cambrian near Tejrovic, Bohemia, which on development showed
+beneath the test of the axial lobe, certain structures which he
+believed represented the casts of proximal segments of appendages.
+On the basis of this specimen he produced a new restoration of the
+ventral surface of the trilobite, in which he showed three short wide
+segments in the place occupied by the coxopodite of an appendage of
+_Triarthrus_. He also made the mouth parts considerably different from
+those of the latter genus. Beecher (1902) showed that the structures
+which Jaekel took for segments of appendages were really the fillings
+between stiffening plates of chitin on the ventral membrane, and
+demonstrated the fact that similar structures existed in _Triarthrus_.
+It cannot be said, therefore, that any appendages are really known in
+_Ptychoparia striata_, but some knowledge of the internal anatomy of
+the species is supplied by the specimen.
+
+
+=Ptychoparia cordilleræ= (Rominger).
+
+ Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 192,
+ pl. 24, fig. 2;--Ibid., vol. 67, 1918, pl. 21, figs. 3-5 (corrected
+ figure).
+
+Walcott has figured a single individual of this species showing
+appendages, the accompanying description being as follows (1918, p.
+144):
+
+ Ventral appendages. Only one specimen has been found showing the
+ thoracic limbs. This indicates very clearly the general character
+ of the exopodite and that it is situated above the endopodite,
+ although there are only imperfect traces of the latter....
+
+ The exopodites are unlike those of any trilobite now known. They
+ are long, rather broad lobes extending from the line of the union
+ of the mesosternites and the pleurosternites. At the proximal end
+ they appear to be as wide as the axial lobe of each segment, and to
+ increase in width and slightly overlap each other nearly out to the
+ distal extremity.... They are finely crenulated along both the
+ anterior and dorsal margins, which indicates the presence of fine
+ setæ.
+
+The specimen is quite imperfectly preserved, but seems to indicate
+that the exopodite of Ptychoparia had a long, rather narrow
+unsegmented shaft.
+
+_Measurements_ (from Walcott's figure): The specimen is a small one,
+about 9.5 mm. long, an individual exopodite is about 2 mm. long and
+the shaft 0.33 mm. wide.
+
+_Horizon and locality:_ Middle Cambrian, Burgess shale, between Mount
+Field and Wapta Peak, above Field, British Columbia.
+
+
+=Ptychoparia permulta= Walcott.
+
+ Illustrated: Walcott, Smithson. Misc. Coll., vol. 67, 1918, p. 145,
+ pl. 21, figs. 1, 2.
+
+Walcott figured one individual of this species showing long slender
+antennules projecting in front of the cephalon. It is of especial
+interest because one of the antennules shows almost exactly the same
+sigmoid curvature which is so characteristic of the related
+_Triarthrus_. The individual segments are not visible.
+
+_Measurements:_ The specimen is 23 mm. long and the direct distance
+from the front of the head to the anterior end of the more perfect
+antennule is 9.5 mm. Measured along the curvature, the same antennule
+is about 11 mm. long.
+
+_Horizon and locality:_ Same as the preceding.
+
+
+
+
+The Appendages of Kootenia.
+
+
+=Kootenia dawsoni= Walcott.
+
+ Illustrated: Walcott, Smithson. Misc. Coll., vol. 67, 1918, pl. 14,
+ figs. 2, 3.
+
+One specimen figured by Doctor Walcott shows the distal ends of some
+of the exopodites and endopodites of the right side. He compares the
+exopodites with those of Neolenus, stating that the shaft consists
+of two segments, the proximal section being long and flat, fringed
+with long setæ, while the distal segment has short fine setæ. The
+endopodite best shown is very slender, and the segments are of uniform
+width and only slightly longer than wide.
+
+Measurements (from Walcott's figures): Length of specimen, about 41
+mm. Length of five distal segments of an endopodite, 7.5 mm. Since
+the pleural lobe is only 7 mm. wide, the endopodites, and probably
+the exopodites also, must have projected a few millimeters beyond the
+dorsal test when extended straight out laterally.
+
+Formation and locality: Burgess shale, Middle Cambrian, on the west
+slope of the ridge between Mount Field and Wapta Peak, above Field,
+British Columbia.
+
+
+
+
+The Appendages of Calymene and Ceraurus.
+
+
+HISTORICAL.
+
+All of the work on these species has been done by Doctor Walcott, who
+summarized his results in 1881.
+
+In the first of his papers (1875, p. 159), Walcott did not describe
+any appendages but paved the way for further work by a detailed and
+accurate description of the ventral surface of the dorsal shell of
+Ceraurus. He demonstrated the presence in this species of strongly
+buttressed processes which extend directly downward from the test just
+within the line of the dorsal furrows. One pair of these is seen
+beneath each pair of the glabellar furrows, each segment of the thorax
+has a pair, and there are four pairs on the pygidium. He pointed out
+also that these projections were but poorly developed on that part of
+the glabella which is covered by the hypostoma. He called them axial
+processes, the only name which appears to have been suggested thus
+far.
+
+The first announcement of the discovery of actual appendages in
+_Ceraurus_ and _Calymene_ was made by the same investigator in a
+pamphlet published in 1876 in advance of the 28th Report of the New
+York State Museum of Natural History, the publication of the whole
+report being delayed till 1879. The results were obtained by the
+process of cutting translucent slices of enrolled trilobites derived
+from the Trenton limestone at Trenton Falls, New York. Since he
+summarized all the results of this study in one paper at a later
+date, it is not necessary to follow the stages of the work.
+
+A second preliminary paper was published in pamphlet form in
+September, 1877, and in final form in 1879, when the first figures
+were presented.
+
+In his important paper of 1881, Walcott reviewed all that was known of
+the appendages of trilobites to that time, and gave the results of
+seven years of study of sections of enrolled specimens. Slices had
+been made of 2,200 individuals from Trenton Falls, which resulted in
+obtaining 270 which were worthy of study. Of these, 205 were from
+_Ceraurus pleurexanthemus_, 49 from _Calymene senaria_, 11 from
+_Isotelus gigas_, and 5 from _Acidaspis trentonensis_.
+
+Walcott's views on certain portions of the anatomy can best be set
+forth in the form of a few extracts (1881, pp. 199-208):
+
+_The Ventral Membrane._--In those longitudinal sections in which the
+ventral membrane is most perfectly preserved, it is shown to have been
+a thin, delicate pellicle or membrane, strengthened in each segment by
+a transverse arch, to which the appendages were attached. These arches
+appear as flat bands separated by a thin connecting membrane, somewhat
+as the arches in the ventral surface of some of the Macrouran
+Decapods....
+
+In by far the greater number of sections, both transverse and
+longitudinal, the evidence of the former presence of an exterior
+membrane, protecting the contents of the visceral cavity, rests on the
+fact that the sections show a definite boundary line between the white
+calcspar, filling the space formerly occupied by the viscera, and the
+dark limestone matrix. Even the thickened arches are rarely seen.
+
+The mode of attachment of the leg to the ventral surface is shown [in
+transverse and longitudinal sections of _Ceraurus_ and _Calymene_].
+These illustrations are considered as showing that the point of
+articulation was a small, round process projecting from the posterior
+surface of the large basal joint, and articulating in the ventral arch
+somewhat as the legs of some of the Isopods articulate with the arches
+in the ventral membrane. The arches of the ventral membrane in the
+trilobite ... afford a correspondingly firm basis for the attachment
+of the legs.
+
+Branchial appendages.--The branchiæ have required more time and labor
+to determine their true structure than any of the appendages yet
+discovered. They were first regarded as small tubes arranged side by
+side, like the teeth in a rake; then as setiferous appendages, and
+finally as elongate ribbon-like spirals and bands attached to the side
+of the thoracic cavity, the epipodite being a so-called branchial arm.
+All of these parts are now known to belong to the respiratory system,
+but from their somewhat complex structure, and the various curious
+forms assumed by the parts when broken up and distorted, it was a long
+time before their relations were determined.
+
+The respiratory system is formed of two series of appendages, as found
+beneath the thorax. The first is a series of branchiæ attached to the
+basal joints of the legs, and the second, the branchial arms, or
+epipodites.
+
+The branchiæ, as found in _Calymene_, _Ceraurus_, and _Acidaspis_,
+have three forms. In the first they bifurcate a short distance from
+the attachment to the basal joint of the leg, and extend outward and
+downward as two simple, slender tubes, or ribbon-like filaments.
+In the second form they bifurcate in the same mariner, but the two
+branches are spirals. These two forms occur in the same individual
+but, as a rule, the more simple ribbon-like branchia is found in the
+smaller or younger specimens, and the spiral form in the adult.... The
+spiral branchiæ of Ceraurus are usually larger and coarser than those
+of _Calymene_.
+
+The third type of the branchiæ [consists of rather long straight
+ribbons arranged in a digitate manner on a broad basal joint]. As far
+as yet known, this is confined to the anterior segments of the thorax.
+
+The epipodite or branchial arm was attached to the basal joints of the
+thoracic legs and formed of two or more joints. This has been called a
+branchial arm, not that it carried a branchia, but on account of its
+relation to the respiratory system. It is regarded as an arm or
+paddle, that, kept in constant motion, produced a current of water
+circulating among the branchiæ gathered close beneath the dorsal
+shell. . . .
+
+Of the modification the respiratory apparatus underwent beneath the
+pygidium, we have no evidence.
+
+In his latest publication (1918, pp. 147-153, pls. 26-28, 33), Walcott
+has reviewed his earlier work on _Calymene_ and _Ceraurus_, and
+presented a new restoration of the former. The coxopodites are now
+interpreted as being similar to those of _Triarthrus_ and Neolenus,
+but the exopodites are still held to be spiral and the setiferous
+organs labelled as epipodites rather than exopodites.
+
+
+
+
+Comparison of the Appendages of Calymene and Ceraurus with those of
+Triarthrus.
+
+
+As one may see by reading the above quotations from Doctor Walcott's
+descriptions, he found certain branchial organs in _Ceraurus_ and
+_Calymene_ which have not been found in other trilobites but otherwise
+the essential features of the appendages of all are in agreement.
+
+Spiral Branchiæ.
+
+It is now necessary to inquire if the thin sections can not be
+interpreted on the basis of trilobites with the same organs as
+_Triarthrus_. The interpretation of the structures seen in these
+translucent slices is exceedingly difficult, and Doctor Walcott
+deserves the utmost praise for the acumen with which he drew his
+deductions. Even with the present knowledge of _Triarthrus_,
+_Isotelus_, and _Neolenus_ as a guide, I do not think it is safe to
+speak dogmatically about what one sees in them.
+
+Walcott has summarized his results in his restoration of the
+appendages of _Calymene_ (1918, pl. 33). The coxopodite supports a
+slender six-jointed endopodite as in _Triarthrus_, dorsal to which is
+a short setiferous epipodite which differs from the exopodite of
+_Triarthrus_, in being less long, unsegmented, and in having shorter
+setæ. Arising from the same part of the coxopodite with this epipodite
+is the bifurcate spiral branchia which has not been seen in this form
+in other trilobites. The evidence on which the existence of this organ
+is postulated consists of a series of sections across the thorax, the
+best of them figured by Walcott in his plates 2 and 3 (1881) and plate
+27 (1918).
+
+The specimens sliced were all partially or quite enrolled, and in that
+position one would expect to find the appendages so displaced that it
+would be only rarely that a section would be cut, either by chance or
+design, in such a direction as to show any considerable part of any
+one appendage. This expectation has proved true in regard to the
+endopodites, the sections rarely showing more than two or three
+consecutive segments. Sections like those shown in figures 1 and 2
+in plate 2 (1881) seem to be unique. On the other hand, there are
+numerous slices showing the so-called spiral branchiæ. They show for
+the most part as a succession of rectangular to kidney-shaped spots
+of clear calcite.[1] Usually these clear spots are isolated, not
+confluent, but in a small number of specimens, perhaps three or four,
+the spots are connected in such a way as to show a zig-zag band which
+suggests a spiral. Such an explanation is of course entirely
+reasonable, but it would be surprising if so slender a spiral should
+be cut in such a way as to exhibit the large series of successive
+turns shown in many of these thin sections. Continuous sections of
+such organs should be no more common than continuous sections of
+endopodites.
+
+[Footnote 1: In looking at Walcott's figures of 1881, it should be
+remembered that the dark portions of the figures are clear calcite in
+the specimens, while the light part is the more or less opaque
+matrix.]
+
+One of the arguments against the interpretation of these series of
+spots as sections across spiral arms is that of probabilities. It
+is known from flattened specimens that _Neolenus_, _Kootenia_,
+_Ptychoparia_, _Triarthrus_, and _Cryptolithus_ all have a single type
+of exopodite, consisting of a simple setiferous shaft. All these
+genera have been examined in a way that permits no doubt about the
+structure, and no trace of spiral arms has been detected. On the other
+hand, Walcott found spiral arms in three unrelated genera, _Calymene_,
+_Ceraurus_, and _Acidaspis_, all of the trilobites in which he found
+exopodites by the method of sectioning. What are the probabilities
+that genera of three different families, studied by means of sections,
+should agree in having a type of exopodite different from that of the
+five genera about whose interpretation there can be no doubt?
+
+Another argument against the interpretation of the sections as spirals
+is that in any one line the individual spots are of roughly uniform
+size. This means of course that the spiral has been cut by a plane
+parallel to the tangent plane. This might happen once, just as once
+Doctor Walcott cut all six segments of a single endopodite, but that
+it should happen repeatedly is highly improbable. Moreover, there is
+a limit to the diameter of the section which may be made from these
+slender spirals. Most of the spots have one diameter about one half
+greater than the other, but others are from three to six times as long
+as wide. These last could obviously be cut only from a very large
+spiral, and they are therefore interpreted by Walcott as setæ of
+epipodites. Yet all gradations are found among the sections, from the
+long setæ to the short dots. (See pl. 27, 1918.) In referring to one
+slice, Walcott says (1918, p. 152):
+
+In the latter figure and in figure 13, plate 27, the setæ of several
+epipodites appear to have been cut across so as to give the effect
+of long rows of setæ. The same condition occurs in specimens of
+_Marrella_ when the setæ of several exopodites are matted against each
+other.
+
+[Illustration: Fig. 12.--A slice of _Ceraurus pleurexanthemus_ in
+which the exopodite happened to be cut in such a way as to show a part
+of the shaft and some of the setæ in longitudinal section. Specimen
+80. × 4.]
+
+This is certainly an apt comparison, and equally true if _Neolenus_,
+_Triarthrus_, or _Cryptolithus_ were substituted for _Marrella_.
+
+Now consider the "epipodites." They are well shown in _Calymene_ in
+the specimens illustrated on plate 27, figure 11 (1918), and plate 3,
+figure 3 (1881), and less clearly in one or two others. Slices 22 (pl.
+27, fig. 12, 1918) and 80 (our fig. 12) show what is called the same
+organ in Ceraurus. It will be noted that all of these slices are cut
+in the same way, that is, more or less parallel to the under surface
+of the head, or, at any rate, on a plane parallel to a plane which
+would be tangent to the axial portion of the coiled shell. The
+sections which show the spirals best are those which are cut by a
+plane perpendicular to the long axis of the body. If one were to
+attempt to cut an enrolled _Triarthrus_ in such a way as to get a
+section showing the length of the setæ, one would not cut a section
+perpendicular to the axis of the animal, nor, in fact, would he cut
+one parallel to the ventral plane, but it is obvious that in this
+latter type of section he would stand a better chance of finding a
+part of the plane of the exopodite coincident with the plane of his
+section than in the former. And that seems to be what has happened in
+these sections of _Calymene_ and _Ceraurus_. If the exopodites were
+preserved, transverse sections were bound to cut across many sets of
+fringes, and the resultant slice would show transverse sections of the
+setæ as a series of overlapping spots. A few fortunately located
+sections in a more nearly horizontal plane might cut the setæ and
+occasionally the shaft of one or more exopodites in the longitudinal
+plane, and the resulting effect would produce the so-called
+"epipodites." A careful study has shown that no one of these
+epipodites is complete, and they do not have the palmate form shown in
+Walcott's figures.
+
+And the last and most important argument against the spiral appendages
+is that certain slices, of both _Calymene_ and _Ceraurus_, show
+definitely exopodites of exactly the type found in other trilobites.
+These are discussed later in the detailed description of the various
+slices.
+
+If these series of spots are interpreted on the basis of the known
+structure of _Triarthrus_, they are of course a series of sections
+through the setæ of the exopodites. It will be shown in Part IV
+that these setæ are not circular in section, but flattened, in
+_Cryptolithus_ even blade-like, and that they overlap one another. A
+section across them would give the same general appearance as, for
+instance, that shown in figures 4, 6, 9, and 10 of Walcott's plate 3
+(1881).
+
+When both endopodites and the "spiral branchiæ" are present in the
+same section (pl. 1, fig. 4; pl. 2, figs. 1, 2), the "spiral branchiæ"
+are dorsal to the endopodites, as the setæ of the exopodites would be
+expected to be. The specimens which show the clear spots connected,
+and which suggest a spiral (pl. 3, fig. 5), may seem at first sight to
+bear evidence against this interpretation, but one has only to think
+of the effect of cutting a section along the edge where the setæ are
+attached to the shaft of the exopodite of _Triarthrus_ to see that
+such a zig-zag effect is entirely possible. One would expect to cut
+just this position only rarely, and, in fact, the zig-zags are seen in
+only three or four sections. The bifurcation of the basal segment of
+the "spiral branchiæ" (pl. 3, fig. 10, 1881) is probably more apparent
+than real, if indeed these basal segments have anything to do with the
+succeeding one.
+
+A second peculiarity of _Calymene_, shown in Walcott's restoration, is
+the great enlargement of the coxopodites and of the distal segments of
+the endopodites of the fifth pair of appendages of the cephalon. This
+is based on the sections of plate 3, figures 6, 7, 8, 9, 10 (1881).
+After a study of the specimens I regret to find myself still
+unconvinced that the posterior cephalic appendages were any larger
+than those in front.
+
+Ventral Membrane.
+
+The most striking value of the thin sections of _Ceraurus_ and
+_Calymene_, and therein they have a great superiority over all the
+other forms so far investigated, is that they show the extent of the
+body cavity and the position, though not the substance, of the ventral
+membrane. Transverse sections through _Ceraurus_ (Walcott's pl. 1.
+figs. 1-5; pl. 2, figs. 1, 3, 1881) and _Calymene_ (pl. 3, figs. 9,
+10, 1881) show that the body cavity was almost entirely confined to
+the axial lobe. The longitudinal sections of _Ceraurus_ (pl. 2, figs.
+6, 8; pl. 4, fig. 8) and of _Calymene_ (pl. 2, figs. 5, 7; pl. 5,
+figs. 1-4) show that the ventral membrane was exceedingly thin and was
+wrinkled transversely when the shell was enrolled.
+
+The specimens of figures 1-3, plate 5 (1881) show the form of the
+ventral membrane more distinctly than any of the others. The section
+of figure 1 was cut just inside the dorsal furrow on the right side,
+and figure 2, which is on the opposite side of the same slice, is
+almost exactly on the median line. Figure 3 shows a section just
+inside the left dorsal furrow. Section 2 did not cut any of the
+appendages, and the ventral membrane is shown as a thickened,
+probably chitinous sheet thrown into low sharply crested folds equal
+in number to, and pointing in a direction just the reverse of, the
+crests of the segments of the thorax. Under the pygidium, where there
+would of course be less wrinkling, the folds are hardly noticeable. In
+the actual specimens one sees more plainly than in the figures the
+line of separation between the ventral membrane and the appendages,
+but the state of preservation of everything beneath the dorsal shell
+is so indefinite that one does not feel sure just what the connection
+between the appendages and the membrane was. In the original of figure
+5, plate 2, which seems to have been cut so as to cross the appendages
+at their line of junction with the ventral membrane, there appear to
+be narrow chitinous (?) plates extending from the ventral membrane to
+the dorsal test.
+
+Appendifers.
+
+In Ceraurus there are regular calcareous processes which extend down
+from the dorsal test just inside the line of the dorsal furrow, and
+which undoubtedly serve as points of attachment of the appendages.
+These processes, which for convenience I have designated as
+"appendifers," are broken off in most specimens showing the lower
+surface of _Ceraurus pleurexanthemus_, but on certain ones cleaned
+with potash they are well preserved. Doctor Walcott showed them well
+in his figures of the lower surface of this species (1875, pl. 11;
+1881, pl. 4, fig. 5), while the attempt of Raymond and Barton (1913,
+pl. 2, fig. 7) to show them by photography was not so successful.
+
+There is one pair of appendifers on each of the thoracic segments and
+four pairs on the pygidium. On the cephalon there is one pair under
+the neck furrow, and a pair under the posterior glabellar furrows.
+These are not concealed by the hypostoma. Further forward, and
+completely covered by the hypostoma, are two much less strongly
+developed but similar ones, so that there are in all four pairs of
+appendifers on the cephalon, though it is extremely doubtful if the
+appendages were articulated directly to all of them. On a specimen of
+_Ceraurus pleurexanthemus_ 30 mm. long on the median line, the dorsal
+furrows are 7.5 mm. apart at the anterior end of the thorax, and the
+tips of the appendifers of this segment are only 4 mm. apart. Each
+consists of a straight slender rod with a knoblike end projecting
+directly downward from the dorsal test, and supported by a thin
+calcareous plate which runs diagonally forward to the anterior edge of
+the segment directly under the dorsal furrow. On the pygidium three
+pairs of the appendifers have this form, while the fourth pair consist
+of low rounded tubercles which are concealed by the doublure. These
+appendifers are probably cut in many of Walcott's sections of
+Ceraurus, but owing to the state of preservation it is not always
+possible to determine what part is appendage, what part is body
+cavity, and what part is appendifer.
+
+Nearly forty years ago Von Koenen (1880, p. 431, pl. 8, figs. 9, 10)
+described and figured the appendifers of Phacops latifrons. He found
+them to be calcareous projections on the hinder margin of each
+segment, converging inward, and about 1.5 mm. long. He correctly
+considered them as supports (Stützpunkte) for the feet.
+
+Appendifers are well developed also in Pliomerops, and in well
+preserved specimens of _Calymene senaria_ from Trenton Falls they are
+present, but instead of being rod-like processes, they are rather
+thick, prominent folds of the shell. They are also well shown in some
+of the thin sections. A specimen of _Triarthrus_ (No. 229, our pl. 5,
+fig. 2) has broad processes extending downward from the lower side of
+the test below the dorsal furrows, much as in _Calymene_, and the
+individual of _Cryptolithus_ shown in plate 8, figure 1, possesses
+slender appendifers. Two other specimens (Nos. 237 and 242) show them
+quite well. They were probably present in all trilobites, but seldom
+preserved. The appendifers have the same origin as the entopophyses of
+_Limulus_, and like them, may have relatively little effect on the
+dorsal surface.
+
+_Calymene senaria_ Conrad.
+
+(Text figs. 13-16, 23.)
+
+ Illustrated: Walcott, Bull. Mus. Comp. Zool., Harvard Coll., vol.
+ 8, 1881, pl. 1, figs. 6-10; pl. 2, figs. 5-7, 10; pl. 3, figs. 1,
+ 3, 8-10; pl. 4, figs. 3, 7; pl. 5, figs. 1-6; pl. 6, figs. 1
+ (restoration), 2;--Proc. Biol. Soc. Washington, vol. 9, 1894, pl.
+ 1. fig. 7 (restoration);--Geol. Mag., dec. 4, vol. 1. 1894, pl. 8,
+ figs. 7, 8;--Smithson. Misc. Coll., vol. 67, 1918, pl. 26, figs.
+ 1-7, 9-13; pl. 27, figs. 4, 5 (not 5a), 11 (not 12, _Ceraurus_),
+ 13, 14, 15 (not _Ceraurus_); pl. 28, figs. 7, 8; pl. 33, fig. 1
+ (restoration); pl. 34, fig. 2; pl. 35, fig. 6.--Dames, N. Jahrb. f.
+ Min., etc., vol. 1, 1880, pl. 8, figs. 1-5.--Milne-Edwards, Ann.
+ Sci. Nat., Zoologie, ser. 6, vol. 12, 1881, pl. 11, figs. 19-32;
+ pl. 12, figs. 33-41.--Packard, Amer. Nat., vol. 16, 1882, p. 796,
+ fig. 12.--Bernard, The Apodidæ, 1892, text figs. 50, 52,
+ 54;--Quart. Jour. Geol. Soc., London, vol. 50, 1894, text figs. 13,
+ 15, 17.--Oehlert, Bull. Soc. Géol. France, ser. 3, vol. 24, 1896,
+ fig. 12.--Beecher, Amer. Jour. Sci., vol. 13, 1902, pl. 5, fig. 7.
+
+In both of Walcott's accounts (1881, 1918) of the appendages of
+_Calymene_ and _Ceraurus_, he has described them together, so that
+those who have not taken time to study the illustrations and
+disentangle the descriptions are very apt to have a confused notion in
+regard to them. I have therefore selected from the original specimens
+those slices of _Calymene_ which are most instructive, and bearing in
+mind the probable appearance of the appendages of an enrolled
+_Triarthrus_, have tried to interpret them. In such a method of study,
+I have of course started with a pre-formed theory of what to expect,
+but have tried to look for differences as well as likenesses.
+
+_Cephalic Appendages._
+
+_Antennules._--The evidence of antennules rests on a single slice (No.
+78). The appendage in question is exceedingly slender and arises at
+the side of the hypostoma near its posterior end. It shows fine,
+slender segments, and curves first outward and then forward. If it is
+in its natural position, it is not an antennule, but the endopodite of
+the second or third pair of cephalic appendages. It is short, only
+about one-third the length of the hypostoma, but is doubtless
+incomplete. The two distal segments show a darker filling, indicating
+that they were hollow. Judging from analogy with other trilobites, the
+appendage is probably an endopodite and not an antennule. There can be
+no reasonable doubt, however, that _Calymene_ possessed antennules.
+
+Some idea of the form of the coxopodites of the cephalic appendages
+may be obtained from sections which cut in approximately the plane of
+the hypostoma. Such sections are shown in Walcott's photographs (pl.
+26, figs. 4, 6, 11, 1918). Specimens 50 (fig. 4, our fig. 13), 51
+(fig. 6), 6 (fig. 11), and 40 (our fig. 14) agree in showing two
+pairs of slender coxopodites which are attached at the sides of the
+hypostoma and run backward parallel and close to it, and two pairs of
+larger coxopodites which are behind the hypostoma, although the point
+of attachment of the third pair is in front of its tip. The anterior
+pair are apparently under-developed and no longer function as mouth
+parts, while the posterior two pairs are large and armed on their
+inner ends with spines. Specimen 78, which has already been mentioned
+in connection with the antennules, shows a second very slender
+appendage back of the so-called antennule, which is equally slender,
+but is directed outward instead of forward. It seems not improbable,
+from their position and similarity, that these two are the endopodites
+of the first two appendages on one side of the hypostoma. Specimen 6
+shows rather inadequately the endopodites of the second and third
+cephalic appendages. I have not found other slices showing endopodites
+of the cephalon. Walcott, in both his restorations, has shown
+enlarged, paddle-shaped dactylopodites on the distal ends of the
+fourth cephalic endopodites. The evidence for this rests principally
+on three slices, No. 38 (pl. 26, figs. 9, 10), 53 (pl. 26, fig. 12),
+and 43 (pl. 26, fig. 13). Of these, No. 43 may be dismissed at once as
+too poorly preserved to be interpreted. No. 53 does show a section of
+an appendage which seems to have an unusually wide dactylopodite, but
+this slice presents no evidence at all as to the appendage to which
+the dactylopodite appertains, nor can one even be sure that there has
+not been a secondary enlargement. Specimen 43 shows this feature
+much less definitely than is indicated by the published photograph
+and drawing. The segment in question is strongly curved, with a
+constriction possibly dividing it into two. If it is in its natural
+position in this section, it obviously belongs to one of the thoracic
+segments and not to the cephalon. With evidence of difference so
+unsatisfactory, I prefer to reconstruct the posterior cephalic
+endopodites on the same plan as those of the thorax.
+
+[Illustration: Fig. 13.--Slice through _Calymene senaria_ in the plane
+of the hypostoma, showing the very slender coxopodites beside that
+organ, the spines on the inner end of one of the maxillulæ, and the
+anterior position of the attachment of all these appendages. From a
+photographic enlargement. Specimen 50. × 4.]
+
+[Illustration: Fig. 14.--Slice through the hypostoma and thorax of
+_Calymene senaria_ Conrad, showing the small size of the coxopodites
+nearest the hypostoma. Shell in black, appendages and filling of
+abdominal cavity dotted. From a photographic enlargement. Specimen 40.
+× 3.8.]
+
+[Illustration: Fig. 15.--Transverse section of _Calymene_, showing
+method of articulation with the appendifer. The shell is in solid
+black, the filling of the appendage and appendifer stippled. Traced
+from a photographic enlargement of the slice. Specimen 63. × 7.]
+
+_Exopodites._--Walcott admits that there is no direct evidence of spiral
+exopodites in the cephalon of _Calymene_. No one of the sections
+cutting through the plane of the hypostoma shows any trace of
+appendages which could be interpreted as exopodites.
+
+_Thoracic Appendages._
+
+The large coxopodites of the anterior thoracic appendages are well
+shown in many specimens cut longitudinally, of which Nos. 23, 50, and
+55 may be mentioned, since photographs of them have been published by
+Walcott (pl. 26, figs. 1-4, 1918). The endobases of all taper toward
+the proximal ends. Transverse slices show sections of the coxopodites
+which are no wider than those in longitudinal sections, indicating
+that they were not compressed but probably cylindrical. This is borne
+out by an individual (pl. 28, fig. 7, 1918) which is not a slice but
+an actual specimen, the body cavity of which was hollow, and, opened
+from above, shows the impressions of the last two coxopodites of the
+cephalon, and the first four of the thorax.
+
+One transverse section (No. 63, see our fig. 15) is especially
+valuable, as it shows the method of articulation of the coxopodites
+with the dorsal skeleton. Another specimen (No. 73) shows that
+appendifers are present in _Calymene_, and while the appendifer does
+not retain its original form in slice No. 63, the section does show
+clearly that there was a notch in the inner (upper) side of the
+coxopodite into which the lower end of the appendifer fitted, thus
+giving a firm, articulated support for the appendage. This notch
+appears to be slightly nearer the outer than the inner end of the
+coxopodite, and since it must have made a kind of ball-and-socket
+joint, considerable freedom of movement was allowed. The appendage
+must have been held in place by muscles within the coxopodite and
+attached to the appendifer.
+
+No slice which I have seen shows a continuous section through all the
+segments of an endopodite, but many, both longitudinal and transverse,
+show one, two, or as many as three segments.
+
+Such sections as No. 120 show that the endopodites of the thorax
+were slender and composed of segments of rather uniform diameter.
+Other sections, notably No. 83, 154, and in, show that they tapered
+distally, and bore small spines at the outer end of each segment.
+
+The exopodites of course furnish the chief difficulty in
+interpretation. Doctor Walcott finds two sets of structures attached
+to the coxopodite, a long, slender, spiral exopodite, and a short,
+broad epipodite with a fringe of long setæ. Since he has given the
+same interpretation for _Calymene_, _Ceraurus_, and _Acidaspis_, I
+have considered the question of all three together on a preceding page
+(p. 48), and given my reasons for regarding both structures as due to
+sections in different directions across setiferous exopodites.
+
+Sections like those shown in figures 11, 13, and 14 of plate 27 (1918)
+happen to be cut in or near the plane of the setæ of an exopodite, and
+so show hairs of considerable length. Such sections are, as would be
+expected, very few in number, while sections like those shown on
+figures 4, 5, 7, and 9 of plate 27, which cut the setæ more nearly at
+right angles, are very common. Slices which give any definite idea of
+the form of the shaft of the exopodite are exceedingly rare. Perhaps
+the most satisfactory one is No. 23 (pl. 3, fig. 3, 1881), which shows
+the proximal part of a long, slender, unsegmented shaft, with the
+bases of a number of slender setæ. The organ is not complete, as would
+be inferred from the published figure, but the section cuts diagonally
+across it, and the total length is unknown. It is directed forward,
+like the exopodites of Neolenus, but whether or not this is a natural
+position is yet to be learned.
+
+The proximal, non-setiferous portion of the exopodite is evidently
+at an angle with the setiferous part. Another similar exopodite is
+apparently shown by specimen 29 (pl. 3, fig. 9, 1881), which has a
+similar angulated shaft and just a trace of the bases of the setæ.
+
+_Pygidial Appendages._
+
+That appendages were present under the pygidium is shown by
+longitudinal sections, but nothing is known of the detail of
+structure.
+
+[Illustration: Fig. 16. Restoration of _Calymene senaria_ Conrad,
+based upon data obtained from the study of the translucent sections
+made by Doctor Walcott. Prepared by Doctor Elvira Wood, under the
+supervision of the author. About twice natural size.]
+
+_Relation of Hypostoma to Cephalon in Calymene._
+
+In _Calymene_ the shape of the hypostoma bears little relation to the
+shape of the glabella, and it is relatively smaller, both shorter and
+narrower, than in Ceraurus. In shape, neglecting the side lappets at
+the front, it is somewhat rectangular, but rounded at the back, where
+it is bifurcated by a shallow notch. The anterior edge has a narrow
+flange all across, which is turned at almost right angles to the plane
+of the appendage, and which fits against the doublure of the free
+cheeks at the sides and against the epistoma in the middle. The side
+lappets show on their inner (upper) surface shallow pits, one on each
+lappet, which fit over projections that on the dorsal surface show as
+deep pits in the bottom of the dorsal furrows in front of the anterior
+glabellar furrows. The appendifers on the head in _Calymene_ take the
+form of curving projections of shell underneath the glabellar and neck
+furrows, and owing to the narrowness of the hypostoma, all these are
+visible from the ventral side, even with it in position. This shield
+extends back about 0.6 of the length of the cephalon, and to a point
+a little behind the second glabellar furrow from the back of the head.
+
+In Doctor Walcott's restoration of _Calymene_ he has represented
+all four pairs of biramous appendages as articulating back of the
+posterior end of the hypostoma. I think his sections indicate that
+the gnathobases of two pairs of these appendages rested alongside or
+beneath it, and in particular, the longitudinal sections (1881, pl. 5)
+would appear to show that the mouth was some distance in advance of
+its posterior end.
+
+_Restoration of Calymene._
+
+(Text fig. 16.)
+
+From what has been said above, it is evident that for a restoration of
+the appendages of _Calymene_ considerable dependence must be placed
+upon analogy with other trilobites. Nothing is positively known of the
+antennules, the exopodites of the cephalon, or any appendages, other
+than coxopodites, of the pygidium, but all were probably present. It
+is inferred from the slices that the first two pairs of cephalic
+appendages were poorly developed, the endopodites short and very
+slender, the coxopodites lying parallel to the sides of the hypostoma
+and nearly or quite functionless. The gnathites of the last two pairs
+of cephalic appendages are large, closely approximated at their inner
+ends, and bear small tooth-like spines. The endopodites are probably
+somewhat better developed than the anterior ones and more like those
+on the thorax.
+
+The coxopodites of the thorax appear to have had nearly cylindrical
+endobases which tapered inward. The endopodites were slender, tapering
+gradually outward, and probably did not extend beyond the dorsal test.
+Small spines were present on the distal end of each segment. Each
+exopodite had a long, slender, unsegmented shaft, to which were
+attached numerous long, overlapping, flattened setæ. The shaft may
+have been angulated near the proximal end, and may have been directed
+somewhat forward and outward as in Neolenus, but the evidence on this
+point is unsatisfactory. The number of pairs of appendages is that
+determined by Walcott from longitudinal sections, namely, four pairs
+on the cephalon beside the antennules, thirteen pairs in the thorax,
+and nine pairs on the pygidium.
+
+
+=Calymene= sp. ind.
+
+(pl. 6, figs. 4, 5.)
+
+ Illustrated: Walcott, Bull. Mus. Comp. Zool., Harvard Coll., vol.
+ 8, 1881, pl. 6, figs. 5a, b;--Proc. Biol. Soc. Washington, vol. 9,
+ 1894, pl. 1, fig. 10;--Geol. Mag., dec. 4, vol. 1, 1894, pl. 8,
+ fig. 10;--Smithson. Misc. Coll., vol. 67, 1918, pl. 36, figs. 1, 2,
+ 2a-d.--Milne-Edwards, Ann. Sci. Nat., Zoologie, ser. 6, vol. 12,
+ 1881; pl. 12, figs. 44a, b.
+
+In the United States National Museum there is a thin piece of
+limestone, about 3 inches square, which has on its surface eight
+jointed objects that have been called legs of trilobites. Two of these
+were figured by Walcott (1881, pl. 6, fig. 5). The slab contains
+specimens of _Dalmanella_ and _Cryptolithus_, in addition to the
+appendages of trilobites, and is said by Doctor Ulrich to have come
+from the tipper part of the Point Pleasant formation (Trenton) on the
+bank of the Ohio River below Covington, Kentucky.
+
+The specimens are all endopodites of long slender form, similar to
+those of _Triarthrus_, but since that genus does not occur in the
+Point Pleasant, it is necessary to look upon some other trilobite as
+the former possessor of these organs. Both _Isotelus_ and _Calymene_
+occur at this horizon, and as the specimens obviously do not belong
+to _Isotelus_ or _Cryptolithus_, it is probable that they were
+formerly part of a _Calymene_.
+
+All the endopodites are of chitinous material, and the various
+specimens show, according to the perfection of their preservation,
+from four to six segments. The endopodite as a whole tapers but
+slightly outward, and the individual segments are of nearly equal
+length. They appear to be but little crushed, and are oval in section,
+with a crimped anterior and posterior margin. One or two show a median
+longitudinal ridge, such as is seen in some appendages of
+_Triarthrus_. Each segment is parallel-sided, with a slight expansion
+at the distal end, where the next segment fits into it.
+
+Under the heading "Ordovician Crustacean Leg," Walcott (1918, p. 154,
+pl. 36, figs. 1,2) has recently redescribed these specimens, and
+thinks that they do not belong to _Calymene_, nor, indeed, to any
+trilobite. He concludes that they were more like what one would expect
+in an isopod. Passing over the fact that the oldest isopod now known
+is Devonian, the fossils in question seem to me quite trilobite-like.
+Walcott says:
+
+ The legs are associated with fragments of _Calymene meeki_ but it
+ is not probable that they belong to that species; if they did, they
+ are unlike any trilobite leg known to me. The very short coxopodite
+ and basopodite are unknown in the trilobites of which we have the
+ legs, as they are fused into one joint forming the long protopodite
+ in the trilobite. The distal joint is also unlike that of the
+ trilobite legs known to us.
+
+A great deal of Doctor Walcott's difficulty probably arises from his
+homology of the coxopodite of the trilobite with the protopodite of
+the higher Crustacea. The coxopodite of the trilobite is not fused
+with the basipodite, this latter segment always remaining free.
+Indeed, Walcott himself says of _Neolenus_ (1918, p. 128):
+
+ Each thoracic leg (endopodite) is formed of a large elongate
+ proximal joint (protopodite), four strong joints each about 1.5
+ times as long as wide (basopodite, ischiopodite, meropodite and
+ carpopodite); two slender elongate joints (propodite and
+ dactylopodite) and a claw-like, more or less tripartite
+ termination.
+
+Walcott's drawing (pl. 36, fig. 1) is a composite one, and while it
+shows eight segments, I was not able to count more than seven on any
+of the specimens themselves. In regard to the terminal segment,
+the dactylopodite of the limb shown in his plate 36, figure 2, is
+unusually long, and a comparison with other photographs published on
+the same plate shows that such long segments are unusual.
+
+Proof that these are appendages of a _Calymene_ is of course wanting,
+but there is no particular reason so far to say that they are not.
+
+_Measurements:_ Two of the more complete specimens, each showing six
+segments, are each 8 mm. long.
+
+Somewhat similar to the specimens from Covington are the ones
+described by Eichwald (1825, p. 39, 1860, pl. 21), the specimens being
+from the Silurian of Gotland. The figure copied by Walcott (1881, pl.
+6, fig. 4) has never been looked upon as entirely satisfactory
+evidence of the nature of the specimen, and so far as I know, the
+fossil has not been seen by any modern investigator.
+
+
+=Ceraurus pleurexanthemus= Green.
+
+(pl. 11; text figs. 12, 17-19, 21, 22, 24, 29, 30.)
+
+ Illustrated: Walcott, Ann. Lye. Nat. Hist. New York, vol. II, 1875,
+ pl. 11;--31st Ann. Rept. New York State Mus. Nat. Hist, 1879, pl.
+ 1, fig. 3;--Bull. Mus. Comp. Zool., Harvard Coll., vol. 8, 1881,
+ pl. 1, figs. 1-5; pl. 2, figs. 1-4, 6-8; pl. 3, figs. 2, 4-7; pl.
+ 4, figs. 1, 2, 4-6, 8; pl. 6, fig. 3; Smithson. Misc. Coll., vol.
+ 67, 1918, pl. 26, figs. 8, 14, 15; pl. 27, figs. 1-3, 5a, 6-9, 12
+ (not _Calymene_), (not 15, _Calymene_); pl. 28, figs. 1-5; pl. 34,
+ fig. 1; pl. 35, fig. 7.--Milne-Edwards, Ann. Sci. Nat., Zoologie,
+ ser. 6, vol. 12, 1881, pl. 10, figs. 1-18.--Bernard, The Apodidæ,
+ 1892, text figs. 46, 51.
+
+_Cephalic Appendages._
+
+No trace of antennules has yet been found.
+
+I find only three sections cut through the plane of the hypostoma of
+Ceraurus which show anything of the cephalic appendages, and no one of
+them is very satisfactory. The best is No. 22, the one figured by
+Walcott (pl. 3, fig. 2, 1881; pl. 27, fig. 12, 1918), but one should
+remember that this section is not actually cut in the plane of the
+hypostoma but is a slice diagonally through the head, cutting through
+one eye and the posterior end of the hypostoma. It shows what seem to
+be the coxopodites of the second, third, and fourth pairs of cephalic
+appendages, the exopodites of the third and fourth pairs, and the
+metastoma. If this interpretation is correct, the first pair of
+gnathites lay alongside the hypostoma or under its edge, and were
+feebly developed, the second pair were attached in front of the tip of
+the hypostoma, curved back close to it, and their inner ends reached
+the sides of the metastoma. The third and fourth pairs were back of
+the metastoma, the third pair was stronger than the second, and the
+fourth probably like the third.
+
+[Illustration: Fig. 17. Transverse section of _Ceraurus
+pleurexanthemus_, showing the relation of the coxopodite to the
+appendifer. Traced from a photographic enlargement of the slice.
+Specimen 128. × 4/5.]
+
+[Illustration: Fig. 18. Slice of _Ceraurus pleurexanthemus_, showing a
+nearly continuous section of an endopodite and an exopodite above it.
+The latter is so cut as to show only the edge of the shaft and the
+bases of a few setæ. Traced from a photographic enlargement. Specimen
+in. × 4.]
+
+Specimen 92 shows traces of the slender endopodites belonging to the
+cephalon, but no details. Specimen 22 shows on one side exopodites
+(epipodites of Walcott) belonging to the third and fourth cephalic
+appendages. That belonging to the third shows some long setæ and a
+trace of the shaft, while the one on the fourth appendage (third
+coxopodite) has a portion of a broad shaft and a number of long setæ.
+It should again be remembered that the slice does not cut through the
+plane of the exopodite, but across it at a low angle, so that a part
+but not all of the shaft is shown. On the other side of this slice
+there is a fairly good section of one of the thoracic exopodites. It
+is, however, turned around in the opposite direction from the others,
+as would be expected in an enrolled specimen.
+
+Specimens 4 and 5 (pl. 1, figs. 4, 5, 1881) are slices cut diagonally
+through the head of Ceraurus, in front of the posterior tip of the
+hypostoma. They show fragments of endopodites and exopodites which may
+be interpreted as practically identical in form with those of the
+thorax. Due to the diagonal plane in which the section is cut, slice 5
+shows the coxopodites of two pairs of appendages, one lying nearer
+the median cavity than the other. It is extremely difficult to
+visualize the interpretation of such sections.
+
+_Thoracic Appendages._
+
+A transverse section through a thoracic segment (No. 128, our fig. 17)
+shows the relation of coxopodite to appendifer to be the same as in
+_Calymene_, the upper side of the coxopodite having a notch a little
+outward from the middle. After seeing that specimen, it is possible to
+understand slice No. 168, which shows longitudinal sections through a
+number of coxopodites of the thorax, with fragments of both exopodites
+and endopodites articulated at the distal ends. These and longitudinal
+vertical sections like No. 18 (pl. 2, fig. 8, 1881) show that the
+endobases taper inward, and the general uniformity in width in
+sections taken at various angles indicates that the coxopodites were
+not greatly flattened.
+
+A unique slice (No. 111, pl. 2, fig. 2, 1881; pl. 27, fig. 1, 1918;
+our fig. 18) shows a nearly complete thoracic endopodite, and above it
+a part of the proximal end of the exopodite of the same segment. When
+one considers that out of over two thousand sections only this one
+shows the six successive segments of an endopodite, one realizes how
+futile it is to expect that dozens of the equally slender "spirals"
+should be cut so as to show practically all their turns.
+
+This endopodite is slender, all the segments have nearly the same
+length and diameter, though there is a slight taper outward, each
+segment is expanded distally for the articulation of the next, and
+there are small spines on the distal ends of some of them. There is
+probably a terminal spine present, though it is neither so long nor so
+plainly visible as in Walcott's photograph.
+
+The exopodite on this same specimen was evidently cut diagonally
+across near the setiferous edge, showing a section through the shaft
+and the bases of seven setæ (fig. 18). This section is so exactly what
+would be obtained by cutting similarly an exopodite of either Neolenus
+or _Triarthrus_ that it should in itself dispose of the
+"spiral-exopodite" theory.
+
+Several sections have already been illustrated showing sections across
+the setæ of the exopodites (pl. 3, figs. 4-6, 1881; pl. 27, figs. 3,
+4, 9, 1918), and similar sections are not uncommon. Only a very few,
+however, show sections in the plane of the exopodite. If only No. 111,
+described above, were known, it would be inferred that the exopodite
+had a slender shaft as in _Calymene_, but another good slice, No. 80
+(fig. 12, ante) shows that the blade was rather broad, though not so
+broad as in Neolenus. The other specimen is No. 22, which has already
+been discussed. The thoracic exopodite of this specimen has been very
+incorrectly figured by Walcott, as it shows no such palmate shaft as
+he has indicated, but a long blade-like one is outlined, though its
+entire width is not actually shown.
+
+_Pygidial Appendages._
+
+Sections 14 and 18 (pl. 2, figs. 4, 8, 1881) prove the presence under
+the pygidium of three pairs of appendages, the coxopodites and
+fragments of endopodites of which are shown. Nothing is known of the
+exopodites.
+
+_Relation of Hypostoma to Cephalon._
+
+In Ceraurus the body portion and posterior end of the hypostoma are
+roughly oval, about as wide as the glabella at its broadest part, and
+the posterior edge extends back to within 0.5 to 1 mm. of the neck
+furrow. The posterior pair of appendifers are behind the hypostoma,
+while the second pair are in front of its posterior end but escape
+being covered by it on account of its oval shape. At the anterior end
+the hypostoma is widened by the presence of two side lappets which
+extend beyond the boundaries of the glabella. In both Ceraurus and
+Cheirurus the anterior edge of the hypostoma fits against the doublure
+at the anterior margin of the head and the epistoma is either entirely
+absent or is so narrow as not to be seen in specimens in the ordinary
+state of preservation. A section across the cephalon of _Ceraurus
+pleurexanthemus_ at the horizon of the eyes shows the sides of the
+hypostoma fitting closely against the sides of the glabella (Walcott's
+pl. 1, fig. 1). Further back on the head it is not in contact with the
+dorsal test, and the gnathobases extend beneath it.
+
+Restoration of _Ceraurus pleurexanthemus_. (pl. 11; text fig. 19.)
+
+The restoration of the appendages of _Ceraurus pleurexanthemus_ is a
+tentative one, based upon a careful study of the translucent sections
+prepared by Doctor Walcott. In no case among these sections is the
+actual test of any appendage preserved, and the real form of each part
+is generally obscured by the crystallization of the calcite which
+fills the spaces formerly occupied by animal matter.
+
+[Illustration: Fig. 19. Restoration of a transverse section of the
+thorax of _Ceraurus pleurexanthemus_ Green, showing the relation of
+the appendages to the appendifers and the ventral membrane. The
+probable positions of the heart and alimentary canal are indicated.]
+
+No section shows anything which can be identified as any part of the
+antennules, so that these organs have been supplied from analogy with
+_Triarthrus_.
+
+There are undoubtedly four pairs of biramous Cephalic appendages, but
+their points of attachment are not so obvious. There are two pairs of
+conspicuous appendifers on the posterior part of the cephalon and
+another pair almost concealed by the hypostoma. It is probable that
+the appendages of the cephalon were not attached directly beneath
+them, as the four pairs have to be placed within the space occupied by
+the three pairs of appendifers. As the mouth is in front of the
+posterior end of the hypostoma, the gnathites of the first pair of
+biramous appendages may have extended beneath that organ, or they may
+have lain beside it, and only become functional when the hypostoma was
+dropped down in the feeding position. The second pair of gnathites
+reached just to the tip of the hypostoma, and the other two pairs
+seemingly curved backward behind it.
+
+The points of attachment on the thorax, as shown clearly in sections,
+were directly beneath the lower ends of the appendifers. The
+endopodites were long enough to reach to or a little beyond the outer
+extremities of the pleural spines, while the exopodites were
+apparently somewhat shorter. Each endopodite consisted of six short,
+fairly stout segments, each with at least two spines on the somewhat
+expanded distal ends. The exact form of the exopodites could not be
+made out. The shaft was apparently rather short, unsegmented, and
+fairly broad. The setæ appear from the sections to have been more or
+less blade-shaped and to have overlapped, as do those of the
+exopodites of _Cryptolithus_. Judging from their position in the
+sections, the setæ not only bordered the posterior side of the shaft,
+but radiated out from the end as well.
+
+The pygidium shows three pairs of functional appendifers, hence three
+pairs of appendages have been supplied. There is a fourth pair of
+rudimentary appendifers, but as they are beneath the doublure they
+could not have borne ambulatory appendages.
+
+
+The Appendages of Acidaspis trentonensis Walcott.
+
+(pl. 6, fig. 6.)
+
+A single individual of _Acidaspis trentonensis_, obtained from the
+same locality and horizon as the specimens of _Triarthrus_ and
+_Cryptolithus_, when cleaned from the ventral side shows a number
+of poorly preserved endopodites which seem very similar in shape and
+position to those of _Triarthrus_. One endopodite on the right side
+of the head and the first five on the right side of the thorax are the
+best shown. All are slender, are directed first forward at an angle of
+about 45 with the axis, then, except in the case of the cephalic
+appendage, turn backward on a gentle curve and extend a little
+distance beyond the margin of the test, but not as far as the tips of
+the lateral spines of the thoracic segments.
+
+The individual segments of the endopodites can not be seen clearly
+enough to make any measurements. On the fourth and fifth endopodites
+of the thorax, some of the segments seem to be broad and triangular as
+in _Triarthrus_. All that can be seen indicates that _Acidaspis_ had
+appendages entirely similar to those of _Triarthrus_, but perhaps not
+quite so long, as they seem not to have projected beyond the limits of
+the lateral spines. There are no traces of antennules nor,
+unfortunately, of exopodites.
+
+_Measurements:_ Length 8 mm.
+
+Walcott (1881, p. 206) stated that his sections had shown the presence
+in this species of legs "both cephalic and thoracic" and also the
+"spiral branchiæ." His specimens were from the Trenton at Trenton
+Falls, New York.
+
+
+
+
+The Appendages of Cryptolithus.
+
+
+=Cryptolithus tessellatus= Green.
+
+(pl. 6, fig. 7; pls. 7-9; text figs. 20, 25, 45, 46.)
+
+(See also Part IV.)
+
+ Illustrated: Beecher, Amer. Jour. Sci., vol. 49, 1895, pl. 3.
+
+When Professor Beecher wrote his short article on the "Structure
+and Appendages of _Trinucleus_" (1895), he had only three specimens
+showing appendages. In his later work he cleaned several more, so that
+there are now thirteen specimens of _Trinucleus_ = _Cryptolithus_
+available for study, though some of these do not show much detail. In
+his last and unpublished study, Beecher devoted the major part of his
+attention to this genus, and summarized his findings in the drawings
+which he himself made of the best individuals (text figs. 45, 46).
+Valiant (1901) stated that he had found a _Trinucleus_ with antennæ in
+the Frankfort shale south of Rome, New York. The specimen has not been
+figured.
+
+None of the specimens shows much more of the appendages of the
+cephalon than, the hypostoma and the antennules, so that we are still
+in ignorance about the mouth parts.
+
+The most striking characteristics of the appendages are as follows:
+the antennules are long, and turn backward instead of forward; none
+of the limbs projects beyond the margin of the dorsal test; the
+exopodites extend beyond the endopodites, reaching very nearly to the
+margin of the test; the endopodites are not stretched out at right
+angles to the axis, but the first three segments have a forward and
+outward direction as in _Triarthrus_, while the last four turn back
+abruptly so that they are parallel to the axis; the limbs at the
+anterior end of the thorax are much more powerful than the others; the
+dactylopodites of the endopodites show a fringe of setæ instead of
+three spines as in _Triarthrus_ and _Neolenus_. All these would, as
+Beecher has already suggested, seem to be adaptations to a burrowing
+habit of life, the antennules being turned backward and the other
+appendages kept within the shelter of the dorsal test in order to
+protect them, and the anterior endopodites enlarged and equipped with
+extra spines to make them more efficient digging and pushing organs.
+
+_Restoration of Cryptolithus._
+
+(Text fig. 20.)
+
+It should be definitely understood that the present figure is a
+restoration and not a drawing of a specimen, and that there are many
+points in the morphology of _Cryptolithus_ about which no information
+is available, especially about the appendages under the central
+portion of the cephalon. The information afforded by all the figures
+published in this memoir is combined here. As gnathites are preserved
+on none of the specimens, those represented in the figure are purely
+conventional.
+
+A person who is acquainted only with _Cryptolithus_ preserved in
+shale, or with figures, usually has a very erroneous idea of the
+fringe It is not a flat border spread out around the front of the
+head, but stands at an angle about 45 in uncrushed specimens of most
+species. When viewed from the lower side, there is a single outer,
+concentric row of the cup-shaped depressions, bounded within by a
+prominent girder. This row is in an approximately horizontal plane,
+while the remainder of the doublure of the fringe rises steeply into
+the hollow of the cephalon. Since the front of the hypostoma is
+attached to this doublure, it stands high up within the vault and
+under the glabella. Two specimens, Nos. 231 and 233, show something of
+the hypostoma, and they are the only ones known of any American
+trinucleid. That of specimen 233, the better preserved, is very small,
+straight across the front, and oval behind. It seems that it is
+abnormally small in this specimen and I should not be surprised if in
+other specimens it should be found to be larger.
+
+In the Bohemian _Trinucleoides reussi_ (Barrande), the oldest of the
+trinucleids, the hypostoma is very commonly present, and is of the
+proper size to just cover the cavity of the glabella, seen from the
+lower side, and has, toward the anterior end, side flaps which reach
+out under the prominent glabellar lobes. This large size of the
+hypostoma would cause the antennules to be attached outside the dorsal
+furrows, and the position in which they are attached in the American
+species of _Cryptolithus_ may be explained as an inherited one, since
+with the small hypostoma they might have been within the glabella, as
+in _Triarthrus_.
+
+The antennules are seen in three specimens, and in all cases are
+directed backward. The particular course in which they are drawn in
+the restoration is purely arbitrary. The second pair of cephalic
+appendages are represented as directed downward and forward, since in
+one or two specimens fragments of forward-pointing endopodites were
+seen near the front of the cephalon, and because in other trilobites
+the second pair of appendages is always directed forward. The
+remaining three pairs have a more solid basis in observed fact, for
+the two or three specimens retaining fragmentary remains of them
+indicate that they turn backward like those on the thorax, and that
+the individual segments are longer and more nearly parallel-sided than
+those of the more posterior appendages. The gnathites of all the
+cephalic appendages are admittedly purely hypothetical. None of the
+specimens shows them. As drawn, they are singularly inefficient as
+jaws, but if, as is suggested by the casts of the intestines of
+trinucleids found in Bohemia, these trilobites were mud-feeders,
+inefficient mouth-parts would be quite in order.
+
+[Illustration: Fig. 20. _Cryptolithus tessellatus_ Green. A
+restoration of the appendages drawn by Doctor Elvira Wood from the
+original specimens and from the photographs made by Professor Beecher.
+× 9.]
+
+The appendages of the thorax and pygidium can fortunately be taken
+quite directly from the photographs of the dorsal and ventral sides of
+well preserved specimens. There is of course a question as to the
+number and the exact form of those on the pygidium, but I think the
+present restoration is fairly well justified by the specimens. As
+would be expected from the narrow axial lobe, the gnathobases of the
+coxopodites are short and small.
+
+
+
+
+Summary on the Ventral Anatomy of Trilobites.
+
+
+COMPARISON OF APPENDAGES OF DIFFERENT GENERA.
+
+Since the appendages of _Triarthrus_, _Cryptolithus_, _Neolenus_,
+_Calymene_, and _Ceraurus_ are now known with some degree of
+completeness, those of _Isotelus_ somewhat less fully, and something
+at least of those of _Ptychoparia_, _Kootenia_, and _Acidaspis_, these
+forms being representatives of all three orders and of seven different
+families of trilobites, it is of some interest to compare the
+homologous organs of each.
+
+All in which the various appendages are preserved prove to have a pair
+of antennules, four pairs of biramous limbs on the cephalon, as many
+pairs of biramous limbs as there are segments in the thorax, and
+a variable number of pairs on the pygidium, with, in the case of
+_Neolenus_ alone, a pair of tactile organs at the posterior end. Each
+limb, whether of cephalon, thorax, or pygidium, consists of a
+coxopodite, which is attached on its dorsal side to the ventral
+integument and supported by an appendifer, an exopodite, and an
+endopodite. The exopodite is setiferous, and the shaft is of variable
+form, consisting of one, two, or numerous segments. The endopodite
+always has six segments, the distal one armed with short movable
+spines.
+
+_Coxopodite._
+
+The coxopodite does not correspond to the protopodite of higher
+Crustacea, the basipodite remaining as a separate entity. The inner
+end of the coxopodite is prolonged into a flattened or cylindrical
+process, which on the cephalon is more or less modified to assist in
+feeding, and so becomes a gnathobase or gnathite. The inner ends of
+the coxopodites of the thorax and pygidium are also prolonged in a
+similar fashion, but are generally somewhat less modified. These
+organs also undoubtedly assisted in carrying food forward to the
+mouth, but since they probably had other functions as well, I prefer
+to give them the more non-committal name of endobases.
+
+In _Triarthrus_ and _Neolenus_ the endobases are flattened and taper
+somewhat toward the inward end. In _Isotelus_, _Calymene_ and
+_Ceraurus_, they appear to have been cylindrical. In other genera they
+are not yet well known. In all cases, particularly about the mouth,
+they appear to have been directed somewhat backward from the point of
+attachment. As it is supposed that these organs moved freely forward
+and backward, the position in which they occur in the best preserved
+fossils should indicate something of their natural position when
+muscles were relaxed.
+
+_Cephalon._
+
+_Antennules._--Antennules are known in _Triarthrus_, _Cryptolithus_,
+_Neolenus_, and _Ptychoparia_. In all they are long, slender, and
+composed of numerous segments, which are spiniferous in _Neolenus_,
+and very probably so in the other genera.
+
+In _Triarthrus_, _Neolenus_, and _Ptychoparia_ they project ahead of
+the cephalon, emerging quite close together under the front of the
+glabella, one on either side of the median line. In _Cryptolithus_
+they turn backward beneath the body, but since only three or four
+specimens are known which retain them, it is possible that other
+specimens would show that these organs were capable of being turned
+forward as well as backward. The proximal ends of the antennules being
+ball-like, it is probable, as Doctor Faxon has suggested to me, that
+these "feelers" had considerable freedom of motion. The antennules of
+_Triarthrus_ are apparently somewhat less flexible than those of the
+other genera, and have a double curvature that is seen among the
+others only in Ptychoparia. The proximal end of an antennule in
+_Triarthrus_ is a short cylindrical shaft, apparently articulating in
+a sort of ball-and-socket joint. The proximal end in the other genera
+is still unknown. The points of attachment in _Triarthrus_ seem to be
+under the inner part of the second pair of glabellar furrows. In
+_Cryptolithus_ they appear to be beside the anterior lobe of the
+glabella under what have long been known as the antennal pits. In the
+other genera the location is not definitely known, but in _Neolenus_
+it seems to be under the dorsal furrows near the anterior end of the
+glabella. Viewed from the under side, the point of attachment is
+probably always beside the middle or anterior part of the hypostoma,
+just behind the side wings.
+
+_Paired biramous appendages._--Behind the antennules all the appendages
+except those on the anal segment are biramous, consisting of a
+coxopodite with an inward-directed endobase and an outward-directed
+pair of branches, the exopodite above, and the six-jointed endopodite
+beneath. The basipodite really bears the exopodite, but the latter
+also touches the coxopodite. This structure has been seen in
+_Triarthrus_, _Cryptolithus_, _Neolenus_, _Kootenia_, _Calymene_,
+_Ceraurus_, and _Ptychoparia_. In _Triarthrus_, _Neolenus_,
+_Acidaspis_, _Ptyclioparia_, and Kootenia, the appendages extend
+beyond the margins of the dorsal test. In _Cryptolithus_ and
+_Isotelus_ none (other than antennules) does so. In _Isotelus_ and
+_Acidaspis_ only the endopodites have been seen. In _Triarthrus_,
+_Calymene_, _Ceraurus_, and _Neolenus_ there are four pairs of
+appendages behind the antennules. The other genera probably had the
+same number, but the full structure of the under part of their cephala
+is not known. In _Triarthrus_ the endopodites of the cephalon are
+slender, the individual segments parallel-sided, the inner ones
+flattened, the outer ones cylindrical in section. They project
+slightly beyond the edge of the cephalon when fully extended, and each
+terminates in three small spines. In _Cryptolithus_ the endopodites of
+the cephalon are longer than those of the thorax, but with the
+possible exception of the first pair, are bent backward at the
+carpopodite, and do not ordinarily project beyond the brim of the
+test. In _Neolenus_ the endopodites of the cephalon are rather thick
+and wide, but are long, project forward, and extend beyond the brim.
+The individual segments are flattened, probably compressed oval in
+section. The terminal segment of each is furnished with three strong
+spines at its distal end. In _Calymene_ and _Ceraurus_ the endopodites
+appear to consist of slender segments which are oval or circular in
+section. In _Calymene_ Walcott believed the three distal segments of
+the last endopodites of the head to be greatly enlarged, giving these
+appendages a paddle-like form similar to some of the appendages of
+eurypterids. The evidence for this does not seem to me to be good. The
+cephalic endopodites of _Isotelus_ are entirely similar to those of
+the thorax, and are rather short, consisting of a series of short
+cylindrical segments which do not taper greatly toward the distal end.
+The endopodites of the cephalon of _Acidaspis_, _Kootenia_, and
+_Ptychoparia_ are still unknown.
+
+The exopodites of the cephalon seem in all known cases (_Triarthrus_,
+_Cryptolithus_, _Neolenus_, and Ceraurus) to be like those of the
+thorax. They point more directly forward in most cases, project beyond
+the margin of the head normally only in Triarthrus, and usually occupy
+the region under the cheeks (fixed and free).
+
+The endobases of the coxopodites of the appendages of the cephalon
+probably in all cases function as mouth-parts (gnathites), and are
+especially modified for this purpose in Triarthrus, being flattened,
+shoe-shaped in outline, and so arranged that they work over one
+another in a shearing fashion. While the more anterior of the
+coxopodites are attached in front of the posterior tip of the
+hypostoma, the gnathites of Triarthrus bend backward so that all are
+behind the hypostoma. In _Calymene_ and _Ceraurus_, two or three pairs
+of the gnathites are back of the hypostoma, and one or more pairs may
+be beside or under the hypostoma. In these genera the mouth is
+probably in front of the tip of the upper lip. In _Isotelus_, the
+mouth seems to have been situated in the notch between the two
+branches of the hypostoma, and the gnathites of two or three pairs of
+the appendages probably worked under its forks. Since the length of
+the hypostoma differs in the various species of _Isotelus_, there
+would be a variable number of gnathites projecting under its forks,
+according to the species. In this genus the gnathites are of the same
+long form, cylindrical in cross-section, as the endobases of the
+thoracic segments, but each is bowed back considerably from the point
+of attachment.
+
+The gnathites of _Neolenus_ are like the endobases of the thorax, but
+broader. The great length of the hypostoma makes it probable that the
+mouth was far back and that some of the gnathites were in front of it.
+The gnathites of _Cryptolithus_ are unknown. Professor Beecher in his
+drawing shows some fragments with toothed ends near the hypostoma, and
+it may be that they are inner ends of gnathites, but I see nothing
+to substantiate such an interpretation. If, as some suppose,
+_Cryptolithus_ was a mud feeder, the gnathites were probably poorly
+developed. Of the gnathites of _Kootenia_, _Ptychoparia_, and
+_Acidaspis_ also nothing is known.
+
+_Thorax._
+
+In each genus there is a pair of appendages for each segment of
+the thorax. When the axial lobe is narrow, the endobases of the
+coxopodites are small and short (_Cryptolithus_, _Ceraurus_,
+_Calymene_). When the axial lobe is wide, the endobases are long and
+stout (_Isotelus_, _Triarthrus_). The exopodites always lie above
+and in front of the corresponding endopodites. In Triarthrus the two
+branches are of practically equal length. In _Cryptolithus_ the
+exopodites are much the longer. In _Neolenus_, _Calymene_, _Ceraurus_,
+_Kootenia_, and _Ptychoparia_, the exopodites are shorter than the
+endopodites.
+
+The exopodites in Triarthrus consist of a proximal shaft, succeeded by
+numerous short segments, and ending distally in a long, grooved,
+somewhat spatula-shaped segment. Along the anterior margin of the
+shaft there are many small spines. Along the posterior margin there
+are numerous flattened setæ, which all lie in one plane and which seem
+to be more or less united to one another like the barbs of a feather.
+The setæ are short, not much longer than the width of one of the
+thoracic segments, and point backward and outward. In _Cryptolithus_
+the shaft does not seem to be made up of small segments, and is
+narrow, with a decided backward curve. The setæ are considerably
+longer and much more flattened than in Triarthrus. In _Calymene_ the
+state of preservation does not allow a very full knowledge of the
+exopodites, but they appear to have a slender, unjointed shaft and
+short and delicate setæ. The coiled branches of the exopodites as
+described by Walcott seem to me to be only ordinary Triarthrus-like
+organs, and this, as I understand from Schuchert, was also the view of
+Beecher. In _Ceraurus_ the exopodite seems to have been somewhat
+paddle-shaped, expanded at the distal end, and to have had rather
+thick, blade-like setæ.
+
+The exopodite of _Neolenus_ is decidedly leaf-like, and reminds one
+somewhat of the exites of some of the phyllopods. The shaft is a
+broad unsegmented blade. The setæ are slender, delicate, flattened,
+and a little longer than the width of the shaft. The exopodites
+of this genus point forward all along the body. In _Kootenia_ the
+exopodites are like those of _Neolenus_, but with a narrower shaft.
+The exopodites of _Ptychoparia_ appear to be very much like those of
+Triarthrus, but the shaft is probably not segmented.
+
+The endopodites of the thorax of _Triarthrus_, _Cryptolithus_, and
+_Acidaspis_ show progressive modification from front to back in the
+broadening of the individual segments and the assumption by them of
+a triangular form. Not only do the individual segments become more
+triangular from front to back, but more of the segments of each
+endopodite become triangular. This modification has so far been seen
+in these three genera only. The individual segments, except the distal
+ones, seem to be flattened in all these genera. The distal end of the
+terminal segment of each endopodite of _Triarthrus_ bears three small
+movable spines, and each of the segments usually bears three or
+more spines, located in sockets along the dorsal surface and at
+the anterior distal angle of each segment. The endopodite of
+_Cryptolithus_ is bent backward at the carpopodite and this segment
+is always thickened. At the distal end of the dactylopodite there
+is a tuft of spines, the triangular segments have tufts of spines on
+their posterior corners, and there are groups of spines also in the
+neighborhood of the articulations.
+
+The endopodites of _Ceraurus_, _Calymene_, and _Isotelus_ are all
+relatively slender, the segments are parallel-sided, and there seems
+to be no particular modification from front to back of the thorax. The
+endopodites of _Isotelus_ are short, the entire six segments of one
+being but little longer than the coxopodite of the same appendage. The
+segments of the endopodites of _Neolenus_ are mostly short and wide,
+and at the distal end of the terminal segment there are three stout
+spines. In _Kootenia_ the endopodites are long and very slender. The
+endopodites of Ptychoparia are too poorly preserved to show details,
+and those of the thorax of _Acidaspis_ likewise reveal little
+structure, but they seem to have the triangular modification, and to
+turn back somewhat sharply at about the position of the carpopodite.
+
+_Pygidium._
+
+Beecher showed that in _Triarthrus_ there was a pair of appendages on
+the pygidium for every segment of which it is composed except the last
+or anal segment (protopygidium). Walcott has since shown that in
+_Neolenus_ this segment bears a pair of cerci, and Beecher's drawings
+show that in his later studies he recognized a spinous plate, the
+possible bearer of cerci, on the anal segment of _Triarthrus_. The
+appendages of the anal segment have not yet been seen on other species
+of trilobites.
+
+The appendages of the pygidium do not show any special modifications,
+but seem in all cases to be similar to those of the posterior part of
+the thorax. In _Cryptolithus_ all the pygidial appendages are short
+and remain beneath the cover of the dorsal test, while in _Triarthrus_
+and _Neolenus_ they extend behind it.
+
+In the latter genus the endopodites of the pygidial appendages appear
+to be practically identical in form with those of the thorax, the
+individual segments being perhaps a little more nearly square in
+outline. Like those of the thorax, the segments of the pygidial
+endopodites bear numerous short spines. The caudal cerci are richly
+segmented, slightly flexible, spinous tactile organs. They are
+symmetrically placed, nearly straight when in their natural position,
+and make an angle of about 75 with one another. They appear to be
+attached to a narrow rim-like plate which seems to fit in just ahead
+of the doublure of the pygidium, or perhaps over it.
+
+In _Ceraurus_, _Calymene_, and _Isotelus_, the endopodites of the
+pygidium are similar to those of the thorax, but seemingly more
+slender, with less well developed coxopodites, and with, in the
+last-named genus, slender cylindrical segments. Exopodites are not
+known on the pygidia of any of these genera, but since they are
+present and like those of the thorax in _Triarthrus_, _Cryptolithus_,
+_Neolenus_, and _Ptychoparia_, there is little reason to think that
+they were absent in _Ceraurus_ or _Calymene_, though there is some
+question about _Isotelus_.
+
+The limbs are largest and longest on the anterior part of the thorax
+of a trilobite, and diminish regularly in length and strength to the
+posterior end of the pygidium. This regular gradation shows, as
+Beecher was the first to point out, that the growing point of the
+trilobites is, as in other arthropods, in front of the anal segment.
+New _free_ segments are introduced into the thorax at the anterior end
+of the pygidium, and this has led to some confusion between the
+growing point and the place of introduction of free segments.
+
+If a new segment were introduced at a moult in front of the pygidium,
+that segment would probably have less fully developed appendages than
+those adjacent to it, and so make a break in the regular succession.
+The condition of the appendages corroborates the evidence derived from
+the ontogeny of the pygidium, and proves that the new segments are
+introduced at the same growing point as in other Arthropoda.
+
+_Caudal Rami._
+
+Bernard, who believed that the Crustacea had been derived through an
+_Apus_-like ancestor (1892, pp. 20, 85, 274), pointed out that four or
+less than four anal cirri were to be expected. Two well developed
+cirri and two rudimentary ones are present in _Apus_, and they are
+also to be found in other phyllopods and some isopods. It is, however,
+characteristic of the Crustacea as a whole to lack appendages on the
+anal segment. Caudal cirri (cerci) are much more freely developed in
+the hexapods than in the Crustacea, particularly in the more primitive
+orders, Palæodictyoptera, Apterygota, Archiptera, and Neuroptera. They
+are supposed, in this case, to be modified limbs, and therefore not
+homologous with the bristles on the anal segment of an annelid. Doctor
+W. M. Wheeler of the Bussey Institution has kindly allowed me to quote
+the following excerpt from a letter to me, as expressing the opinion
+of one who has made an extensive study of the embryology of insects:
+
+ I would say that I have no doubt that the cerci of insects are
+ directly inherited from the insect ancestors. They are always
+ highly developed in the lower insects, and only absent or vestigial
+ in a few of the most highly specialized orders such as the
+ Hemiptera, Diptera, and Hymenoptera. I have further no doubt
+ concerning their being originally ambulatory in function. They are
+ certainly not developed independently in insects. Embryologically
+ they arise precisely like the legs, and each cercus contains a
+ diverticulum of the mesoblastic somite precisely as is the case
+ with the ambulatory legs and mouth parts.
+
+The "pygidial antennæ" seem to be as fully developed in _Neolenus_ as
+in any of the other arthropods, and may suggest a common ancestry of
+the phyllopods, isopods, and hexapods, in the trilobites. They were
+doubtless tactile organs, and while the evidence is chiefly negative,
+it would seem that they proved useless, and were lost early in the
+phylogeny of this group. Possibly the use of the pygidium as a
+swimming organ proved destructive to them.
+
+
+HOMOLOGY OF THE CEPHALIC APPENDAGES WITH THOSE OF OTHER CRUSTACEA.
+
+The head of the typical crustacean bears five pairs of appendages,
+namely, the antennules, antennas, mandibles, and first and second
+maxillæ, or, as they are more properly called, the maxillulæ and
+maxillæ.
+
+As Beecher has pointed out, the "antennæ" of the trilobites, on
+account of their pre-oral position and invariably uniramous character,
+are quite certainly to be correlated with the antennules.
+
+The second pair of appendages, the first pair of biramous ones,
+Beecher homologized with the antennæ of other crustaceans, and that
+homology has been generally accepted, though Kingsley (1897) suggested
+that it was possible that no representatives of the true antennæ were
+present.
+
+In preparing the restorations in the present study, the greatest
+difficulty has been to adjust the organs about the mouth. In
+_Triarthrus_, numerous specimens show that without question there are
+four pairs of gnathites back of the hypostoma, and that all four
+belong to the cephalon. In forms with a long hypostoma, however, there
+was no room on the cephalon for the attachment of four pairs of
+gnathites, neither were there enough appendifers to supply the
+requisite fulcra. At first I supposed I had solved the difficulty by
+assuming the mouth to be in front of the posterior tip of the
+hypostoma, as it really is in Ceraurus and _Calymene_, and allowing
+the gnathites to play under the hypostoma as Walcott (1912) has shown
+that they do in _Marrella_. Finally, when I came to study in greater
+detail the slices of _Calymene_ and _Ceraurus_, they seemed to show
+that the anterior one or two pairs of appendages became degenerate and
+under-developed. This was probably a specialization due to the great
+development of the hypostoma in trilobites, that organ being much
+more prominent in this than in any other group. As the hypostoma
+lengthened to accommodate the increasing size of sub-glabellar organs
+(stomach, heart, etc.), the mouth migrated backward, leaving the
+anterior appendages ahead of it, with their gnathobases, at least,
+functionless. That such migration has taken place, even in Triarthrus,
+is shown by the fact that the points of articulation of the first
+biramous appendages are pre-oral, and it is more obviously true of
+_Ceraurus_. Correlated with the weakening of the appendages on the
+lower surface is the loss of glabellar furrows on the upper surface.
+The glabellar furrows mark lines of infolding of the test to form the
+appendifers and other rugosities for the attachment of tendons and
+muscles. It is conceivable that this migration backward of the mouth
+began very early in the history of the race, and that even before
+Cambrian times, the antennæ, probably originally biramous appendages
+like those on the remainder of the body, had dwindled away and become
+lost. If this is the case, then the first pair of biramous appendages
+of _Triarthrus_ would be mandibles, the second pair maxillulæ, and the
+third pair maxillæ.
+
+There remain the last pair of cephalic appendages, and they bring up
+the whole head problem of the trilobites. Beecher has stated (1897 A,
+p. 96) his conviction that the head of the trilobite is made up of
+five segments, representing the third, fourth, fifth, sixth, and
+seventh neuromeres of the theoretical crustacean. As a matter of fact,
+he really made up the head of seven segments, since he stated that the
+first neuromere was represented by the hypostoma and the second by the
+epistoma and free cheeks.
+
+Jaekel (1901, p. 157) nearly agreed with Beecher, but made eight
+segments, as he saw five segments in the glabella of certain
+trilobites. In his table (p. 165) he has listed the segments with
+their appendages as follows: 1. Acron, with hypostoma; 2, rostrum
+(epistoma), with free cheeks; 3, first frontal lobe, with (?)
+antennules; 4, second frontal lobe, with antennæ; 5, mandibles; 6,
+first, or pre-maxillæ; 7, second maxillæ; 8, occipital segment with
+maxillipeds.
+
+Jaekel refused to believe that the antennæ of trilobites were really
+entirely simple, and so homologized them with the antennæ and not the
+antennules of other Crustacea. In this he was obviously incorrect, but
+it accounts for his homology of the remainder of the cephalic
+appendages.
+
+It is, at present, impossible to demonstrate the actual number of
+somites in the cephalon of the trilobite, but I believe that Beecher
+was correct in holding that the glabella was composed of four
+segments. There are, it is true, a number of trilobites (Mesonacidæ,
+Paradoxidæ Cheiruridæ, etc.) which show distinctly four pairs of
+glabellar furrows, indicating five segments in the glabella. This is,
+however, probably due to a secondary division of the first lobe.
+
+The correspondence of the five segments on the dorsal side with the
+five pairs of appendages makes it unlikely that any pair of limbs has
+been lost. The condition in _Marrella_, where a trilobite-like
+cephalon bears five pairs of appendages, the second pair of which are
+tactile antennæ, is favorable to the above interpretation. In spite of
+the apparent degeneration of the first two pairs of appendages in
+_Calymene_, no limbs are actually missing, and if some are dropped out
+in the later trilobites it would not affect the homology of those now
+known. I therefore agree with Beecher in homologizing the appendages,
+pair for pair, with those of the higher Crustacea.
+
+
+FUNCTIONS OF THE APPENDAGES.
+
+_Antennules._
+
+The antennules were obviously tactile organs, probably freely movable
+in most trilobites, but in the case of Triarthrus perhaps rather
+rigid, judging from the great numbers of specimens which show the
+characteristic sigmoid curve made familiar by Professor Beecher's
+restoration. The proximal end of the shaft of each antennule of
+Triarthrus is hemispheric and doubtless fitted into a socket, thus
+suggesting great mobility of the whole organ. In spite of this, I have
+seen no specimens in which they did not turn in toward each other and
+cross the anterior margin very near the median line. In front of the
+margin, various specimens show evidence of flexibility, but from the
+proximal end to the margin the position is the same in all specimens.
+
+In all the few specimens of _Cryptolithus_ retaining the antennules,
+these organs are turned directly backward, but it is entirely within
+the range of probabilities that while its burrowing habits made this
+the more usual position, the animal had the power of turning them
+around to the front when they could be used to advantage in that
+direction.
+
+_Exopodites._
+
+It has been the opinion of most observers that the exopodites of
+trilobites were swimming organs, while others have thought that they
+functioned also in aerating the blood. To the present writer it seems
+probable that the chief function was that of acting as gills, for
+which the numerous thin, flattened or blade-like setæ are particularly
+adapted. That they were also used in swimming is of course possible,
+but that was not their chief function. It should be remembered that
+the exopodites are always found dorsal to or above the endopodites,
+and in a horizontal plane. For use in swimming it would have been
+necessary to rotate each exopodite into a plane approximately
+perpendicular to or at least making a considerable angle with the
+dorsal test. In this position, the exopodites would have been thrust
+down between the endopodites, and one would expect to find some
+specimens in which a part at least of the exopodites were ventral to
+the endopodites. Specimens in this condition have not yet been seen
+among the fossils. To avoid having the exopodites and endopodites
+intermingled in this way, the animal would have to bring all the
+endopodites together along the axial line in a plane approximately
+perpendicular to the dorsal test, in which case the exopodites would
+be free to act as swimming organs. The fact that the setæ of an
+exopodite stay together like the barbs on a feather would of course
+tend to strengthen the idea that the exopodites could be used in
+swimming, but that is not the only possible explanation of this
+condition. The union of the basipodite and exopodite shows that the
+two branches of the appendage acted together. Every movement of one
+affected the other, and the motion of the endopodites in either
+swimming or crawling produced a movement of the exopodites which
+helped to keep up a circulation of water, thus insuring a constant
+supply of oxygen.
+
+Although _Neolenus_ is usually accounted a less primitive form than
+_Ptychoparia_ or _Triarthrus_, it has much the most primitive type
+of exopodite yet known. It would appear that the exopodites were
+originally broad, thin, simple lamellæ, which became broken up, on the
+posterior side, into fine cylindrical setæ. As development progressed,
+more and more of the original lamella was broken up until there
+remained only the anterior margin, which became thickened and
+strengthened to support the delicate filaments. The setæ in turn
+became modified from their original simple cylindrical shape to form
+the wide, thin, blade-like filaments of _Cryptolithus_ and _Ceraurus_.
+
+Another possible use of the exopodites is suggested by the action of
+some of the barnacles, which use similar organs as nets in gathering
+food and the endopodites as rakes which take off the particles and
+convey them to the mouth. The exopodites of the trilobite might well
+set up currents which would direct food into the median groove, where
+it could be carried forward to the mouth.
+
+_Endopodites._
+
+The endopodites were undoubtedly used for crawling; in some
+trilobites, probably most of them, for swimming; in the case of
+_Cryptolithus_, and probably others, for burrowing; and probably in
+all for gathering food, in which function the numerous spines with
+which they are arrayed doubtless assisted.
+
+Various trails have been ascribed to the action of trilobites, and
+many of them doubtless were made by those animals (see especially
+Walcott, 1918). Some of these trails seem to indicate that in crawling
+the animal rested on the greater part of each endopodite, while
+others, notably the _Protichnites_ recently interpreted by Walcott
+(1912 B, p. 275, pl. 47), seem to have touched only the spinous tips
+of the dactylopodites to the substratum. The question of the tracks,
+trails, and burrows which have been ascribed to trilobites is discussed
+briefly on a later page; but can not be taken up fully, as it would
+require another monograph to treat of them satisfactorily.
+
+The flattened, more or less triangular segments of the endopodites
+of the posterior part of the thorax and pygidium in _Triarthrus_,
+_Cryptolithus_, and _Acidaspis_ probably show an adaptation of the
+endopodites of the posterior part of the body both as more efficient
+pushing organs and as better swimming legs. The fact that these
+segments are pointed below enabled them to get a better grip on
+whatever they were crawling over, and the flattening allowed a much
+greater surface to be opposed to the water in swimming. In this
+connection it might be stated that it seems very probable that the
+trilobites with large pygidia at least, perhaps all trilobites, had
+longitudinal muscles which allowed them to swim by an up and down
+motion of the fin-like posterior shield, the pygidium acting like the
+caudal fin of a squid. Such a use would explain the function of the
+large, nearly flat pygidia seen in so many of the trilobites beginning
+with the Middle Cambrian, and of those with wide concave borders. It
+should be noted here that it is in trilobites like _Isotelus_, with
+pygidia particularly adapted to this method of swimming, that the
+endopodites are most feebly developed, and show no flattening or
+modification as swimming organs.
+
+The relatively strong, curved, bristle-studded endopodites of
+_Cryptolithus_, combined with its shovel-shaped cephalon, indicate
+_Limulus_-like burrowing habits for the animal, and the mud-filled
+casts of its intestine corroborate this view. That it was not,
+however, entirely a mud groveller is indicated by its widespread
+distribution in middle Ordovician times.
+
+_Use of the Pygidium in Swimming._
+
+The idea that the use of the pygidium as a swimming organ is a
+possible explanation of that caudalization which is so characteristic
+of trilobites has not been developed so far as its merits seem to
+deserve. Two principal uses for a large pygidium of course occur
+to one: either it might form a sort of operculum to complete the
+protection when the trilobite was enrolled, or it might serve as a
+swimming organ. That the former was one of its important functions is
+shown by the nicety with which the cephalon and pygidium are adapted
+to one another in such families as the Agnostidæ, Asaphidæ, Phacopidæ,
+and others. That a large pygidium is not essential to perfect
+protection on enrollment is shown by an equally perfect adjustment of
+the two shields in some families with small pygidia, notably the
+Harpedidæ and Cheiruridæ That the large pygidial shields are not for
+protective purposes only is also shown by those forms with large
+pygidia which are not adjusted to the conformation of the cephalon, as
+in the Goldiidæ and Lichadidæ. It is evident that a large pygidium,
+while useful to complete protection on enrollment, is not essential.
+
+It would probably be impossible to demonstrate that the trilobites
+used the pygidium in swimming. The following facts may, however, be
+brought forward as indicating that they probably did so use them.
+
+1. The appendages, both exopodites and endopodites, are relatively
+feebly developed as swimming organs. This has been discussed above,
+and need not be repeated. It must in fairness be observed, however,
+that many modern Crustacea get about very well with limbs no better
+adapted for swimming than those of the trilobites.
+
+2. The articulations of the thoracic segments with each other and with
+the two shields are such as to allow the pygidium to swing through an
+arc of at least 270, that is, from a position above the body and at
+right angles to it, around to the plane of the bottom of the cephalon.
+Specimens are occasionally found in which the thorax and pygidium are
+so flexed that the latter shield stands straight above the body. A
+well preserved _Dipleura_ in this position is on exhibition in the
+Museum of Comparative Zoology, and Mr. Narraway and I have figured a
+_Bumastus milleri_ in the same attitude (Ann. Carnegie Mus., vol. 4,
+1908, pl. 62, fig. 3).
+
+3. What little can be learned of the musculature (see under
+musculature, seq.) indicates that the trilobites had powerful extensor
+and flexor muscles, such as would be required for this method of
+swimming. It may be objected that the longitudinal muscles were too
+small to permit the use of a caudal fin. In the lobster, where this
+method of progression is most highly developed, there is a large
+mass of muscular tissue which nearly fills the posterior segments.
+Trilobites have not usually been thought of as powerfully muscled, but
+it may be noted that in many cases broad axial lobes accompany large
+pygidia. As the chief digestive region appears to have been at the
+anterior end, and other organs are not largely developed, it seems
+probable that the great enlargement of the axial lobe was to
+accommodate the increased muscles necessary to properly operate the
+pygidium. It may be noted that in all these genera the axial lobe of
+the pygidium is either short or narrow.
+
+4. The geological history of the rise of caudalization favors this
+view. With the exception of the Agnostidæ and Eodiscidæ, all Lower
+Cambrian trilobites had small pygidia, and the same is true of
+those of the Middle Cambrian of the Atlantic realm (except for the
+_Dorypyge_ of Bornholm). In Pacific seas, however, large-tailed
+trilobites of the families Oryctocephalidæ, Bathyuridæ, and Asaphidæ
+then began to be fairly common, though making up but a small part of
+the total fauna of trilobites. In the Upper Cambrian of the Atlantic
+province the Agnostidæ were the sole representatives of the isopygous
+trilobites, while in the Pacific still another family, the
+Dikelocephalidæ, was added to those previously existing.
+
+With the Ordovician, caudalization reached its climax and the fashion
+swept all over the world. It is shown not so much in the proportion of
+families with large pygidia, as in the very great development of the
+particular trilobites so equipped. Asaphidæ and Illænidæ were then
+dominant, and the Proëtidæ, Cyclopygidæ Goldiidæ, and Lichadidæ had
+begun their existence. A similar story is told by the Silurian record,
+except that the burden of the Asaphidæ has been transferred to the
+Lichadidæ and Goldiidæ. All the really old (Cambrian) families of
+trilobites with small pygidia had now disappeared. In the general
+dwindling of the subclass through the Devonian and later Palæozoic,
+the few surviving species with small pygidia were the first to go, and
+the proëtids with large abdominal shields the last.
+
+The explanation of this history is probably to be found in the rise of
+the predatory cephalopods and fishes, the natural enemies of the
+trilobites, against whom they could have no other protection than
+their agility in escaping. While the records at present known carry
+the fishes back only so far as the Ordovician (fishes may have arisen
+in fresh waters and have gone to sea in a limited way in the
+Ordovician and more so in Silurian time) and the cephalopods to the
+Upper Cambrian, the rise of the latter must have begun at an earlier
+date, and it is probably no more than fair to conjecture that the
+attempt to escape swimming enemies caused an increase in the swimming
+powers of the trilobites themselves. At any rate, the time of the
+great development of the straight cephalopods coincided with the time
+of greatest development of caudalization; both were initiated in the
+Pacific realm, and both spread throughout the marine world during the
+middle Ordovician. And since, in the asaphids, a decrease in swimming
+power of the appendages accompanied the increase in the size of the
+pygidium, it seems probable that the swimming function of the one had
+been transferred to the other. A high-speed, erratic motion which
+could be produced by the sudden flap of a pygidium would be of more
+service in escape than any amount of steady swiftness produced by the
+oar-like appendages of an animal of the shape of a trilobite.
+
+_Coxopodites._
+
+The primary function of the endobases of the coxopodites was doubtless
+the gathering, preparation, and carrying of food to the mouth.
+Although the endobases of opposite sides could not in all cases meet
+one another, they were probably spinose or setiferous and could
+readily pass food from any part of the axial groove forward to the
+mouth, and also send it in currents of water. The endobases of the
+cephalic coxopodites were probably modified as gnathites in all cases,
+but little is known of them except in Triarthrus, where they were
+flattened and worked over one another so as to make excellent shears
+for slicing up food, either animal or vegetable. In some cases the
+proximal ends of opposed gnathites were toothed so as to act as jaws,
+but a great deal still remains to be learned about the oral organs of
+all species.
+
+The writer has suggested (1910, p. 131) that a secondary function
+of the endobases of the thorax of _Isotelus_ and probably other
+trilobites with wide axial lobes was that of locomotion. In _Isotelus_
+the endobases of the thorax are greatly over-developed, each being
+much stouter and nearly as long as the corresponding endopodite, and
+the explanation seemed to me to lie in the locomotor or crawling use
+of these organs instead of the endopodites. Certain trails which I
+figured seemed to support this view.
+
+
+POSITION OF THE APPENDAGES IN LIFE.
+
+In almost all the specimens so far recovered the appendages are either
+flattened by pressure or lie with their flat surfaces in or very near
+the plane of stratification of the sediment. This flattening is
+extreme in Neolenus, Ptychoparia, and Kootenia, moderate in
+_Triarthrus_ and _Cryptolithus_, and apparently slight or not
+effective in _Isotelus_, _Ceraurus_, and _Calymene_. These last are,
+however, from the conditions of preservation, least available for
+study.
+
+In Part IV, attention is called to a specimen of Triarthrus (No. 222)
+in which some of the endopodites are imbedded nearly at right angles
+to the stratification of the shale. This specimen is especially
+valuable because it shows that the appendages in the average specimen
+of Triarthrus have suffered very little compression, and it also
+suggests the probable position of the endopodites when used for
+crawling.
+
+In considering the position of the appendages in life, one must always
+remember one great outstanding feature of trilobites, the thinness and
+flexibility of the ventral membrane. The appendages were not inserted
+in any rigid test but were held only by muscular and connective
+tissue. Hence we must premise for them great freedom of motion, and
+also relatively little power. The rigid appendifers, and the
+supporting apodemes discovered by Beecher, supplied fulcra against
+which they could push, but their attachment to these was rather loose.
+
+Considering, first, the position of the appendages in crawling, it
+appears that different trilobites used their appendages in different
+ways. _Neolenus_ had compact stocky legs, which allowed little play of
+one segment on another, as is shown by the wide joints at right angles
+to the axis of the segment. Such limbs were stiff enough to support
+the body when the animal was crawling beneath the water, where of
+course it weighed but little. That such a crawling attitude was
+adopted by trilobites has been shown by Walcott in his explanation of
+the trails known as _Protichnites_ (1912 B, p. 278). Many trilobites
+probably crawled in this way, on the tips of the toes, so to speak.
+In such the limbs would probably extend downward and outward, with the
+flattened sides vertical.
+
+The limb of _Triarthrus_, however, is of another type. The endopodites
+are long, slender, flexibly jointed, the whole endopodite probably too
+flexible to be used as a unit as a leg must be in walking on the
+"toes." The proximal segments of the thoracic and pygidial endopodites
+are, however, triangular instead of straight-sided, and, the
+spine-bearing apex of the triangle being ventral, it enabled the
+endopodites to get a grip on the bottom and thus push the animal
+forward. This method of progression was more clumsy and less rapid
+than that of Neolenus, but it sufficed. The natural position of the
+endopodite when used in this way would seem to be with the flattened
+sides of the segments standing at an angle of 30 to 45 with the
+vertical, thus allowing a good purchase on the bottom and at the same
+time offering the minimum resistance to the water when moving the
+appendages forward.
+
+_Isotelus_ has endopodites different from those of either _Neolenus_
+or _Triarthrus_. They are composed of cylindrical segments, the joints
+indicating a certain amount of flexibility. Since there is no method
+by which the segments may get a purchase on the bottom other than by
+pushing with the distal ends, it would seem at first thought that
+_Isotelus_, like Neolenus, crawled on its "toes." The endopodites
+of _Isotelus_ are however, short and feeble when compared with
+the size of the test, while the endobases of the coxopodites are
+extraordinarily developed. These facts, together with certain trails,
+strongly suggest the use of the coxopodites as the primary ambulatory
+organs, the endopodites probably assisting. In this event, the
+position of the endopodites and coxopodites would be downward, both
+outward and inward from the point of attachment, and the motion both
+backward and forward. The fact that in the specimens as preserved the
+coxopodites point backward and the endopodites forward indicates that
+the limb as a whole swung on a pivot at the appendifer. It is of
+course natural to suggest that the coxopodites and endopodites of all
+the trilobites with wide axial lobes, _Nileus_, _Bumastus_,
+_Homalonotus_, etc., were developed in this same way.
+
+_Cryptolithus_ presents still another and very peculiar development of
+the endopodites where ability to get purchase on the sea floor is
+obtained by a stout limb of slight flexibility, bowed and turned
+backward in the middle, where an enlarged segment insures stiffness.
+The segments are flattened, and since the greatest strength when used
+in pushing and crawling is in the long axis of the oval section of
+the flattened limb, it seems probable that these limbs did not hang
+directly down, with their sides vertical, but that their position in
+life was very much the same as that in which they are preserved as
+fossils. By moving these bowed legs forward and backward in a plane at
+a small angle to the surface of the body, a powerful pushing impetus
+could be obtained. They may, however, have occupied much the same
+position as do those of _Limulus_.
+
+In the case of the endopodites, therefore, it is necessary to study
+the structure and probable method of their use in each individual
+genus before suggesting what was the probable position in life. In
+the act of swimming, the position was probably more uniform. When
+the endopodites were used in swimming, as they undoubtedly could be
+with more or less effect in all the trilobites now known, those with
+flattened surfaces probably had them at such an angle as to give the
+best push against the water on the back stroke, while on the forward
+stroke the appendage would be turned so that' the thin edge opposed
+the water. The great flexibility of attachment would certainly permit
+this, though unfortunately nothing is as yet known of the
+musculature. The coxopodites of course had less freedom of movement
+in this respect, and probably could not turn their faces. For this
+reason, it seems to me likely that those coxopodites which are
+compressed did not stand with their flattened faces vertical, but in a
+position which was nearly horizontal or at least not more than 45 from
+the horizontal. If the flattened faces were vertical, they would be in
+constant opposition to the water during forward movements and would be
+of no use in setting up currents of water toward the mouth, as every
+back stroke would reverse the motion.
+
+The position of the exopodites in life seems to have been rather
+uniform in all the genera now known. I have set forth on a previous
+page my reasons for thinking that they took little part in swimming,
+and I look upon them as being, in effect, leaf-gills. It seems
+probable that in all genera the exopodites were held rather close
+to the test, the shaft more or less rigid, the filamentous setæ
+gracefully pendent, but pendent as a sheet and not individually, there
+having been some method by which adjoining setæ were connected
+laterally. Free contact with the water was thus obtained without the
+mingling of endopodites and exopodites which would have been so
+disastrous to progression.
+
+
+
+
+PART II.
+
+
+
+
+Structure And Habits Of Trilobites.
+
+
+INTERNAL ORGANS AND MUSCLES.
+
+Granting that the trilobite is a simple, generalized, ancient
+crustacean, it appears justifiable to attribute to it such internal
+organs as seem, from a study of comparative anatomy, to be primitive.
+
+The alimentary canal would be expected to be straight and simple,
+curving downward to the mouth, and should be composed of three
+portions, stomodæum, mesenteron, and proctodæum, the first and last
+with chitinous lining. In modern Crustacea, muscle-bands run from the
+gut to part of the adjacent body wall, so that scars of attachment of
+these muscles may be sought. At the anterior end of the stomodæum,
+they are usually especially strong. From the mesenteron there might be
+pouch-like or tubular outgrowths.
+
+The heart would probably be long and tubular, with a pair of ostia for
+each somite.
+
+In modern Crustacea, the chief organs of renal excretion are two pairs
+of glands in the head, one lying at the base of the antennæ and one at
+the base of the maxillæ. Only one pair is functional at a time, but
+these are supposed to be survivors of a series of segmentally arranged
+organs, so that there might be a pair to each somite of a trilobite.
+
+The nervous system might be expected to consist of a supracesophageal
+"brain," comprising at least two pairs of ganglionic centers, and a
+double ventral chain of ganglia with a ladder-like arrangement.
+
+Besides these organs, a variety of glands of special function might be
+predicted.
+
+Reproductive organs probably should occur in pairs, and more than one
+pair is to be expected. There is little to indicate the probable
+location of the genital openings, but they may have been located all
+along the body back of the cephalon.
+
+It may be profitable to summarize present knowledge of such traces of
+these organs as have been found in the fossils, if only to point out
+what should be sought.
+
+
+ALIMENTARY CANAL.
+
+Beyrich (1846, p. 30) first called attention to the alimentary canal
+of a trilobite, (_Cryptolithus goldfussi_,) and Barrande (1852, p.
+229) confirmed his observations. A number of specimens of this species
+have been found which show a straight cylindrical tube or its filling,
+extending from the glabella back nearly to the posterior end of the
+pygidium. It lies directly under the median line of the axial lobe,
+and less than its own diameter beneath the dorsal test. At the
+anterior end it apparently enlarges to occupy the greater part of the
+space between the glabella and the hypostoma, but was said by the
+early observers to extend only a little over halfway to the front.
+Beyrich thought the position of the median tubercle indicated the
+location of the anterior end.
+
+Walcott (1881, p. 200) stated that in his experience in cutting
+sections of trilobites it was a very rare occurrence to find traces of
+the alimentary canal. The visceral cavity was usually filled with
+crystalline calcite and all vestiges of organs obliterated. There
+were, however, some slices which showed a dark spot under the axial
+lobe, which probably represented the canal. In his restoration he
+showed it as of practically uniform diameter throughout, and extending
+but slightly in front of the mouth.
+
+Jaekel (1901, p. 168, fig. 28) has produced a very different
+restoration. His discussion of this point seems so good, and has been
+so completely overlooked, that I will append a slightly abridged
+version of a translation made some years ago for Professor Beecher.
+The idea was, however, not original with Jaekel, as it was suggested
+by Bernard (1894, p. 417), but not worked out in detail.
+
+ While considering the problem as to what organ could have lain
+ beneath the glabella of the trilobite, and while studying the
+ organization of living Crustacea for the purpose of comparison, I
+ found in the collections of the Geological Institute preparations
+ of _Limulus_ which seemed to me to directly solve the entire
+ question.
+
+ From the mouth, which lies at about the middle of the head shield,
+ the oesophagus bends forward, swells out at the frontal margin of
+ the animal at a sharp upward bend in order to take a straight
+ course backward after the formation of an enlarged stomach. Still
+ within the head shield there branch out from each' side of the
+ canal two small vessels which pass over into the richly branched
+ mass of liver lying under the broad lateral parts of the head
+ shield. After seeing this specimen, I no longer had the least doubt
+ that the head shield of the trilobites is to be interpreted in a
+ similar manner. The position of the hypostoma and gnathopods makes
+ it necessary to assume that the position of the mouth of the
+ trilobite lay pretty far back. If, therefore, this depends upon the
+ secondary ventral deflection of the oral region, as seems to be the
+ case, then it is a priori probable that the anterior part of the
+ canal has also shared in this ventral inflection.
+
+ The posterior part of the canal in the region of the segmented
+ thorax and pygidium is comparatively narrow, as shown long ago by
+ Beyrich; he represents only a thin tube which shows no swellings
+ whatever, and such are usually missing in Arthropoda.
+
+ As the glabella of most trilobites is regularly convex, there must
+ lie beneath it an organ running from front to back, which presses
+ the bases of the cephalic legs away from each other and down from
+ the dorsal test. An organ so extensive and unpaired, running thus
+ from front to back, can, among the Arthropoda, be regarded only as
+ an alimentary canal, for the swellings of the cephalic ganglia and
+ the heart are by far too small to produce such striking elevations
+ on the front and upper surface of the glabella. The canal might
+ then have consisted of a gizzard belonging to the oesophagus,
+ and astomach proper or main digestive canal.
+
+ ... Among the trilobites there are two pairs of vessels on both
+ sides of the glabella which have precisely the same position with
+ reference to the supposed course of the alimentary canal as the
+ ducts of the hepatic lobes in _Limulus_. One observes in numerous
+ trilobites, although in different degrees of clearness and under
+ various modifications, a dendritic marking of the inner surface
+ of the cheeks which takes its rise at the lateral margins of the
+ glabella and spreads thence like a bush over the entire surface
+ of the cheeks. Exactly the same position is taken by the richly
+ branched hepatic lobes of _Limulus_ on the lower surface of the
+ head shield; a fact of special weight in favor of the homology
+ and similar significance of the two phenomena, is that in the
+ trilobites also, the anterior of the two main ducts is the larger,
+ the posterior the smaller. The striking similarity of the two
+ structures is shown by a comparison of the head shield of
+ _Eurycare_ [_Elyx_] from the Cambrian of Sweden, in which the
+ course of the canals is shown with remarkable clearness [with
+ those of _Limulus_].
+
+ I have been able to convince myself that the existence of the two
+ canals on each side is also the rule in other genera, even though
+ the posterior pair is frequently but feebly developed or completely
+ obscured by the anterior pair. In _Dionide formosa_, for example, I
+ find only the anterior pair, which is very large and divided into
+ two principal branches. From all these considerations it seems to
+ me no longer doubtful that the median elevation was caused by the
+ stomach and gizzard, and that the cheeks have principally served to
+ cover the hepatic appendages of the alimentary canal.
+
+ The cause of the incomplete development of the glabellar lobes
+ lies, hence, in the intrusion of the alimentary canal, and it makes
+ naturally the most effect where the gizzard spreads out and bends
+ into the stomach. This spot lies behind the frontal lobe, which is
+ hence increased in size according as the stomach increases in size;
+ in this way not only the foremost segments of the glabella become
+ enlarged, but also the following ones more or less pressed aside.
+ This process is easily followed phylogenetically and
+ ontogenetically.
+
+ From the latter point of view, the development of _Paradoxides_ is
+ very instructive. In a head shield 2.5 mm. long the whole anterior
+ part of the glabella is broadened, but the five pairs of lateral
+ impressions are clearly marked and the six segments of the head
+ bounded by them are all of about the same size. In a head shield
+ about 13 mm. long, the foremost segment is very much increased in
+ size, the jaw lobes pressed still further apart; in adult forms
+ both anterior segments are combined into the frontal swellings
+ of the glabella. In other groups this process proceeds
+ phylogenetically still further, so that among the Phacopidæ and in
+ _Trinucleus_, behind the frontal swelling of the glabella only the
+ last cephalic segment retains a certain independence. The frontal
+ lobe is thus no definite part, although it is as a rule composed of
+ the mesotergites of the first two cranidial segments.
+
+
+This idea of an enlarged mesenteron certainly has much to commend it,
+and such actual evidence as exists seems in favor of rather than
+against it. The strongest, firmest, best-protected place in the whole
+body of the trilobite is the cavity between the vaulted glabella and
+the hypostoma. As Jaekel has said, it is far too large a cavity for
+the brain, larger than would seem to be required for a heart, and what
+else could be there but a stomach? As has already been pointed out,
+Beyrich and Barrande found a pear-shaped enlargement of the alimentary
+canal under the glabella of _Cryptolithus_. Longitudinal sections
+through the glabella of _Calymene_ and _Ceraurus_ practically always
+show the cavity there filled with clear crystalline calcite. One
+actual specimen of _Ceraurus_ (Walcott 1881, pl. 4, fig. 1) shows the
+cavity between the glabella and hypostoma entirely empty. The vacant
+spaces in these two classes of specimens do not, however, necessarily
+mean anything more than imperfect preservation.
+
+[Illustration: Fig. 21.--Transverse slice through _Ceraurus
+pleurexanthemus_, to show the dorsal sheath above the abdominal
+cavity. Specimen 118. Traced from a photographic enlargement. × 4.]
+
+[Illustration: Fig. 22.--Transverse section through the cephalon of
+_Ceraurus pleurexanthemus_, showing the abdominal sheath and the large
+mud-filled alimentary canal (clear white). Traced from a photographic
+enlargement. Specimen 97. × 3.3.]
+
+[Illustration: Fig. 23.--Transverse section of the thorax of _Calymene
+senaria_, showing the large size of the mud-filled alimentary canal
+(clear white). Traced from a photographic enlargement One appendifer
+(also clear white) is shown. Specimen 153. × 3.3.]
+
+_Ceraurus pleurexanthemus._
+
+This species is taken up first, as it is the one shown in Walcott's
+often-copied figure (1881, pl. 4, fig. 6). It is to be feared that too
+many have looked at this figure without reading the accompanying
+explanation, and have taken it for a copy of an actual specimen and
+not a mere diagram, which it admittedly is. The evidence on which it
+is based is comprised in eight transverse slices, one through the
+glabella and seven through the thorax. Three of these have been
+figured by Walcott: No. 27, 1881, pl. 3, fig. 7; No. 13, 1881, pl. 2,
+fig. 3, 1918, pl. 26, fig. 14; No. 202, 1918, pl. 27, fig. 8. In all,
+as can be seen by reference to the figures, the canal is partially
+collapsed, and is much larger than is indicated in Walcott's
+restoration. The other sections bear out the testimony of those
+figured. One of these figured specimens (No. 27) and another figured
+herewith (No. 118, see fig. 21) show an exceedingly interesting
+structure which has previously escaped notice. The body cavity seems
+to have had, in this region at least, a chitinous sheath on the dorsal
+side. As shown especially in figure 21, this sheath impinges dorsally
+and laterally against the axial lobe and thus furnishes a special
+protection for the soft organs beneath, probably protecting them from
+the strain of the dorsal muscles.
+
+While there is no way in which the location of these sections in the
+thorax can be positively determined, it is probable that they came
+from the anterior end. In sections further back, supposed to be in the
+posterior region of the mesenteron, no sheath is shown, but the canal
+is nearly if not quite as large in relation to the size of the axial
+lobe.
+
+The single section through the glabella (specimen 97) is of course
+important and fortunately well preserved (fig. 22). It shows the
+dorsal sheath pressed against the inner surface of the axial lobe
+along its middle portion, but diverging from it at the sides. The
+section of the canal is oval, nearly twice as wide as high, but it is
+obviously somewhat depressed. The original canal evidently filled
+nearly the whole of the dorsal part of the glabella in this particular
+region. Unfortunately, the connection with the mouth is not shown, and
+the form of the hypostoma indicates that the section cut the glabella
+diagonally, either in the anterior or posterior part, probably the
+latter. In all these cases it should be remembered that the specimens
+were found lying on their backs, and the canal has fallen in
+(dorsally) since death.
+
+The sections show that in _Ceraurus pleurexanthemus_ the anterior part
+of the alimentary canal was large, filling the part of the glabella
+below the heart; that the body cavity was provided with a chitinous
+dorsal sheath extending back into the thorax; and that the posterior
+portion of the mesenteron was likewise large and oval in section.
+Since the alimentary canal must be connected with the mouth and anus,
+some such restoration as that of Jaekel is indicated. No chitinous
+lining of the stomodæum or proctodæum was found, but it is not certain
+that any of the sections cut either of those regions.
+
+_Calymene senaria._
+
+Ten transverse sections and one longitudinal slice show the form of
+the alimentary canal in _Calymene_. One of these has been figured by
+Walcott (1881, pl. 1, fig. 9) but without showing the organ in
+question.
+
+The only section cutting the cephalon which shows any trace of the
+canal is a longitudinal one (No. 141), which is not very satisfactory.
+It has a large, nearly circular, opaque spot under the anterior part
+of the glabella which may or may not represent a section across the
+anterior end of the mesenteron. Three sections (No. 9, 115, 143) show
+the dorsal sheath, the latter having the mud-filled canal beneath it.
+The sheath arches across the axial lobe as in Ceraurus, leaving room
+for the dorsal muscles at the sides and above it. In this region the
+canal is large and oval in section. Six slices cut the mesenteron
+behind the abdominal sheath (Nos. 39, 117, 148, 153, 62, 65) (see fig.
+23). In the first four of these it is oval in section and large, but
+not so large as in No. 143. In the last two, it is small and circular
+in section, from which it is inferred that the canal tapers
+posteriorly.
+
+_Cryptolithus goldfussi_ (Barrande).
+
+ Illustrated: Beyrich, Untersuch. über Trilobiten, Berlin, 1846, pl.
+ 4, fig. 1c.--Barrande, Syst. Sil. Bohême, vol. 1 1852, pl. 30,
+ figs. 38, 39.
+
+Both Beyrich and Barrande have shown that from the posterior end of
+the axial lobe to the neck-ring on the cephalon, the alimentary canal
+in _Cryptolithus_ has a nearly uniform diameter of less than half the
+width of the axial lobe. In front of the neck-ring, it enlarges, and
+while its original describers state that it extends only about halfway
+to the front of the glabella, Barrande's figure 39 shows it extending
+quite to the front, and his figure 38 shows it fully two thirds of the
+distance to the anterior end, as does Beyrich's figure of 1846.
+
+The Museum of Comparative Zoology contains a single specimen of this
+species from Wesela, Bohemia, which shows the course of the canal from
+the middle of the pygidium to the anterior part of the glabella. The
+enlargement appears to begin about halfway to the front of the
+glabella and to be greatest at the anterior end. At the anterior end
+of the glabella, the anterior end of the thorax, and the posterior end
+of the pygidium, the canal is still packed full of a material somewhat
+darker in appearance than the matrix, while the remainder of it is
+open. A well defined constriction is present under the middle of the
+next to the last thoracic segment, but whether this is accidental or
+whether it indicates the point where the mesenteron discharges into
+the proctodæum can not be determined. The inside of the canal has
+somewhat of a lustre and there are three conical projections into it
+on the median ventral line, a very small one in front of the neck
+furrow, a larger one under the anterior part of the second segment,
+and a third between the fourth and fifth segments.
+
+_Summary._
+
+The specimens of _Cryptolithus_ from Bohemia and of _Ceraurus_ and
+_Calymene_ from New York seem to substantiate the claim of Bernard and
+Jaekel that at the anterior end of the canal there was an enlarged
+organ which occupied the greater part of the cavity of the glabella.
+It appears that it extended into the thorax, and that above it and the
+heart was a chitinous dorsal sheath. Behind the enlarged portion, the
+mesenteron appears to have been of practically uniform diameter in
+_Cryptolithus_, but to have tapered posteriorly in Ceraurus and
+_Calymene_. The proctodæum can not yet be differentiated from the
+mesenteron, and only in _Cryptolithus_ has the posterior portion of
+the alimentary canal been seen. It is, there, merely a continuation of
+the mesenteron. The stomodæum likewise has not been identified, but
+was probably a short gullet leading up from the mouth into the
+enlarged digestive cavity.
+
+[Illustration: Fig. 24. Longitudinal section of _Ceraurus
+pleurexanthemus_, showing the probable outline of the alimentary canal
+and the heart above it. A restoration based on the slices described
+above.]
+
+The principle of the enlargement of the latter and its influence on
+the dorsal shell once established, the significance of different types
+of glabellæ becomes apparent. It will be remembered that the glabella
+of the protaspis of most trilobites is narrow, and that the same is
+true of the glabellæ of most ancient and all primitive trilobites. The
+free-swimming larvæ and the free-swimming ancestors of the trilobites
+were probably strictly carnivorous, lived on concentrated food, and
+needed but a small digestive tract. As the animals "discovered the
+ocean bottom" and began to be omnivorous or herbivorous, larger
+stomachs were required, and so in the later and more specialized
+trilobites the glabella became expanded latterally or dorsally, or
+both, to meet the requirement for more space, until, in such Devonian
+genera as _Phacops_, the cephalon was nearly all glabella.
+
+
+GASTRIC GLANDS.
+
+Jaekel's suggestion, quoted above, that the so-called "nervures" seen
+on the under surfaces of the heads of some trilobites are really
+glands for the secretion of digestive juices, is at least worthy of
+consideration. Moberg, however (1902, p. 299), suggested that these
+markings probably had something to do with the eyes rather than the
+stomach. He says in part (translation):
+
+ In general we can now say that such features are common to all the
+ eyeless Conocoryphidæ. With the conocoryphs I include _Elyx_ and
+ consider Harpides as at least closely related. Similar impressions
+ are also found in forms with eyes, as, for instance, in the
+ Olenidæ, but here such radiate partly from the border of the eye,
+ partly from the front end of the glabella, partly from the [visual
+ surface of the] eye, and sometimes from the angle between the
+ occipital ring and the glabella. They therefore go out from such
+ different points that they can not possibly be branches of the
+ liver. It would also be very remarkable if such an important organ
+ should have been developed in a few eyeless forms, but have failed
+ to leave the least trace in the rest of the trilobites.
+
+Lindstroem (1901, pp. 18, 19, 33; pl. 5. figs. 29, 31; pl. 6,
+figs. 43-45) has discussed these markings and given beautiful
+figures showing their appearance in _Olenus_, _Parabolina_, _Elyx_,
+_Conocoryphe_, and _Solenopleura_. He decided that they were to be
+explained as branches of the circulatory system, comparing them with
+the veins and arteries of _Limulus_. He pointed out that there was a
+coincidence between these markings and the position of the eyes, and
+suggested a causal connection with the latter.
+
+Beecher (1895 B, p. 309), also from a comparison with _Limulus_,
+suggested that the eye-lines of _Cryptolithus_, _Harpes_,
+_Conocoryphe_, _Olenus_, _Ptychoparia_, _Arethusina_, etc., probably
+represented the optic nerves, and since the eye-lines are usually the
+main trunks of the dendritic markings, it is fair to assume that he
+considered the whole as due to branches of nerves.
+
+Reed has recently (1916, pp. 122, 173) discussed these lines as
+developed in the Trinucleidæ, and seems to accept Beecher's
+explanation.
+
+Three explanations of the "nervures" are thus current, and the authors
+of all of them refer us to _Limulus_ as proving their claims! So far
+as general appearance goes, the markings on the trilobites more
+closely resemble the veins of a _Limulus_ than either the nerves or
+"liver" of that animal. The veins, however, are not in contact with
+the dorsal shell, but are buried in the liver and muscles, while the
+arrangement of the arteries, which are dorsal in position, is quite
+unlike what is seen in the trilobites.
+
+The term nervures, as applied to these markings, is not only
+misleading, but an incorrect use of one of Barrande's words, for by
+nervures he meant delicate surface markings. Until the real function
+of the organs which made these markings is definitely established, it
+may be well to call them genal cæca, for they obviously were open
+tunnels ending blindly, whatever they contained.
+
+The question of the function of the genal cæca can not, in any case,
+be settled by an appeal to _Limulus_, and it is doubtful if it can be
+settled at all at the present time. Certain things tend to show that
+Jacket's explanation is the most plausible, and these may be briefly
+set forth.
+
+Walcott (1912 A, pp. 176, 179, pls. 27, 28) has described specimens of
+_Naraoia_ and _Burgessia_ in which similar markings are well shown,
+and where they are obviously connected with the alimentary canal just
+at the anterior end of the mesenteron. In _Burgessia_, which seems to
+be a notostracan branchiopod, the trunk sinuses are very wide, and the
+appearance is on the whole unlike that of any known trilobite. In
+_Naraoia_, however, the markings are much finer and directly
+comparable with those of _Elyx_. If my contention that _Naraoia_ is a
+trilobite should be sustained, it might almost settle the question of
+the "nervures." In _Burgessia_ these lateral trunks enter the main
+canal behind the fifth pair of appendages. In the trilobites they
+debouch much further forward.
+
+The principal argument in favor of the interpretation of these
+markings as nerves lies in their connection with the eyes. There is
+considerable evidence to indicate that the eye-lines and the genal
+cæca are two distinct structures, but because both originate from the
+sides of the anterior lobe of the glabella, and both extend outward at
+nearly right angles to the axis, or obliquely backward, they are, when
+both present, coincident. Genal cæca occur on blind trilobites, on
+trilobites with simple eyes, and on trilobites with compound eyes.
+Eye-lines occur on trilobites with both simple and compound eyes, and
+genal cæca may or may not be present in both cases. The morphology
+of the ridge forming the eye-line in trilobites with compound eyes
+is well known. It is abundantly proved by ontogeny that it is the
+continuation of the palpebral lobe, and a development of the pleura of
+the first dorsal segment of the cephalon. Lake, Swinnerton, and Reed
+have tried to show that the eye-lines of the Harpedidæ and Trinucleidæ
+are homologous with the eye-lines of the trilobites with compound
+eyes, and that the ocelli on the cheeks are therefore degenerate
+compound eyes.
+
+The simplest form of the genal cæcum is seen in the blind _Elyx_
+(Lindstroem 1901, pl. 6, fig. 43). The main trunk is at nearly right
+angles to the axis, the increase in its width is gradual in
+approaching the glabella, and an equal number of branches diverge from
+both sides.
+
+_Ptychoparia striata_ (Barrande 1852, pl. 14, figs. 1, 3) is an
+excellent example of a trilobite with compound eyes and genal cæca. It
+will be noted that the main trunk and the eye-line are coincident, and
+that both on the free and fixed cheeks the branches are all on the
+anterior side of the eye-line. Compare this with the condition in
+_Conocoryphe_ (Barrande, pl. 14, fig. 8; Lindstroem, pl. 6, fig. 44),
+and one sees there a main branch having the same direction as in
+_Ptychoparia_ and likewise with all the branches on the anterior side.
+At first sight this would seem to support the contention that these
+lines do lead out to the eyes, since _Conocoryphe_ is blind, and the
+main trunk leads practically to the margin. But although Conocoryphe
+is blind, it has free cheeks, and the main trunk does not lead to the
+point on those free cheeks where eyes are to be expected, but back
+into the genal angles. And this direction holds in such diverse genera
+(as to eyes and free cheeks) as _Harpes_, _Cryptolithus_, _Dionide_,
+and _Endymionia_. In all these the genal cæca fade out in the genal
+angles, and in none of them would compound eyes be expected in that
+region. The coincidence of the eye-lines with the trunks of the
+genal cæca in _Ptychoparia_ seems to be merely a coincidence. That
+the markings which radiate from the eyes of _Ptychoparia_ and
+_Solenopleura_ are not impressions made by nerves is obvious. That
+they are of the same nature as the similar markings in the eyeless
+trilobites is equally obvious. Ergo, they can not be nerves in either
+case, and that they have anything to do with the eyes is highly
+improbable. The eye was merely superimposed upon these structures.
+
+The relation of the genal cæca to the ocelli on the cheeks is best
+shown in the Trinucleidæ. In all species of _Tretaspis_ simple eyes
+are present, and in most of them there are very narrow eye-lines. The
+latter are occasionally continued beyond the ocular tubercle back to
+the genal angle. A similar course is seen in _Harpes_. If the simple
+eye is the homologue of the compound eye, and the eye-line here the
+homologue of the eye-line in _Ptychoparia_, why does it continue
+beyond the eye? In any case, it can not be interpreted as a nerve.
+_Cryptolithus tessellatus_, when the cephalon is 0.45 mm. to 0.65 mm.
+long, shows short eye-lines and a small simple eye on each cheek. In
+some half-grown specimens, traces of the ocelli can be seen, but the
+eye-lines are absent. In the adult, both the eye-lines and the ocelli
+are entirely wanting. Reed states that "nervures" are also absent, and
+so they are from most specimens, but well preserved casts of the
+interior from the Upper Trenton opposite Cincinnati show them, and one
+cheek is here figured (fig. 25). As apparent from the figure, the main
+trunk is very short and gives rise to two principal branches, the
+first of which in its turn sends off lines from the anterior side. It
+was a specimen showing these lines which Ruedemann (1916, p. 147)
+figured as showing facial sutures. The interest lies in the fact that
+while the ocelli and eye-lines were lost in development, the genal
+cæca are present in the adult, showing that they are different
+structures.
+
+[Illustration: Fig. 25.--_Cryptolithus tessellatus_ Green. Side view
+of the cheek of a specimen from the top of the Trenton opposite
+Cincinnati, Ohio, to show the branching genal cæca. These are the
+"facial sutures" of Ruedemann.]
+
+_Harpides_ is another genus in which genal cæca are strikingly shown,
+and in this case they completely cover the huge cheeks, radiating from
+two main trunks to the front and sides. I have seen no good specimens,
+but it would appear from Angelin's figure (1854, pl. 41, fig. 7) that
+the rather large, simple eyes are not situated exactly on the vascular
+trunks. In the _Harpides_ from Bohemia, the main trunks extend out
+with many branches beyond the simple eyes. It should be stated that
+the courses of the genal cæca are not correctly figured by Barrande
+(Supplement, 1872, pl. 1, fig. 11), as shown by casts of the original
+specimen in the Museum of Comparative Zoology. From Barrande's figure,
+one would suppose that the eye-lines and their continuation beyond the
+"ocelli" were superimposed upon the genal cæca without having any
+definite connection with them, but as a matter of fact the radial
+markings really diverge from the main trunks as in _Elyx_ and similar
+forms.
+
+_Summary._
+
+As Reed has said, these lines are not mere ornamentation, but rather
+represent traces of structures of some functional importance. They
+probably can not be explained as traces of nerves and more likely
+represent either traces of the gastric cæca or of the circulatory
+system. While they are known chiefly in Cambrian and Lower Ordovician
+trilobites, there is no evidence that the organs represented were not
+present in later forms, even if the shell may not have been affected
+by them. While they indicate very fine, thread-like canals, the
+present evidence seems to be in favor of assigning to them the
+function of lodging the glands which secreted the principal digestive
+fluids.
+
+
+HEART.
+
+_Illænus._
+
+Volborth (1863, pl. 1, fig. 12 = our fig. 26) has described the only
+organ in a trilobite which suggests a heart. A Russian specimen of
+_Illænus_ with the shell removed shows a somewhat flattened, tubular,
+chambered organ extending from under the posterior end of the cephalon
+to the anterior end of the pygidium. The posterior nine chambers were
+each 1.5 mm. long and 1.5 mm. wide, while the two anterior chambers
+were respectively 2.5 mm. and 3 mm. wide. These were all under the
+thorax, and at least two more chambers are shown under the cephalon,
+but rather obscurely. The species of the _Illænus_ is not stated, but
+since no _Illænus_ has more than ten segments in the thorax, and
+this tube has at least thirteen chambers, it is evident that its
+constrictions are inherent in it, and are not due to the segmentation
+of the thorax. Beecher has made a passing allusion to this organ as an
+alimentary canal. This was the original opinion of Volborth. Pander,
+however, suggested to him that it might be a heart. The alimentary
+canal of _Cryptolithus_ does not show any constrictions, while the
+heart of _Apus_ (see fig. 27) and other branchiopods does show them.
+It should be noted, further, that while this heart enlarges toward the
+front, it is everywhere very small as compared with the width of the
+axial lobe, and much narrower than sections of _Ceraurus_ and
+_Calymene_ would lead one to expect the alimentary canal of _Illænus_
+to be. Where the heart is 1.5 mm. to 3 mm. wide, the axial lobe is 11
+mm. wide.
+
+[Illustration: Fig. 26. Copy of Volborth's figure of the heart of
+_Illænus_.]
+
+[Illustration: Fig. 27. Heart of _Apus_. Copied from Gerstäcker.]
+
+While this may be merely a cast of the alimentary canal it is
+sufficiently like a heart to deserve consideration as such an organ.
+
+_Ceraurus and Calymene._
+
+Nothing suggesting a heart has been seen in the sections of _Ceraurus_
+and _Calymene_. The mesenteron and its sheath crowd so closely against
+the dorsal test in the anterior part of the thorax that there seems
+to be no room for the heart, but it must have been located beneath the
+sheath and above the alimentary canal. If the latter were filled with
+mud, and the animals lay on their backs, as most of them did at death,
+the canal would drop down into the axial lobe and the soft heart would
+naturally disappear and leave 110 trace of its presence in the
+fossils.
+
+_The Median "Ocellus" or "Dorsal Organ."_
+
+Many trilobites, otherwise smooth, bear on the glabella a median
+pustule which is usually referred to as a simple eye or median
+ocellus, but whose function can not be said to have been certainly
+demonstrated. Ruedemann (1916, p. 127), who has recently made a
+careful study of this problem, lists about thirty genera, members
+of ten families, Agnostidæ, Eodiscidæ Trinucleidæ, Harpedidæ,
+Remopleuridæ, Asaphidæ Illænidæ, Goldiidæ, Cheiruridæ, and Phacopidæ,
+in which this tubercle is present, and had he wished he might have
+cited more, for it is of almost universal occurrence in Ordovician
+trilobites.
+
+I have not especially searched the literature for references to this
+median tubercle. It is often mentioned by writers in descriptions of
+species, but apparently few have tried to explain it. Beyrich (1846,
+p. 30) suggested that it indicated the beginning of the alimentary
+canal. Barrande mentioned it, but if he gave any explanation, it has
+escaped me. McCoy (Syn. Pal. Foss. 1856, p. 146) called it an ocular
+(?) tubercle, and that seems to have been the interpretation which
+most writers on trilobites have assigned to it, if they suggested any
+function at all. Beecher (1895 B, p. 309) concurred in this opinion.
+
+Bernard (1894, p. 422) ascribed to this tubercle, as well as to the
+median tubercle on the nuchal segment, an excretory function,
+comparing it with the "dorsal organ" in _Apus_.
+
+Reed (1916, p. 174) states that it may be either the representative of
+the "dorsal" organ of the branchiopods, or a median unpaired ocellus.
+
+Ruedemann (1916) has made the only real investigation of the subject.
+He came to the conclusion that it was a parietal eye, without a
+crystalline lens, but corresponding to the "parietal eye of other
+crustaceans, and especially of the phyllopods, which is a lens-shaped
+or pear-shaped sac, usually filled with sea water." He found that
+above the "ocellus" the test was usually thin or even absent, and in a
+few cases a dark line beneath seemed to outline the original form of
+the sac. His summary follows:
+
+ It is claimed that most, if not all, trilobites possessed a median
+ or parietal eye on the glabella. [In proof of this assertion the
+ following facts are stated:]
+
+ 1. A great number of species, belonging to more than thirty genera,
+ possess a distinct tubercle on the glabella. This tubercle occurs
+ alone in many genera, otherwise smooth, as in the Asaphidæ, and is
+ hence of functional importance.
+
+ 2. In certain cases, as in _Cryptolithus tessellatus_, distinct
+ lenticular bodies [not lenses] were recognized; in others, as in
+ _Asaphus expansus_, only a thinner, probably transparent test.
+ Many other species show a distinct pit in interior casts of the
+ tubercle, indicating a lens-like thickening of the top of the
+ tubercle. The median eye therefore probably possessed all the
+ different stages of development seen in other crustaceans.
+
+ 3. As in the parietal eyes of the crustaceans and the eurypterids,
+ the tubercles are most prominent and distinct in the earlier
+ growth-stages, notably so in _Isotelus gigas_.
+
+ 4. The tubercle is especially well developed in the so-called blind
+ forms where the lateral eyes are abortive, as in _Cryptolithus_
+ (_Trinucleus_), _Dionide_, _Ampyx_.
+
+ 5. The tubercles always appear on the apex on the highest part of
+ the glabella, where their visual function would be most useful.
+
+ 6. The tubercle is generally situated between the lateral eyes,
+ like the parietal eye in crustaceans and eurypterids, on account of
+ its close connection with the brain.
+
+ 7. Frequently it forms the posterior termination of a short crest,
+ also as in certain eurypterids (_Stylonurus_), indicating the
+ direction of the nerve.
+
+ 8. The median eye is borne on a tubercle or mound in the Ordovician
+ and Silurian trilobites, while the tubercle is rarely noticed in
+ the Devonian and in few Cambrian forms. In the Devonian forms,
+ similarly as in many crustaceans and in later growth-stages of some
+ asaphids, the strong development of the lateral eyes may have led
+ to a loss of the parietal eyes. In the Cambrian genera evidence is
+ present to suggest that the parietal eyes consisted only of
+ transparent spots or lens-like thickenings of the exoskeleton,
+ hardly noticeable from the outside.
+
+ 9. It is _a priori_ to be inferred that the trilobites should, as
+ primitive crustaceans, have possessed median or parietal eyes.
+
+As a student, I accepted Professor Beecher's dictum that this tubercle
+represented a median _ocellus_, but more recently a number of things
+have led me to the view that it is the point of attachment of the
+ligament by which the heart is supported.
+
+The chief arguments against its interpretation as a parietal eye seem
+to be that its structure is not absolute proof, being capable of other
+explanation; its position is variable, in front, between, or back of
+the eyes; it is exactly like other tubercles on the median line,
+especially the nuchal spine or tubercle, and the similar ones along
+the axial lobe of the thorax; and it is not present in the protaspis
+or very young trilobites.
+
+1. The structure disclosed by Ruedemann's sections, a sort of sac-like
+cavity beneath a thinned test, can be explained as a gland, a
+ligamentary attachment, or a vestigial spine, as well as an eye. In a
+section of _Asaphus expansus_, which I made some years ago when trying
+to get some light on this problem, there is a similar cavity under the
+pustule, but a secondary layer of shell lay beneath it and apparently
+cut it off from the glabellar region, thus indicating that it had
+lost its function in the adult of this animal. Sections through the
+tubercles of the glabella of _Ceraurus_ show all of them hollow, with
+very thin upper covering or none at all, and their structure is not
+unlike that of the tubercle of _Cryptolithus_. In fact, sections can
+be seen in Doctor Walcott's slices which are practically identical
+with the one Ruedemann obtained from _Cryptolithus_. Since it is
+obvious that not all of the pustules of a _Ceraurus_ could have been
+eyes, the evidence from structure is rather against than for the
+interpretation of the median pustule as such an organ.
+
+2. The position of the tubercle varies greatly in different genera.
+Where furthest forward (_Tretaspis_, _Goldius_), it is just back of
+the frontal lobe, while in some species of asaphids it is in the neck
+furrow. In species with compound eyes it is frequently between the
+eyes, but more often back of them. If its history be traced in a
+single family, it is generally found farthest forward in the more
+ancient species and moves backward in the more recent ones. The eyes
+do this same thing, but the median tubercle goes back further than the
+eyes. This can be seen, for example, in the American Asaphidæ, where
+the pustule is up between the eyes of _Hemigyraspis_ and _Symphysurus_
+of the Beekmantown and back of the eyes of the _Isotelus_ of the
+Trenton. Turning now to the under side of the head, it appears that
+the tubercle bears a rather definite relation to the hypostoma. If the
+hypostoma is short, the tubercle is well forward. If long, it is far
+back on the head. It seems in many cases to be just back of the
+posterior tip of the hypostoma, or just behind the position of the
+mouth, while in others it is not as far back as the tip of the
+hypostoma.
+
+The median tubercle is in many cases developed into a long spine.
+This is usually in an ancient member of a tubercle-bearing family,
+and suggests that in most cases the tubercle is a vestigial organ.
+An example of this occurs in _Trinucleoides_, the most ancient of the
+Trinucleidæ. _Trinucleoides reussi_ (Barrande) (Supplement, 1872, pl.
+5, figs. 17, 18) has a very long slender spine in this position. It
+could be explained as an elevated median eye, but it also very
+strongly suggests the zoæal spine of modern brachyuran Crustacea.
+Gurney (Quart. Jour. Mic. Sci., vol. 46, 1902, p. 462) supports
+Weldon in the conclusion that the long spines of the zoæa are
+directive, and states that the animal swims in the direction of the
+long axis of the spine. He also suggests that, since the period of
+their presence corresponds to the period before the development of the
+"auditory" organs, the spines may perform the functions of balancing
+and orientation. It is generally admitted that the spine of the zoæa
+is also protective, and the obvious function, first pointed out by
+Spence Bate in 1859, is that it contains a ligament which helps
+suspend the heart, which lies beneath the spine. This latter function
+may have been that of the median tubercle in the trilobite. Such an
+explanation would account for the backward migration mentioned above,
+for as the stomach enlarged and the mouth moved backward on the
+ventral side, the heart may have been pushed backward on the upper
+side.
+
+There is also a curious parallelism between the ontogenetic history of
+the zoæal spine and the phylogenetic history of the Trinucleidæ or
+Cheiruridæ (Nieszkowskia is the ancient member of this family in which
+the spine replaces the tubercle). When first hatched, the larval crab
+shows no trace of the spine, but very quickly it evaginates, lying
+dorsally on the median line, pointing forward (Faxon, Bull. Mus. Comp.
+Zool., vol. 6, 1880, pl. 2). With the splitting of the original
+envelope, the spine becomes erect, but persists only a short time, and
+is reduced to a vestigial tubercle toward the end of the zoæal stages,
+its disappearance being, as pointed out by Gurney, coincident with the
+development of the balancing organs. This manner of suspension of the
+heart by a long tendon certainly does suggest that Gurney is right in
+his interpretation of the function. Briefly, the zoæal spine served
+for a short time a function later taken over by other organs. It was
+not present in the youngest stages, it became prominent at a very
+early stage, was soon vestigial, and then lost.
+
+Take now the trilobites. There is no trace of the median pustule in
+the protaspis of any form, and in many primitive trilobites it is
+absent. It appears first as a long spine in certain families, and
+later becomes vestigial and disappears. Very few trilobites of
+Silurian and later times show it at all.
+
+In the particular case of the Trinucleidæ, which were burrowers, the
+spine is present on only the oldest and most primitive of the group, a
+form which has only a most rudimentary fringe. It is obvious from the
+large size of the pygidium in the larval trinucleid that this family
+is derived from a group of free swimmers. _Trinucleoides reussi_ was
+perhaps in the transitional stage, just leaving the swimming mode of
+life, and belonged to a group which had not developed any other
+"statocyst" than the median spine. Among the later Trinucleidæ the
+spine became a vestigial tubercle, and in some cases entirely
+disappeared. A similar history can be traced in the Cheiruridæ,
+starting from some such forms as the American Lower Ordovician
+_Nieszkowskia_ (_N. perforator_ p. ex.).
+
+Another example of a median spine instead of a tubercle is in Goldius
+rhinoceros (Barrande). Since this species is not from the oldest
+Goldius-bearing rocks, but from the Lower Devonian, it does not follow
+what seems to be the general rule, but makes an interesting exception.
+Goldius rhinoceros (Barrande) (Supplement, 1872, pl. 9, figs. 12, 13)
+has the median tubercle elevated into a stubby, recurved spine very
+suggestive of the horn of a rhinoceros. Since the eyes of this species
+are very well developed, there seems no especial reason for the
+elevation of a parietal eye, and the example certainly does not
+support that interpretation.
+
+3. This tubercle is essentially similar to other tubercles on the
+median line of cephalon, thorax, and even pygidium. This has been
+discussed sufficiently under section 1 above, but it may perhaps be
+justifiable to point out that in some of the Agnostidæ there is a
+median tubercle on both shields, and since it has not yet been
+demonstrated beyond question which shield is the cephalon, to say
+which one is a parietal eye and which one is a tubercle is impossible.
+In other words, the parietal eye can not be differentiated from any
+other tubercle except by its position.
+
+4. One of the as yet unexplained features of the protaspis of
+trilobites is the absence of the "nauplius eye." Beecher (1897 B, p.
+40) explained this on the ground of the extremely small size of the
+protaspis and the imperfection of the preservation. If the median
+tubercle were really a median eye, it should be present in the
+protaspis and the earlier stages of the ontogeny, even if not in the
+adult, and should certainly appear before the compound eyes. (In
+_Limulus_, however, the compound eyes appear first.) The median eye
+has not so far been seen in any young trilobite in any stage previous
+to that in which compound eyes are present. The full ontogeny is not
+known of any species with compound eyes in which the median tubercle
+is present in the adult, but theoretically the median eye should be
+most prominent in the young of just those primitive trilobites about
+whose development most is known.
+
+
+NERVOUS SYSTEM.
+
+There has been a rather general impression among students of
+trilobites that the eye-lines, which should be differentiated from the
+genal cæca, denote the course of the optic nerves, but no other
+evidence of the nervous system has been found, save the so-called
+nervures which have been discussed above. In _Apus_ the nerves leading
+to the eyes come off from the anterior ganglion or "brain" and run
+directly to the eyes. If conditions were similar in the trilobites,
+the "brain" was beneath the anterior glabellar lobe, provided, of
+course, that the eye-lines do indicate the course of the optic nerve.
+
+The ontogenetic history of the eye-lines of trilobites with compound
+eyes is instructive, and has already been discussed by Lindstroem
+(1901, pp. 12-25), but he did not cite the case of _Ptychoparia_,
+which is particularly interesting, because in this genus both
+eye-lines and "nervures" are present. Beecher (1895 C, p. 171, pl. 8,
+figs. 5-7) has shown that in _Ptychoparia kingi_ the eye-lines of a
+specimen in the metaprotaspis stage run forward at a low angle with
+the glabella, while in the adult their course is nearly at right
+angles to it. They have therefore swung through an arc of at least 60
+and in so doing have had ample opportunity to become coincident with
+the primary trunks of the genal cæca. Once that was accomplished, it
+is quite likely that the one fold in the shell would continue to house
+both structures. In other trilobites, there is a similar backward
+progression of the eye-lines.
+
+As would be expected, the ventral ganglia and the longitudinal cords
+left no trace in the test. Since each segment has appendages, there
+was probably a continuous chain of ganglia back to the posterior end
+of the pygidium.
+
+
+
+VARIOUS GLANDS.
+
+_Dermal glands._--The surface of many trilobites is "ornamented" with
+pustules and spines which on sectioning are nearly always found to be
+hollow, and in many cases have a fine opening at the tip. While it is
+generally believed that the purpose of these spines was protective,
+yet it is possible that many of them were merely outgrowths which
+increased the area through which the respiratory function could be
+carried on. It will be recalled that most of the smooth trilobites
+are punctate, some of them very conspicuously so, and the spines and
+pustules of ornamented trilobites may merely subserve the same
+function as the pores of smooth ones.
+
+If the spines were protective, it would not be surprising if some of
+them, hollow and open at the top, were poisonous also, and had glands
+at the base. These are, however, purely matters of speculation so far.
+
+_Renal excretory organs._--Nothing has been seen of any such organs,
+unless the genal cæca may possibly be of that nature. The main trunks
+of these always lead to the sides of the anterior glabellar lobe,
+which is not the point of attachment of either antennæ or biramous
+limbs, so that there seems little chance that they will bear this
+interpretation.
+
+_Reproductive organs._--Nothing is yet positively known about the
+reproductive organs or the position of their external openings. If the
+"exites" of _Neolenus_ could be interpreted as brood-pouches, which
+does not seem probable, then the genital openings were located near
+the base of some pair of anterior thoracic appendages.
+
+_The Panderian Organs: Internal Gills or Poison Glands?_
+
+At a meeting of the Mineralogical Society at St. Petersburg, Volborth
+(1857) announced that Doctor Pander had two years before discovered
+certain organs on the lower side of the doublure of the pleural lobes
+of the thorax of a specimen of _Asaphus expansus_. These organs were
+oval openings in the doublure, one near the posterior margin of the
+cephalon, and one on each thoracic segment of the half-specimen
+figured by Volborth in 1863. They were explained by Volborth and by
+Eichwald (1860, 1863) as the points of attachment of appendages.
+Billings (1870) described and figured the "Panderian organs" of
+"_Asaphus platycephalus_" and stated that he had seen them in
+_Asaphus_ [_Ogygites_] _canadensis_ and _A. megistos_ [_Isotelus
+maximus_] as well. He thought some sort of organ was attached to them,
+but could not suggest its function. Woodward (1870) thought that the
+openings were "only the fulcral points on which the pleuræ move."
+Their position outside the fulcra shows that this explanation is
+impossible.
+
+So far as I am aware, the Panderian organs have been seen only in
+the Asaphidæ. Barrande figured them in "_Ogygia_" [_Hemigyraspis_]
+_desiderata_ (1872) and Schmidt in two species of _Pseudasaphus_. They
+seem to occupy the same position in Bohemian, Russian, and American
+specimens. There is always one pair of openings on each thoracic
+segment, and one pair in line with them on the posterior margin of the
+cephalon. They occur near the anterior margin of the segment, and near
+the inner end of the doublure. In some cases they are surrounded by a
+ventrally projecting rim, while in others they have a thin edge. There
+seem to be no markings on the interior of the shell which are
+connected with them.
+
+While thinking over the trilobites in connection with the origin of
+insects, it occurred to me that these hitherto unexplained Panderian
+organs might possibly be openings to internal gills and that the
+Asaphidæ might have been tending toward an amphibious existence.
+On mentioning this to Doctor R. V. Chamberlin of the Museum of
+Comparative Zoology, he called my attention to the possibility that
+they might be openings similar to those of the repugnatorial glands of
+Diplopoda. While no definite decision as to the function can be made,
+the explanation offered by Doctor Chamberlain seems more plausible
+than my own, and has suggested still a third, namely, that they might
+be the openings of poison glands.
+
+If one were to argue that these apertures are really connected with
+respiration, it might be pointed out that they are ventral in
+position, while the _foramina repugnatoria_ are always dorsal or
+lateral, even in diplopods with broad lateral expansions. If offensive
+secretions were poured out beneath a concave shell like that of a
+trilobite, they would be so confined as to be but slightly effective
+against an enemy. This would indicate that if these openings were the
+outlets of glands, the substance secreted might be a poison used to
+render prey helpless. On the other hand, openings to gills are
+normally ventral in position, and if the pleural lobes were folded
+down against the body, they would be brought very close to the bases
+of the legs.
+
+A further curious circumstance is that so far no traces of exopodites
+have been found on _Isotelus_. The endopodites of both _Isotelus
+latus_ and _I. maximus_ are fairly well preserved in the single known
+specimen of each, yet no authentic traces of exopodites have been
+found with them. Moreover, Walcott sliced specimens of _Isotelus_ from
+Trenton Falls and found only endopodites. It may also be recalled that
+the finding of the specimen of _Isotelus arenicola_ at Britannia and
+the tracks which I attributed to it, suggested to me that it was a
+shore-loving animal (1910). It offers a field for further inquiry,
+whether the Asaphidæ may not have had internal gills, and whether some
+primitive member of the family may not have given rise to tracheate
+arthropods.
+
+[Illustration: Fig. 28. Side view of a specimen of _Isotelus gigas_
+Dekay, from which the test of the pleural lobes has been broken to
+show the position of the Panderian organs. Natural size. Specimen in
+the Museum of Comparative Zoology.]
+
+The explanation of the Panderian organs as openings of poison glands
+is less radical than the one just set forth, and so possibly lies
+nearer the truth. One would expect poison glands to lie at the bases
+of fangs, and so they do in specialized animals like chilopods and
+scorpions, but the trilobites may have had the less effective method
+of pouring out the poison from numerous glands. The purpose may have
+been merely to paralyze the brachiopod or pelecypod which was
+incautious enough to open its shell in proximity to the asaphid.
+
+
+MUSCULATURE.
+
+This is a field which is rather one for investigation than for
+exposition. Very little has been done, though probably much could be.
+The chief obstacle to a clearer understanding of the muscular system
+lies in the difficulty of getting at the inner surface of the test
+without obscuring the faint impressions in the process.
+
+There exist in the literature a number of references to scars of
+attachment of muscles, and any study of the subject should of course
+begin by the collection of such data. I shall at this time refer to
+only a few observations on the subject.
+
+The structure and known habits of trilobites make it obvious that
+strong flexor and extensor muscles must have been present, and some
+trace of them and of their points of attachment should be found. It is
+likely that their proximal ends were tough tendons. The muscles
+holding up the heart and alimentary canal would be less likely to
+reveal their presence by scars, but there must have been at least one
+pair of strong muscles extending from the under side of the head
+across to the hypostoma. Judging from the method of attachment, the
+muscles moving the limbs were short ones, chiefly within the segments
+of the legs themselves.
+
+_Flexor Muscles._
+
+Since the majority of trilobites had the power of enrollment, and seem
+also to have used the pygidia in swimming, the flexors must have been
+important muscles. Beecher (1902, p. 170) appears to have been the
+only writer to point out any tangible evidence of their former
+presence. Walcott (1881, p. 199) had shown that the ventral membrane
+was reinforced in each segment by a slightly thickened transverse
+arch. Beecher showed that on this thickened arch in _Triarthrus_,
+_Isotelus_, _Ptychoparia_, and _Calymene_, there are low longitudinal
+internal ridges or folds. One of these is central, and there is a pair
+of diagonal ridges on either side. Beecher interpreted these ridges as
+separating the strands of the flexor muscles, and believed that a line
+of median ridges divided a pair of longitudinal muscles, while the
+outer ridges showed the place of attachment of the pair of strands
+which was set off to each segment. He did not discuss the question as
+to where the anterior and posterior ends were attached. In trilobites
+with short pygidia, the attachment would probably have been near the
+posterior end, and it is possible that the two scars beneath the
+doublure and back of the last appendifers in _Ceraurus_ may indicate
+the point of attachment in that genus.
+
+There is as yet no satisfactory evidence as to where the anterior ends
+of the flexors were attached. In _Apus_ these muscles unite in an
+entosternal sinewy mass above the mouth, but no evidence of any
+similar mass has been found in the trilobites and it is likely that
+the muscles were anchored somewhere on the test of the head.
+
+_Extensor Muscles._
+
+The exact position of these muscles has not been previously discussed.
+The interior of the dorsal test shows no such apodemes as are found on
+the mesosternites, but, as I have shown in the discussion of the
+alimentary canal of _Calymene_ and _Ceraurus_, there is an opening
+on either side of the axial lobe between the dorsal test and the
+abdominal sheath, and it is entirely probable that an extensor muscle
+passed through each of these. The abdominal sheath extends only along
+the posterior region of the glabella and the anterior part of the
+thorax, and probably served to protect the soft organs from the strain
+of the heavy muscles. The extensors (see fig. 29) probably lay along
+the top of the axial lobe on either side of the median line of the
+thorax to the pygidium, where they appear to have been attached
+chiefly on the under side of the anterior ring of the axial lobe,
+although strands probably continued further back. This is above and
+slightly in front of the fulcral points on the pleura, and meets the
+mechanical requirements. _Ceraurus_ (Walcott, 1875, and 1881, p. 222,
+pl. 4, fig. 5) shows a pair of very distinct scars on the under side
+of the first ring of the pygidium, and in many other trilobites
+(_Illænus_, _Goldius_, etc.) distinct traces of muscular attachment
+can be seen in this region, even from the exterior. The anterior ends
+were probably attached by numerous small strands to the top of the
+glabella, and, principally, to the neck-ring.
+
+On enrolling, the sternites of all segments are pulled forward and the
+tergites backward. In straightening out, the reverse process takes
+place. The areas available for muscular attachment are so disposed as
+to indicate longitudinal flexor and extensor muscles rather than short
+muscles extending from segment to segment. Indeed, the tenuity of the
+ventral membrane is such as to preclude the possibility of enrollment
+by the use of muscles of that sort, while powerful longitudinal
+flexors could have been anchored to cephalon and pygidium. The
+strongly marked character of the neck-ring of trilobites is probably
+to be explained as due to the attachment of the extensor muscles,
+rather than to its recent incorporation in the cephalon. The same is
+true of the anterior ring on the pygidium.
+
+[Illustration: Fig. 29. Restoration of a part of the internal organs
+of _Ceraurus pleurexanthemus_ as seen from above. At the sides are the
+extensor muscles, and in the middle, the heart overlying the
+alimentary canal. Drawn by Doctor Elvira Wood, under the supervision
+of the author.]
+
+_Possible preservations of extensors and flexors in Ceraurus_.--Among
+Doctor Walcott's sections are four slices which I should not like to
+use in proving the presence of longitudinal muscles, but which may be
+admitted as corroborative evidence. Two of these transverse sections
+(Nos. 114 and 199) show a dorsal and a ventral pair of dark spots in
+positions which suggest that they represent the location of the dorsal
+and ventral muscles, while two others (Nos. 131 and 140) show only the
+upper pair of spots. The chief objection to this interpretation is
+that it is difficult to imagine how the muscles could be so replaced
+that they happen to show in the section. Both the sections showing all
+four spots are evidently from the anterior part of the thorax, as they
+show traces of the abdominal sheath, which seems to be squeezed
+against the inside of the axial lobe, with the muscles (?) forced out
+to the sides. The ventral pair lie just inside the appendifers, but
+even if they are sections of muscles, all four are probably somewhat
+out of place.
+
+_Hypostomial Muscles._
+
+The hypostoma fits tightly against the epistoma, or the doublure when
+the epistoma is absent, but in no trilobite has it ever been seen
+ankylosed to the dorsal test, and its rather frail connection
+therewith is evidenced by the relative rarity of specimens found with
+it in position. That the hypostoma was movable seems very probable,
+and that it was held in place by muscles, certain. The maculæ were
+always believed to be muscle scars until Lindstroem (1901, p. 8)
+announced the discovery by Liljevall of small granules on those of
+_Goldius polyactin_ (Angelin). These were interpreted as lenses
+of eyes by Lindstroem, who tried to show that the maculæ of all
+trilobites were functional or degenerate eyes. Most palæontologists
+have not accepted this explanation, and since the so-called eyes cover
+only a part of the surface of the maculæ, it is still possible to
+consider the latter as chiefly muscle-scars.
+
+In Lindstroem's summary (1901, pp. 71, 72) it is admitted that the
+globular lenses are found only in _Bronteus_ (_Goldius_) (three
+Swedish species only) and _Cheirurus spinulosus_ Nieszkowski, while
+the prismatic structure supposed to represent degenerate eyes was
+found in eleven genera (Asaphidæ, Illænidæ, Lichadidæ). All of these
+are forms with well developed eyes, and Lindstroem himself points out
+that the appearance of actual lenses in the hypostoma was a late
+development, long after the necessity for them would appear to have
+passed.
+
+The use of the hypostoma has been discussed by Bernard (1892, p. 240)
+and extracts from his remarks are quoted:
+
+ The earliest crustacean-annelids possessed large labra or prostomia
+ projecting backward, still retained in the Apodidæ and trilobites.
+ This labrum almost necessitated a very deliberate manner of
+ browsing. The animal would creep along, and would have to run some
+ way over its food before it could get it into its mouth, the whole
+ process, it seems to us, necessitating a number of small movements
+ backwards and forwards. Small living prey would very often escape,
+ owing to the fact that the animal's mouth and jaws were not ready
+ in position for them when first perceived. The labrum necessitates
+ the animal passing forwards over its prey, then darting backward to
+ follow it with its jaws. We here see how useful the gnathobases of
+ _Apus_ must be in catching and holding prey which had been thus
+ passed over. Indeed the whole arrangement of the limbs of _Apus_
+ with the sensory endites forms an excellent trap to catch prey
+ over which the labrum has passed.
+
+In alcoholic specimens of _Apus_ the labrum is not in a horizontal
+plane, as it is in most well preserved trilobites, but is tipped down
+at an angle of from 30 to 45, and the big mandibles lie under it. It
+has considerable freedom of motion and is held in place by muscles
+which run forward and join the under side of the head near its
+posterior margin. It seems entirely possible that the hypostoma of
+the trilobite had as much mobility as the labrum of _Apus_, and that
+byopening downward it brought the mouth lower and nearer the food. It
+will be recalled that the hypostomata of practically all trilobites
+are pointed at the posterior margin, there being either a central
+point or a pair of prongs. By dropping down the hypostoma until
+the point or prongs rested on or in the substratum, and sending food
+forward to the mouth by means of the appendages, a trilobite could
+make of itself a most excellent trap, and if the animal could dart
+backward as well as forward, the hypostoma would be still more useful.
+There is no reason to suppose that they could not move backward, and
+the "pygidial antennæ" of _Neolenus_ indicate that animals of that genus
+at least did so. This habit of dropping down the hypostoma would also
+permit the use of those anterior gnathobases which seem too far ahead
+of the mouth in the trilobites with a long hypostoma.
+
+For actual evidence on this point, it is necessary to have recourse
+once more to Doctor Walcott's exceedingly valuable slices. From such
+sections of _Ceraurus_ as his Nos. 100, 106, 108, 170, and 173, it is
+evident that the hypostoma of that form could be dropped considerably
+without disrupting the ventral membrane (fig. 30). Sections of
+_Calymene_ already published (Walcott 1881, pl. 5, figs. 1, 2) show
+the hypostoma turned somewhat downward, and the slices themselves show
+sections of the anterior pair of gnathobases beneath the hypostoma.
+When the hypostoma was horizontal, these gnathobases were crowded out
+at the sides.
+
+[Illustration: Fig. 30.--Longitudinal section of cephalon of _Ceraurus
+pleurexanthemus_, to show position of the mouth and folds of the
+ventral membrane between the glabella and the hypostoma. The test is
+in solid black and the part within the ventral membrane dotted. From a
+photographic enlargement. Specimen 169. × 3.9.]
+
+[Illustration: Fig. 31.--A copy of Doctor Moberg's figure of _Nileus
+armadillo_, showing the position of the muscle scars.]
+
+If the hypostoma were used in the manner indicated, the muscles must
+have been more efficient than those of the labrum of _Apus_, and it is
+probable that they crossed to the dorsal test. Just where they were
+attached is an unsolved problem. Barrande (1852, pl. 1, fig. 1) has
+indicated an anterior pair of scars and a single median one on the
+frontal lobe of _Dalmanites_ that may be considered in this connection,
+and also three pairs of scars on the last two lobes of the glabella of
+_Proëtus_ (1852, pl. 1, fig. 7). Moberg (1902, p. 295, pl. 3, figs. 2,
+3, text fig. 1) has described in some detail the muscle-scars of a
+rather remarkable specimen of _Nileus armadillo_ Dalman. While, as I
+shall point out, I do not agree wholly with Professor Moberg's
+interpretation, I give here a translation (made for Professor Beecher)
+of his description, with a copy of his text figure:
+
+ The well preserved surface of the shell permits one to note not
+ only the tubercle (t) but a number of symmetrically arranged
+ glabellar impressions. And because of their resemblance to the
+ muscular insertions of recent crustaceans, I must interpret them as
+ such. They appear partly as rounded hollows (k and i), also as
+ elongate straight or curved areas (a, b, c, e, g, h) made up of
+ shallow impressions or furrows about 1 mm. long, sub-parallel, and
+ standing at an angle to the trend of the areas. Impression e is
+ especially well marked, inasmuch as the perpendicular furrows are
+ arranged in a shallow crescentic depression; and impression d shows
+ besides the obscure furrows a number of irregularly rounded
+ depressions. Larger similar ones occur at f, and in part extend
+ over toward g.
+
+ The meaning of these impressions, or their myologic significance,
+ could be discussed, although such discussion might rather be termed
+ guessing.
+
+ Inner organs, such as the heart and stomach, might have been
+ attached to the shell along impressions a and b. Also along or
+ behind c and h, which both continue into the free cheeks, ligaments
+ or muscular fibers may have been inserted. From d, e, f, and g,
+ muscles have very likely gone out to the cephalic appendages.
+ Against this it may be urged that impression d is too far forward
+ to have belonged to the first pair of feet. Again, the impression h
+ may in reality represent two confluent muscular insertions, from
+ the first of which, in that case, arose the muscles of the fourth
+ pair of cephalic feet. Were this the case, the muscles of the first
+ pair of cheek feet should be attached at e. And d in turn may be
+ explained as the attachment of the muscles of the antennæ, k those
+ of the hypostoma, and from i possibly those of the epistoma. That k
+ is here named as the starting point of the hypostomial muscles and
+ not those of the antennæ, depends partly on the analogous position
+ of i and partly on the fact that the hypostoma of _Nileus
+ armadillo_ (text figure, in which the outline of the hypostoma is
+ dotted), by reason of it? wing-like border, could not have
+ permitted the antennæ to reach forward, but rather only outward or
+ backward.
+
+My own explanation would be that impressions e, f, and g correspond to
+the glabellar furrows, h the neck furrow, and all four show the places
+of attachment of the appendifers. Those at d may possibly be connected
+with the antennæ, although I should expect those organs to be attached
+under the dorsal furrows at the sides of the hypostoma. It will be
+noted that either b, k, or i correspond well with the maculæ of the
+hypostoma and some or all of them may be the points of attachment of
+hypostomial muscles. They correspond also with the anterior scars of
+_Dalmanites_.
+
+
+EYES.
+
+While I have nothing to add to what has been written about the eyes of
+trilobites, this sketch of the anatomy would be incomplete without
+some reference to the little which has been done on the structure of
+these organs.
+
+Quenstedt (1837, p. 339) appears to have been the first to compare the
+eyes of trilobites with those of other Crustacea. Johannes Müller had
+pointed out in 1829 (Meckel's Archiv) that two kinds of eyes were
+found in the latter group, compound eyes with a smooth cornea, and
+compound eyes with a facetted coat. Quenstedt cited _Trilobites
+esmarkii_ Schlotheim (=_Illænus crassicauda_ Dalman) as an example of
+the first group, and _Calymene macrophthalma_ Brongniart (=_Phacops
+latifrons_ Bronn) for the second. Misreading the somewhat careless
+style of Quenstedt, Barrande (1852, p. 133) reverses these, one of the
+few slips to be found in the voluminous writings of that remarkable
+savant.
+
+Burmeister (1843; 1846, p. 19) considered the two kinds of eyes as
+essentially the same, and accounted for the conspicuous lenses of
+Phacops on the supposition that the cornea was thinner in that genus
+than in the trilobites with smooth eyes.
+
+Barrande (1852, p. 135) recognized three types of eyes in trilobites,
+adding to Quenstedt's smooth and facetted compound eyes the groups of
+simple eyes found in Harpes. In his sections of 1852, pl. 3, figs.
+15-25, which are evidently diagrammatic, he shows separated biconvex
+lenses in both types of compound eyes, _Phacops_ and _Dalmanites_ on
+one hand, and _Asaphus_, _Goldius_, _Acidaspis_, and _Cyclopyge_
+on the other. Clarke ( 1888), Exner ( 1891 ) and especially
+Lindstroem (1901) have since published much more accurate figures and
+descriptions. The first person to study the eye in thin section seems
+to have been Packard (1880), who published some very sketchy figures
+of specimens loaned him by Walcott. He studied the eyes of _Isotelus
+gigas_, _Bathyurus longispinus_, _Calymene_, and _Phacops_, and
+decided that the two types of eyes were fundamentally the same.
+He also compared them with the eyes of _Limulus_.
+
+Clarke (1888), in a careful study of the eye of _Phacops rana_, found
+that the lenses were unequally biconvex, the curvature greater on the
+inner surface. The lens had a circular opening on the inner side,
+leading into a small pear-shaped cavity. The individual lenses were
+quite distinct from one another, and separated by a continuation of
+the test of the cheek.
+
+Exner (1891, p. 34), in a comparison of the eyes of Phacops and
+_Limulus_, came to the opinion that they were very unlike, and that
+the former were really aggregates of simple eyes.
+
+Lindstroem (1901, pp. 27-31) came to the conclusion that besides the
+blind trilobites there were trilobites with two kinds of compound
+eyes, trilobites with aggregate eyes, and trilobites with stemmata and
+ocelli. His views may be briefly summarized.
+
+ I. Compound eyes.
+
+ 1. Eyes with prismatic, plano-convex lenses.
+
+ "A pellucid, smooth and glossy integument, a direct continuation of
+ the common test of the body, covers the corneal lenses, quite as is
+ the case in so many of the recent Crustacea. The lenses are closely
+ packed, minute, usually hexagonal in outline, flat on the outer and
+ convex on the inner surface. Such eyes are best developed in
+ _Asaphus_, _Illænus_, _Nileus_, _Bumastus_, _Proëtus_, etc."
+
+ 2. Eyes with biconvex lenses.
+
+ The surface of the eye is a mass of contiguous lenses, covered by a
+ thin membrane which is frequently absent from the specimens, due to
+ poor preservation. The lenses are biconvex, and being in contact
+ with one another, are usually hexagonal, although in some cases
+ they nearly retain their globular shape. Such eyes are found in
+ Bury care, _Peltura_, _Sphæropthalmus_, _Ctenopyge_, _Goldius_,
+ _Cheirurus_, and probably others.
+
+ II. Aggregate eyes.
+
+ The individual lenses are comparatively large, distinct from one
+ another, each lying in its own socket. There is, however, a thin
+ membrane, which covers all those in any one aggregate, and is a
+ continuation of the general integument of the body. This membrane
+ is continued as a thickened infolding which forms the sockets of
+ the lenses.
+
+ Such eyes are known in the Phacopidæ only.
+
+ III. Stemmata and ocelli.
+
+ The stemmata are present only in _Harpes_, where there may be on
+ the summit of the cheek two or three ocelli lying near one another.
+ Each, viewed from above, is nearly circular in outline, almost
+ hemispheric, glossy and shining. In section they prove to be convex
+ above and flat or slightly concave beneath. The test covers and
+ separates them, as in the case of the aggregate eyes.
+
+ The ocelli of the Trinucleidæ and _Eoharpes_ are smaller, and the
+ detailed structure not yet investigated.
+
+ Lindstroem concludes that so far as its facets or lenses are
+ concerned, the eye of the trilobite shows the greatest analogy with
+ the Isopoda, and the least with _Limulus_.
+
+
+SUMMARY.
+
+The simplest eyes found among the Trilobita are the ocelli. These
+consist of a Simple thickening of the test to form a convex surface
+capable of concentrating light. The similarity in position of the
+paired ocelli of trilobites and the simple eyes of copepods has
+perhaps a significance.
+
+The schizochroal eyes may well be compared with the aggregate eyes of
+the chilopods and scorpions. The mere presence of a common external
+covering is not sufficient to prove this a true compound eye,
+especially as the covering is merely a continuation of the general
+test.
+
+The holochroal eyes are of two kinds, one with plano-convex and one
+with biconvex lenses. The latter would seem to be mechanically the
+more perfect of the two, and it is worthy of note that the trilobites
+possessing the biconvex lenses have, in general, much smaller eyes
+than those with the other type.
+
+If, as some investigators claim, the parietal eye of Crustacea
+originates by the fusion of two lateral ocelli, trilobites show a
+primitive condition in lacking this eye, which may have originated
+through the migration toward the median line of ocelli like those of
+the Trinucleidæ.
+
+
+SEX.
+
+That the sexes were separate in the Trilobita there can be very little
+doubt, but the study of the appendages has as yet revealed nothing in
+the way of sexual differences. One of the most important points still
+to be determined is the location of the genital openings.
+
+In many modern Crustacea, the antennæ or antennules are modified as
+claspers, and it is barely possible that the curious double curvature
+of the antennules of Triarthrus indicates a function of this sort. The
+antennules of many specimens have the rather formal double curvature,
+turning inward at the outer ends, and retain this position of the
+frontal appendages, no matter what may be the condition of those on
+the body. Other specimens have the antennules variously displaced,
+indicating that they are quite flexible. It is conceivable that the
+individuals with rigid antennules are males, the others females.
+
+It is interesting to note that the antennules of _Ptychoparia
+permulta_ Walcott (1918, pl. 21, fig. 1) have the same recurved form.
+All the specimens of Neolenus, however, show very flexible antennas.
+
+Barrande and Salter laid great stress upon the "forme longue" and
+"forme large" as indicating male and female. This was based upon the
+supposition that the female of any animal would probably have a
+broader test than the male, a hypothesis which seems to be very little
+supported by fact. In practical application it was found that the
+apparent difference was so often due to the state of preservation or
+the confusion of two or more species, that for many years little
+reference has been made to this supposed sex difference.
+
+
+EGGS.
+
+In his classic work on the trilobites of Bohemia, Barrande described
+three kinds of spherical and one of capsule-shaped bodies which he
+considered to be the eggs of trilobites. After a review of the
+literature and a study of specimens in the collections of the Museum
+of Comparative Zoology, it can be said that none of these fossils has
+proved to be a trilobite egg, but that they may be plants. A full
+account of them will be published elsewhere.
+
+Walcott (1881) and Billings (1870) have described similar bodies
+within the tests of _Calymene_ and _Ceraurus_, but without showing
+positive evidence as to their nature.
+
+
+Methods Of Life.
+
+This is a subject upon which much can be inferred, but little proved.
+Without trying to cover all possibilities, it may be profitable to
+see what can be deduced from what is known of the structure of the
+external test, the internal anatomy, and the appendages. This can, to
+a certain extent, be controlled by what is inferred from the strata
+in which the specimens are found, the state of preservation, and the
+associated animals. (For other details, see the discussion of
+"Function of the Appendages" in Part I.)
+
+
+HABITS OF LOCOMOTION.
+
+The methods of locomotion may be deduced with some safety from a study
+of the appendages, and, as has repeatedly been pointed out, all
+trilobites could probably swim by their use. This swimming was
+evidently done with the head directed forward, and could probably be
+accomplished indifferently well with either the dorsal (gastronectic,
+Dollo) or the ventral (notonectic) side up. If food were sought on the
+bottom by means of sight, the animal would probably swim dorsal side
+up, for by canting from side to side it could see the bottom just as
+easily as though it were ventral side up, and at the same time it
+would be in position to drop quickly on the prey. In collecting food
+at the surface, it might swim ventral side up.
+
+All trilobites could probably crawl by the use of the appendages, and,
+as has already been pointed out, there are great differences in the
+adjustment of the appendages to different methods of crawling. Some
+crawled on their "toes," some by means of the entire endopodites, and
+some apparently used the coxopodites to push themselves along. That
+the normal direction of crawling was forward is indicated by the
+position of the eyes and sensory antennules. There is no evidence that
+their mechanism was irreversible, however, and the position of the
+mouth and the shape of the hypostoma indicate that they usually backed
+into feeding position. The caudal rami of Neolenus were evidently
+sensory, and the animal was prepared to go in either direction.
+
+The use of the pygidium as a swimming organ, suggested by Spencer
+(1903, p. 492) on theoretical grounds, developed by Staff and Reck
+(1911, p. 141) from a mechanical standpoint, and elaborated in
+the present paper by evidence from the ontogeny, phylogeny, and
+musculature, provided the animal with a swifter means of locomotion.
+By a sudden flap of this large fin, a backward darting motion could be
+obtained, which would be invaluable as a means of escape from enemies.
+Staff and Reck seem to think that in this movement the two shields
+were clapped together, and that the animal was projected along
+with the hinge-like thorax forward. This might be a very plausible
+explanation in the case of the bivalve-like Agnostidæ, and it is one
+I had suggested tentatively for that family before I read Staff and
+Reck's paper. In the case of the large trilobites with more segments,
+however, it would be more natural to think of a mode of progression in
+which there was an undulatory movement of the body and the pygidium,
+up-and-down strokes being produced by alternately contracting the
+dorsal and ventral muscles. Bending the pygidium down would tend to
+pull the animal backward, while bringing it back into position would
+push it forward. It follows, therefore, that one of these movements
+must have been accomplished very quickly, the other slowly. If the
+muscle scars have been interpreted properly, the ventral muscles were
+probably the more powerful, an indication that the animal swam
+backward, using the cephalon and antennules as rudders.
+
+The chief objection to the theory of swimming by clapping the valves
+together is that where the thorax consists of several segments it no
+longer acts like the hinge of a bivalve, and a sudden downward flap of
+the pygidium would impart a rotary motion to the animal. Take, for
+example, such nearly spherical animals as the Illænidæ, and it will
+readily be seen that there is nothing to give direction to the motion
+if the pygidium be brought suddenly against the lower surface of the
+cephalon. A lobster, it is true, progresses very well by this method,
+but it depends upon its great claws and long antennæ to direct its
+motions. The whole shape of the trilobite is of course awkward for a
+rapidly swimming animal. It could keep afloat with the minimum of
+effort and paddle itself about with ease, but it was not built on the
+correct lines for speed.
+
+Dollo (1910, p. 406), and quickly following his lead, Staff and Reck
+(1911, p. 130), have published extremely suggestive papers, showing
+that by the use of the principle of correlation of parts, much can be
+inferred about the mode of life of the trilobites merely from the
+structure of the test.
+
+Dollo studied the connection between the shape of the pygidium and the
+position and character of the eyes. As applied by him, and later by
+Clarke and Ruedemann, to the eurypterids, this method seems most
+satisfactory. He pointed out that in Eurypterida like _Stylonurus_ and
+_Eurypterus_, where there is a long spine-like telson, the eyes are
+back from the margin, so that a _Limulus_-like habit of pushing the
+head into the sand by means of the limbs and telson was possible.
+_Erettopterus_ and _Pterygotus_, on the other hand, have the eyes on
+the margin, so that the head could not be pushed into the mud without
+damage, and have a fin-like telson, suggesting a swimming mode of
+life.
+
+In carrying this principle over to the trilobites, Dollo was quite
+successful, but Staff and Reck have pointed out some modifications
+of his results. The conclusions reached in both these papers are
+suggestive rather than final, for not all possible factors have been
+considered. The following are given as examples of interesting
+speculations along this line.
+
+_Homalonotus delphinocephalus_, according to Dollo, was a crawling
+animal adapted to benthonic life in the euphotic region, and an
+occasional burrower in mud. This is shown by well developed eyes in
+the middle of the cephalon, a pointed pygidium, and the plow-like
+profile of the head. This was as far as Dollo went. From the very
+broad axial lobe of _Homalonotus_ it is fair to infer that, like
+_Isotelus_, it had very long, strong coxopodites which it probably
+vised in locomotion, and also very well-developed longitudinal
+muscles, to be used in swimming. From the phylogeny of the group, it
+is known that the oldest homalonotids had broad unpointed pygidia of
+the swimming type, and that the later species of the genus (Devonian)
+are almost all found in sandstone and shale, and all have wider axial
+lobes than the Ordovician forms. It is also known that the epistoma
+is narrower and more firmly fused into the doublure in later than in
+earlier species. These lines of evidence tend to confirm Dollo's
+conclusion, but also indicate that the animals retained the ability to
+swim well.
+
+On the same grounds, _Olenellus thompsoni_ and _Dalmanites limulurus_
+were assigned the same habitat and habits. Both were considered to
+have used the terminal spine as does _Limulus_.
+
+_Olenellus thompsoni_ is generally considered to be unique among
+trilobites in having a _Limulus_-like telson in place of a pygidium.
+This "telson" has exactly the position and characteristics of the
+spine on the fifteenth segment of _Mesonacis_, and so long ago as
+1896, Marr (Brit. Assoc. Adv. Sci., Rept. 66th Meeting, page 764)
+wrote:
+
+ The posterior segments of the remarkable trilobite _Mesonacis
+ vermontana_ are of a much more delicate character than the anterior
+ ones, and the resemblance of the spine on the fifteenth "body
+ segment" of this species to the terminal spine of _Olenellus_
+ proper, suggests that in the latter subgenus posterior segments of
+ a purely membranous character may have existed devoid of hard
+ parts.
+
+This prophecy was fulfilled by the discovery of the specimens which
+Walcott described as _Pædeumias transitans_, a species which is said
+by its author to be a "form otherwise identical with _O. thompsoni_,
+[but] has rudimentary thoracic segments and a _Holmia_-like pygidium
+posterior to the fifteenth spine-bearing segment of the thorax." A
+good specimen of this form was found at Georgia, Vermont, associated
+with the ordinary specimens of _Olenellus thompsoni_, and I believe
+that it is merely a complete specimen of that species. _Olenellus
+gilberti_, which was formerly supposed to have a limuloid telson, has
+now been shown by Walcott (Smithson. Misc. Coll., vol. 64, 1916, p.
+406, pl. 45, fig. 3) to be a _Mesonacis_ and to have seven or eight
+thoracic segments and a small plate-like pygidium back of the
+spine-bearing fifteenth segment. All indications are that the spine
+was not in any sense a pygidium. Walcott states that _Olenellus_
+resulted from the resorption of the rudimentary segments of forms such
+as _Mesonacis_ and _Pædeumias_, leaving the spine to function as a
+pygidium. This would mean the cutting off of the anus and the
+posterior part of the alimentary canal, and developing a new anal
+opening on the spine of one of the thoracic segments!
+
+If the spine of the fifteenth segment is not a pygidium, could it be
+used, as Dollo postulates, as a pushing organ? Presumably not, for
+though in entire specimens of _Olenellus_ (_Pædeumias_) it extends
+back beyond the pygidium, it probably was borne erect, like the
+similar spines in _Elliptocephala_, and not in the horizontal plane in
+which it is found in crushed specimens.
+
+While this removes some of the force of Dollo's argument, his
+conclusion that _Olenellus_ was a crawling, burrowing animal living
+in well lighted shallow waters was very likely correct. The long,
+annelid-like body indicates numerous crawling legs, there is no
+swimming pygidium, and the fusion of the cheeks in the head makes a
+stiff cephalon well adapted for burrowing.
+
+Staff and Reck have pointed out that _Dalmanites limulurus_ was not
+entirely a crawler, but, as shown by the large pygidium, a swimmer
+as well. This kind of trilobite probably represents the normal
+development of the group in Ordovician and later times. The Phacopidæ,
+Proëtidæ, Calymenidæ, and other trilobites of their structure could
+probably crawl or swim equally well, and could escape enemies by
+darting away or by "digging themselves in."
+
+_Cryptolithus tessellatus_ (_Trinucleus concentricus_) is cited by
+Dollo as an example of an adaptation to life in the aphotic benthos,
+permanently buried in the mud. In this case he appealed to Beecher's
+interpretation of the appendages, and pointed out that while the adult
+is blind, the young have simple eyes and probably passed part of their
+life in the lighted zone. It needs only a glance at the very young to
+convince one that the embryos had swimming habits, so that in this
+form one sees the adaptation of the individual during its history to
+all modes of life open to a trilobite. The habits of the Harpedidæ may
+have been similar to those of the Trinucleidæ, but the members of
+this family are supplied with broad flat genal spines. It has been
+suggested that these served like pontoons, runners, or snow-shoes, to
+enable the animal to progress over soft mud without sinking into it.
+Some such explanation might also be applied to the similar development
+in the wholly unrelated Bathyuridæ. The absence of compound eyes and
+the poor development of ocelli in the Harpedidæ suggest that they were
+burrowers, and from these two families, Trinucleidæ and Harpedidæ, it
+becomes evident that a pygidial point or spine is not a necessary part
+of the equipment of a burrowing trilobite. In fact, from the habits of
+_Limulus_ it is known that the appendages are relied upon for digging,
+and that the telson is a useful but not indispensable pushing organ.
+
+_Deiphon_ is an interesting trilobite from many points of view. Its
+pleural lobes are reduced to a series of spines on either side of the
+body, and its pygidium is a mere spinose vestige. Dollo considered
+this animal a swimmer in the euphotic zone, because its eyes are on
+the anterior margin, its body depressed, its glabella globose, and its
+pygidium flat and spinose. That such a method of life was secondary
+in a cheirurid was indicated to him by the fact that the more
+primitive members of the family seemed adapted for crawling. Staff and
+Reck have gone further and shown that the pygidium is only the vestige
+of a swimming pygidium, and that the great development of spines
+suggests a floating rather than a swimming mode of life. They
+therefore argue for a planktonic habitat. A similar explanation is
+suggested for _Acidaspis_ and other highly spinose species.
+
+The Aeglinidæ, or Cyclopygidæ as they are more properly called,
+present the most remarkable development of eyes among the trilobites.
+In this, Dollo saw, as indeed earlier writers have, an adaptation
+to a region of scanty light. The cephalon is not at all adapted to
+burrowing, but the pygidium is a good swimming organ, and one is apt
+to agree that this animal was normally an inhabitant of the ill
+lighted dysphotic region, but also a nocturnal prowler, making trips
+to the surface at night. Similar habits and habitat are certainly
+indicated for _Telephus_ and the Remopleuridæ, but whether _Nileus_
+and the large-eyed _Bumastus_ are capable of the same explanation is
+doubtful.
+
+Finch (1904, p. 181) makes the suggestion that "_Nileus_" (_Vogdesia_)
+_vigilans_, an abundant trilobite in the calcareous shale of the
+Maquoketa, was in the habit of burying itself, posterior end first. He
+found a slab containing fifteen entire specimens, all of which had the
+cephalon extended horizontally near the surface of the stratum, and
+the thorax and pygidium projecting downward. The rock showed no
+evidence that they were in burrows, and the fact that all were in the
+same position indicates that the attitude was voluntarily assumed.
+They appear to have entrenched themselves by the use of the pygidia,
+which are incurved plates readily adapted for such use, and, buried up
+to the eyes, awaited the coming of prey, but were, apparently,
+smothered by a sudden influx of mud. The form of the eye in _Vogdesia
+vigilans_ bears out this supposition of Finch's. Not only are the eyes
+unusually tall, but the palpebral lobe is much reduced, so that many
+of the lenses look upward and inward, as well as outward, forward and
+backward. The particular food required by _V. vigilans_ must have been
+very plentiful in the Maquoketa seas of Illinois and Iowa, for the
+species was very abundant, but that its habits were self-destructive
+is also shown by the great number of complete enrolled specimens of
+all ages now found there. The soft mud was apparently fatal to the
+species before the end of the Maquoketa, for specimens are seen but
+very rarely in the higher beds.
+
+_Vogdesia vigilans_ is shaped much like _Bumastus_, _Illænus_,
+_Asaphus_, _Onchometopus_, and _Brachyaspis_, and it may be that these
+trilobites with incurved pygidia had all adopted the habit of digging
+in backward. As noted above, their pygidia are not very well adapted
+for swimming, and most of them have large or tall eyes.
+
+Dollo's comparison of the Cyclopygidæ to the huge-eyed modern amphipod
+_Cystosoma_ is instructive. This latter crustacean, which has the
+greater part of the dorsal surface of the carapace transformed into
+eyes, is said to live in the dysphotic zone, at depths of from 40 to
+100 fathoms, and to come to the surface at night. It swims ventral
+side down.
+
+The kinds of sediments in which trilobites are entombed have so far
+afforded little evidence as to their habitat. Frech (Lethæa
+palæozoica, 1897-1902, p. 67 _et seq._) who has collected such
+evidence as is available on this subject, places as deeper water
+Ordovician deposits the "Trinucleus-Schiefer" of the upper Ordovician
+of northern Europe and Bohemia, the "Triarthrus-Schiefer" of America,
+the "Asaphus-Schiefer" of Scandinavia, Bohemia, Portugal, and France,
+and the Dalmania quartzite of Bohemia. .
+
+_Cryptolithus_ and _Triarthrus_, although not confined to such
+deposits, are apt to occur chiefly in very fine-grained shales, in
+company with graptolites. These latter are distributed by currents
+over great distances within short periods. It is somewhat curious that
+the nearly blind burrowing Trinucleidæ, the dysphotic, large-eyed
+Remopleuridæ and Telephus, the blind nektonic Agnostidæ and Dionide,
+and the planktonic graptolites should go together and make up almost
+the entire fauna of certain formations. Yet, when the life history of
+each type is studied, a logical explanation is readily at hand, for
+all have free-swimming larvæ.
+
+A list of the methods of life noted above is given by way of summary,
+with examples.
+
+ {Planktonic {Primarily Earliest protaspis of all trilobites
+ { {Secondarily _Deiphon_, _Odontopleura_, etc.
+ {
+ Pelagic { {Primarily Later protaspis of all trilobites.
+ { { _Naraoia_
+ { {
+ { { {Probably many thin-shelled
+ { { { trilobites with large pygidia
+ { { { (only partially nektonic)
+ {Nektonic {Secondarily {Cyclopygidæ }
+ {Remopleuridæ } (nektonic dysphotic)
+
+ {Crawlers and
+ { slow swimmers Most trilobites with small pygidia.
+ { _Triarthrus_, _Paradoxides_, etc.
+ Benthonic {Crawlers and Most trilobites with large pygidia.
+ { active swimmers _Isotelus_, _Dalmanites_, etc.
+ {
+ {Crawlers, slow
+ { swimmers, and Trinucleidæ, Harpedidæ,
+ { burrowers some Mesonacidæ, etc.
+
+
+FOOD AND FEEDING METHODS.
+
+This subject has been less discussed than the methods of locomotion.
+The study of the appendages has shown that while the mouth parts were
+not especially powerful, they were at least numerous, and sufficiently
+armed with spines to shred up such animal and vegetable substances as
+they were liable to encounter. It having been ascertained that the
+shape of the glabella and axial lobe furnishes an indication of the
+degree of development of the alimentary canal it is possible to infer
+something of the kind of food used by various trilobites.
+
+The narrow glabellæ and axial lobes of the oldest trilobites would
+seem to indicate a carnivorous habit, while the swollen glabellæ and
+wider lobes of later ones probably denote an adaptation to a mixed or
+even a vegetable diet. This can not be relied upon too strictly, of
+course, for the swollen glabellæ of such genera as Deiphon or
+Sphærexochus may be due merely to the shortening up of the cephalon.
+
+Walcott (1918, p. 125) suggests that the trilobites lived largely upon
+worms and conceives of them as working down into the mud and prowling
+around in it in search of such prey. While there can be no doubt that
+many trilobites had the power of burying themselves in loose sand or
+mud, a common habit with modern crustaceans, most of them were of a
+very awkward shape for habitual burrowers, and how an annelid could be
+successfully pursued through such a medium by an animal of this sort
+is difficult to understand. In fact, the presence of the large
+hypostoma and the position of the mouth were the great handicaps of
+the trilobite as a procurer of live animal food, and coupled with the
+relatively slow means of locomotion, almost compel the conclusion that
+errant animals of any size were fairly safe from it. This restricts
+the range of animal food to small inactive creatures and the remains
+of such larger forms as died from natural causes. The modern Crustacea
+are effective scavengers, and it is probable that their early
+Palæozoic ancestors were equally so. It is a common saying that in the
+present stressful stage of the world's history, very few wild animals
+die a natural death. In Cambrian times, competition for animal food
+was less keen, and with the exception of a few cephalopods, a few
+large annelids, and a few Crustacea like _Sidneyia_, there seem to
+have been no aggressive carnivores. In consequence, millions of
+animals must have daily died a natural death, and had there been no
+way of disposing of their remains, the sea bottom would soon have
+become so foul that no life could have existed. For the work of
+removal of this decaying matter, the carnivorous annelids and the
+Crustacea, mostly trilobites, were the only organisms, and it is
+probable that the latter did their full share. After prowling about
+and locating a carcass, the trilobite established himself over it, the
+cephalon and hypostoma on one end and the pygidium on the other
+enclosing and protecting the prey, which was shredded off and passed
+to the mouth at leisure by means of the spinose endobases.
+
+Even in Middle Cambrian times some trilobites (e. g., _Paradoxides_)
+seem to have enlarged the capacity of the stomach and taken vegetable
+matter, but later, in the Upper Cambrian and Ordovician, when the
+development of cephalopods and fishes caused great competition for all
+animal food, dead or alive, most trilobites seem to have become
+omnivorous. This is of course shown by the swollen glabella, with
+reduced lateral furrows, and, in the case of a few species
+(_Calymene_, _Ceraurus_), the known enlargement of the stomach.
+
+_Cryptolithus_ is the only trilobite which has furnished any actual
+evidence as to its food. From the fact that the alimentary tract is
+found stuffed from end to end with fine mud, and because it is known
+to have been a burrower, it has been suggested by several that it was
+a mud feeder, passing the mud through the digestive tract for the sake
+of what organic matter it contained. Spencer (1903, p. 491) has
+suggested a modification of this view:
+
+ The phyllopods appear to feed by turning over whilst swimming and
+ seizing with their more posterior appendages a little mud which
+ swarms with infusoria, etc. This mud is then pushed along the
+ ventral groove to the mouth. Casts, of the intestine of trilobites
+ are still found filled with the mud.
+
+_Ceraurus_ and _Calymene_ also must have occasionally swallowed mud in
+quantity, otherwise the form of the alimentary canal could not have
+been preserved as it is in a few of Doctor Walcott's specimens.
+
+
+TRACKS AND TRAILS OF TRILOBITES.
+
+Tracks and trails of various sorts have been ascribed by authors to
+trilobites since these problematic markings first began to attract
+attention, but as the appendages were until recently quite unknown,
+all the earlier references were purely speculative. The subject is a
+difficult one, and proof that any particular track or trail could have
+been made in only one way is not easily obtained. Since the appendages
+have actually been described, comparatively little has been done,
+Walcott's work on _Protichnites_ (1912 B, p. 275) being the most
+important. Since the first description of _Protichnites_ by Owen
+(Quart. Jour. Geol. Soc., London, 1852, vol. 8, p. 247), it has been
+thought that these trails were made by crustaceans, and the only known
+contemporaneous crustaceans being trilobites, these animals were
+naturally suggested. Dawson (Canadian Nat. Geol., vol. 7, 1862, p.
+276) ascribed them, with reserve, to _Paradoxides_, and Billings
+(1870, p. 484) suggested _Dikelocephalus_ or _Aglaspis_. Walcott
+secured well preserved specimens which showed trifid tracks, and these
+were readily explained when he found the legs of _Neolenus_, which
+terminated with three large spines. Similar trifid terminations had
+already been described by Beecher, and clearly pictured in his
+restoration of _Triarthrus_, but the spines and the tracks had
+somehow not previously been connected in the mind of any observer.
+Walcott concluded that the tracks had been made by a species of
+_Dikelocephalus_, possibly by _D. hartti_, which occurs both north
+and south of the Adirondacks. In a recent paper, Burling (Amer. Jour.
+Sci., ser. 4, vol. 44, 1917, p. 387) has argued that Protichnites was
+not the trail of a trilobite, but of a "short, low-lying, more or less
+heavy set, approximately 12-legged, crab-like animal," which had an
+oval shape, toed in, and was either extremely flexible or else short
+and more or less flexible in outline. This seems to describe a
+trilobite.
+
+_Climactichnites_, the most discussed single trail of all, has also
+been ascribed to trilobites,--by Dana (Manual of Geology, 1863, p.
+185), Billings (1870, p. 485), and Packard (Proc. Amer. Acad. Arts and
+Sci., vol. 36, 1900, p. 64),--though less frequently than to other
+animals. The latest opinion (see paper by Burling cited above) seems
+to be against this theory.
+
+Miller (1880, p. 217) described under the generic name
+_Asaphoidichnus_ two kinds of tracks which were such as he supposed
+might be made by an _Asaphus_ (_Isotelus_). In referring to the second
+of the species, he says: "Some of the toe-tracks are more or less
+fringed, which I attribute to the action of water, though Mr. Dyer is
+impressed with the idea that it may indicate hairy or spinous feet."
+The type of this species, _A. dyeri_, is in the Museum of Comparative
+Zoology, and while it may be the trail of a trilobite, it would be
+difficult to explain how it was produced.
+
+Ringueberg (1886, p. 228) has described very briefly tracks found in
+the upper part of the Medina at Lockport, New York. These consisted of
+a regularly succeeding series of ten paired divergent indentations
+arranged in two diverging rows, with the trail of the pygidium showing
+between each series. The ten pairs of indentations he considered could
+have been made by ten pairs of legs like those shown by the specimen
+of Isotelus described by Mickleborough, and the intermittent
+appearance of the impression of the pygidium suggested to him that the
+trilobite proceeded by a series of leaps.
+
+Walcott (1918, pp. 174-175, pl. 37-42) has recently figured a number
+of interesting trails as those of trilobites, and has pointed out that
+a large field remains open to anyone who has the patience to develop
+this side of the subject.
+
+
+
+
+PART III.
+
+
+
+
+RELATIONSHIP OF THE TRILOBITES TO OTHER ARTHROPODA.
+
+
+It can not be said that the new discoveries of appendagiferous
+trilobites have added greatly to previous knowledge of the systematic
+position of the group. Probably none will now deny that trilobites are
+Crustacea, and more primitive and generalized than any other group in
+that class. The chief interest at present lies in their relation to
+the most nearly allied groups, and to the crustacean ancestor.
+
+Trilobites have been most often compared with Branchiopoda, Isopoda,
+and Merostomata, the present concensus of opinion inclining toward the
+notostracan branchiopods (Apodidæ in particular) as the most closely
+allied forms. It seems hardly worth while to burden these pages
+with a history of opinion on this subject, since it was not until the
+appendages were fully made out, from 1881 to 1895, that zoologists and
+palæontologists were in a position to give an intelligent judgment.
+The present status is due chiefly to Bernard (1894), Beecher (1897,
+1900, et seq.), and Walcott (1912, et seq.).
+
+The chief primitive characteristics of trilobites are: direct
+development from a protaspis common to the subclass; variability in
+the number of segments, position of the mouth, and type of eyes; and
+serially similar biramous appendages.
+
+The recent study has modified the last statement slightly, since it
+appears that in some trilobites there was a modification of the
+appendages about the mouth, suggesting the initiation of a set of
+tagmata.
+
+In comparing the trilobites with other Crustacea, the condition of the
+appendages must be especially borne in mind, for while these organs
+are those most intimately in contact with the environment, and most
+subject to modification and change, yet they have proved of greatest
+service in classification.
+
+Appendages have been found on trilobites from only the Middle Cambrian
+and Middle and Upper Ordovician, but as the Ordovician was the time of
+maximum development of the group, it is probable that trilobites of
+later ages would show degradational rather than progressive changes.
+All the genera which are known show appendages of the same plan, and
+although new discoveries will doubtless reveal many modifications of
+that plan, general inferences may be drawn now with some assurance.
+
+The chief characteristics of the appendages are: first, simple
+antennules, a primitive feature in all Crustacea, as shown by
+ontogeny; second, paired biramous appendages, similar to each other
+all along the body, the youngest and simplest in front of the anal
+segment, the oldest and most modified on the cephalon. The endobases
+are retained on all the coxopodites, except possibly, in some species,
+the anterior ones, and these gnathobases are modified in some genera
+as mouth-parts, while in others they are similar throughout the
+series. With these few fundamentals in mind, other Crustacea may be
+examined for likenesses. The differences are obvious.
+
+
+
+
+Crustacea.
+
+
+BRANCHIOPODA.
+
+The early idea that the trilobites were closely related to the
+Branchiopoda was rejuvenated by the work of Bernard on the Apodidæ
+(1892) and has since received the support of most writers on the
+subject. Fundamentally, a great deal of the argument seems to be that
+_Apus_ lies the nearest of any modern representative of the class to the
+theoretical crustacean ancestor, and as the trilobites are the oldest
+Crustacea, they must be closely related. Most writers state that the
+trilobites could not be derived from the Branchiopoda (see, however,
+Walcott 1912 A), nor the latter from any known trilobite, but both
+subclasses are believed to be close to the parent stem.
+
+Viewed from the dorsal side, there is very little similarity between
+any of the branchiopods and the trilobites, and it is only in the
+Notostraca, with their sessile eyes and depressed form, that any
+comparison can be made. The chief way in which modern Branchiopoda and
+Trilobita agree is that both have a variable number of segments in
+the body, that number becoming very large in _Apus_ on the one hand and
+_Mesonacis_ and _Pædeumias_ on the other. In neither are the appendages,
+except those about the mouth, grouped in tagmata. Other likenesses
+are: the Branchiopoda are the only Crustacea, other than Trilobita, in
+which gnathobases are found on limbs far removed from the mouth; the
+trunk limbs are essentially leaf-like in both, though the limb of the
+branchiopod is not so primitive as that of the trilobite; caudal cerci
+occur in both groups.
+
+If the appendages be compared in a little more detail, the differences
+prove more striking than the likenesses.
+
+In the Branchiopoda, the antennules are either not segmented or only
+obscurely so. In trilobites they are richly segmented.
+
+In Branchiopoda, the antennæ are variable. In the Notostraca they are
+vestigial, while in the males of the Anostraca they are powerful and
+often complexly developed claspers. Either condition might develop
+from the generalized biramous antennas of Trilobita, but the present
+evidence indicates a tendency toward obsolescence. Claus' observations
+indicate that the antennæ of the Anostraca are developments of the
+exopodites, rather than of the endopodites.
+
+The mandibles and maxillæ of the Branchiopoda are greatly reduced, and
+grouped closely about the mouth. Only the coxopodites of the Trilobita
+are modified as oral appendages.
+
+The trunk limbs of _Apus_ are supposed to be the most primitive among
+the Branchiopoda, and comparison will be made with them. Each
+appendage consists of a flattened axial portion, from the inner margin
+of which spring six endites, and from the outer, two large flat exites
+(see fig. 34). This limb is not articulated with the ventral membrane,
+but attached to it, and, if Lankester's interpretation of the origin
+of schizopodal limbs be correct, then the limb of _Apus_ bears very
+little relation to that of the Trilobita. In _Apus_ there is no
+distinct coxopodite and the endobases which so greatly resemble the
+similar organs in the Trilobita are not really homologous with them,
+but are developments of the first endite. Beecher's comparison of the
+posterior thoracic and pygidial limbs of _Triarthrus_ with those of
+_Apus_ can not be sustained. Neither _Triarthrus_ nor any other
+trilobite shows any trace of phyllopodan limbs. Beecher figured (1894
+B, pl. 7, figs. 3, 4) a series of endopodites from the pygidium of a
+young _Triarthrus_ beside a series of limbs from a larval _Apus_.
+Superficially, they are strikingly alike, but while the endopodites of
+_Triarthrus_ are segmented, the limbs of _Apus_ are not, and the parts
+which appear to be similar are really not homologous. The similarity
+of the thoracic limbs in the two groups is therefore a case of
+parallelism and does not denote relationship.
+
+Geologically, the Branchiopoda are as old as the Trilobita, and while
+they did not have the development in the past that the trilobite
+had, they were apparently differentiated fully as early. Anostraca,
+Notostraca and Conchostraca, three of the four orders, are represented
+in the Cambrian by forms which are, except in their appendages, as
+highly organized as the existing species. Brief notes on the principal
+Middle Cambrian Branchiopoda follow:
+
+
+=Burgessia bella= Walcott.
+
+ Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 177,
+ pl. 27, figs. 1-3; pl. 30, figs. 3, 4.
+
+This is the most strikingly like the modern Branchiopoda of any
+species described by Walcott from the Middle Cambrian, and invites
+comparison with _Apus_. The carapace is long, loosely attached to the
+body, and extends over the greater part of the thorax. The eyes are
+small, sessile, and close to the anterior margin.
+
+The appendages of the head consist of two pairs of antennæ, and three
+pairs of slender, jointed legs. Both pairs of antennæ are slender and
+many-jointed, the antennules somewhat smaller than the antennæ. The
+exact structure of the limbs about the mouth has not yet been made
+out, but they are slender, tapering, endopodite-like legs, with at
+least three or four segments in each, and probably more.
+
+There are eight pairs of thoracic appendages, each limb having the
+form of the endopodite of a trilobite and consisting of seven segments
+and a terminal spine. The proximal three segments of each appendage
+are larger than the outer ones, and have a flattened triangular
+expansion on the inner side. Walcott also states that "One specimen
+shows on seven pairs of legs, small, elongate, oval bodies attached
+near the first joint to the outer side of the leg. These bodies left
+but slight impression on the rock and are rarely seen. They appear to
+represent the gills." They are not figured, but taken in connection
+with the endopodite-like appearance of the segmented limbs, one would
+expect them to be vestigial exopodites.
+
+A small hypostoma is present on the ventral side, and several of the
+specimens show wonderfully well the form of the alimentary canal and
+the hepatic cæca. The main branches of the latter enter the mesenteron
+just behind the fifth pair of cephalic appendages.
+
+Behind the thorax the abdomen is long, limbless, and tapers to a
+point. It is said to consist of at least thirty segments.
+
+Compared with _Apus_, _Burgessia_ appears both more primitive and more
+specialized. The carapace and limbless abdomen are _Apus_-like, but
+there are very few appendagiferous segments, and the appendages are
+not at all phyllopodan, but directly comparable with those of
+trilobites, except, of course, for the uniramous character of the
+cephalic limbs. A closer comparison may be made with _Marrella_.
+
+
+=Waptia fieldensis= Walcott.
+
+ Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 181,
+ pl. 27, figs. 4, 5.
+
+The carapace is short, covering the head and the anterior part of the
+thorax. The latter consists of eight short segments with appendages,
+while the six abdominal segments, which are similar to those of the
+thorax, are without limbs except for the last, which bears a pair
+of broad swimmerets. The eyes are marginal and pedunculate. The
+antennules are imperfectly known, but apparently short, while the
+antennas are long and slender, with relatively few, long, segments.
+The mandibles appear to be like endopodites of trilobites and show
+at least six segments. As so often happens in these specimens from
+British Columbia, the preservation of the other appendages is
+unsatisfactory. As illustrated (Walcott, 1912 A, pl. 27, fig. 5), both
+endopodites and exopodites appear to be present, and the shaft of the
+exopodite seems to be segmented as in _Triarthrus_.
+
+Walcott considers _Waptia_ as a transitional form between the
+Branchiopoda and the Malacostraca.
+
+
+=Yohoia tenuis= Walcott.
+
+ Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 172,
+ pl. 29, figs. 7-13.
+
+This species, though incompletely known, has several interesting
+characteristics. The head shows, quite plainly in some specimens, the
+five segments of which it is composed. The eyes are small, situated in
+a niche between the first and second segments, and are described as
+being pedunculate. The eight segments of the thorax all show short
+triangular pleural extensions, somewhat like those of _Remopleurides_
+or _Robergia_. The abdomen consists of four cylindrical segments, the
+last with a pair of expanded caudal rami.
+
+The antennules appear to be short, while the antennas are large, with
+several segments, ending in three spines, and apparently adapted for
+serving as claspers in the male. The third, fourth, and fifth pairs of
+cephalic appendages are short, tapering, endopodite-like legs similar
+to those of _Burgessia_.
+
+The appendages of the thorax are not well preserved, and there seem to
+be none on the abdomen.
+
+This species is referred by Walcott to the Anostraca.
+
+
+=Opabina regalis= Walcott.
+
+ Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 167,
+ pl. 27, fig. 6; pl. 28, fig. 1.
+
+This most remarkably specialized anostracan is not well enough known
+to allow comparison to be made with other contemporaneous Crustacea,
+but it is worthy of mention.
+
+There is no carapace, the eyes are pedunculated, thorax and abdomen
+are not differentiated, and the telson is a broad, elongate, spatulate
+plate. There seem to be sexual differences in the form of the anterior
+cephalic and caudal appendages, but this is not fully established. The
+most remarkable feature is the long, large, median cephalic appendage
+which is so suggestive of the proboscis of the recent _Thamnocephalus
+platyurus_ Packard. The appendages are not well enough preserved to
+permit a determination as to whether they are schizopodal or
+phyllopodan.
+
+_Summary._
+
+Walcott referred _Burgessia_ and _Waptia_ to new families under the
+Notostraca, while _Yohoia_ and _Opabina_ were placed with the
+Anostraca. Except for the development of the carapace, there is a
+striking similarity between _Waptia_ and _Yohoia_, serving to connect
+the two groups.
+
+The Branchiopoda were very highly specialized as early as Middle
+Cambrian time, the carapace of the Notostraca being fully developed
+and the abdomen limbless. Some (_Burgessia_) had numerous segments,
+but most had relatively few. The most striking point about them,
+however, is that so far as is known none of them had phyllopodan
+limbs. While the preservation is in most cases unsatisfactory, such
+limbs as are preserved are trilobite-like, and in the case of
+_Burgessia_ there can be no possible doubt of the structure. Another
+interesting feature is the retention by _Yohoia_ of vestiges of
+pleural lobes. The Middle Cambrian Branchiopoda are more closely
+allied to the Trilobita than are the modern ones, but still the
+subclass is not so closely related to that group as has been thought.
+Modern _Apus_ is certainly much less like a trilobite than has been
+supposed, and very far from being primitive. The Branchiopoda of the
+Middle Cambrian could have been derived from the trilobites by the
+loss of the pleural lobes, the development of the posterior margin of
+the cephalon to form a carapace, and the loss of the appendages from
+the abdominal segments. Modern branchiopods can be derived from those
+of the Middle Cambrian by the modification of the appendages through
+the reduction of the endopodite and exopodite and the growth of the
+endites and exites from the proximal segments.
+
+Carpenter (1903, p. 334), from his study of recent crustaceans, has
+already come to the conclusion that the Branchiopoda are not the most
+primitive subclass, and this opinion is strengthened by evidence
+derived from the Trilobita and from the Branchiopoda of the Middle
+Cambrian.
+
+
+COPEPODA.
+
+The non-parasitic Eucopepoda are in many ways much nearer to the
+trilobites than any other Crustacea. These little animals lack the
+carapace, and the body is short, with typically ten free segments and
+a telson bearing caudal furcæ. The head is composed of five segments
+(if the first thoracic segment is really the fused first and second),
+is often flattened, and lacks compound eyes. Pleural lobes are well
+developed, but instead of being flattened as in the trilobite, they
+are turned down at the sides or even incurved. A labrum is present.
+
+The antennules, antennæ, and mandibles are quite like those of
+trilobites. The antennules are very long and made up of numerous
+segments. The antennæ are biramous, the junction between the
+coxopodite and basipodite is well marked, and the endopodite consists
+of only two segments.
+
+The mandibles are said to "retain more completely than in any other
+Crustacea the form of biramous swimming limbs which they possess in
+the nauplius." The coxopodites form jaws, while both the reduced
+endopodite and exopodite are furnished with long setæ. The maxillulæ
+are also biramous, but very different in form from those of the
+trilobite, and the maxillæ are phyllopodan.
+
+The first thoracic limb is uniramous and similar to the maxillæ, but
+the five following pairs are biramous swimming legs with coxopodite,
+basipodite, exopodite, and endopodite. Both the exopodite and
+endopodite are shorter than in the trilobites, but bear setæ and
+spines.
+
+The last pair of thoracic limbs are usually modified in the male into
+copulatory organs. In some females they are enlarged to form plates
+for the protection of the eggs, in others they are unmodified. In
+still others they are much reduced or disappear. The abdomen is
+without appendages.
+
+The development in Copepoda is direct, by the addition posteriorly to
+the larval form (nauplius) of segments, and the appendages remain
+nearly unmodified in the adult.
+
+Altogether, the primitive Copepoda seem much more closely allied to
+the Trilobita than any other modern Crustacea, but unfortunately no
+fossil representative of the subclass has been found. This is not so
+surprising when one considers the habits and the habitat of most of
+the existing species. Many are parasitic, many pelagic in both fresh
+and marine waters, and many of those living on the bottom belong to
+the deep sea or fresh water. Most free-living forms are minute, and
+all have thin tests.
+
+The eyes of copepods are of interest, in that they suggest the paired
+ocelli of the Harpedidæ and Trinucleidæ. In the Copepoda there are, in
+the simplest and typical form of these organs, three ocelli, each
+supplied with its own nerve from the brain. Two of these are dorsal
+and look upward, while the third is ventral. In some forms the dorsal
+ocelli are doubled, so that five in all are present (cf. some species
+of Harpes with three ocelli on each mound). In some, the cuticle over
+the dorsal eyes is thickened so as to form a lens, as appears to be
+the case in the trilobites. These peculiar eyes may be a direct
+inheritance from the Hypoparia.
+
+
+ARCHICOPEPODA.
+
+Professor Schuchert has called my attention to the exceedingly curious
+little crustacean which Handlirsch (1914) has described from the
+Triassic of the Vosges. Handlirsch erected a new species, genus,
+family, and order for this animal, which he considered most closely
+allied to the copepods, hence the ordinal name. _Euthycarcinus
+kessleri_, the species in question, was found in a clayey lens in the
+Voltzia sandstone (Upper Bunter). Associated with the new crustacean
+were specimens of _Estheria_ only, but in the Voltzia sandstone itself
+land plants, fresh and brackish water animals, and occasionally,
+marine animals are found. The clayey lens seems to have been of fresh
+or brackish water origin.
+
+All of the specimens (three were found) are small, about 35 mm. long
+without including the caudal rami, crushed flat, and not very well
+preserved. The head is short, not so wide as the succeeding segments,
+and apparently has large compound eyes at the posterior lateral
+angles. The thorax consists of six segments which are broader than the
+head or abdomen. The abdomen, which is not quite complete in any one
+specimen, is interpreted by Handlirsch as having four segments in the
+female and five in the male. Least satisfactory of all are traces of
+what are interpreted by the describer as a pair of long stiff
+unsegmented cerci or stylets on the last segment.
+
+The ventral side of one head shield shows faint traces of several
+appendages which must have presented great difficulty in their
+interpretation. A pair of antennules appear to spring from near the
+front of the lower surface, and the remainder of the organs are
+grouped about the mouth, which is on the median line back of the
+center. Handlirsch sees in these somewhat obscure appendages four
+pairs of biramous limbs, antennæ, mandibles, maxillulæ, and maxillæ,
+both branches of each consisting of short similar segments,
+endopodites and exopodites being alike pediform.
+
+Each segment of the thorax has a pair of appendages, and those on
+the first two are clearly biramous. The endopodites are walking legs
+made up of numerous short segments (twelve or thirteen according to
+Handlirsch's drawing), while the exopodite is a long breathing and
+rowing limb, evidently of great flexibility and curiously like the
+antennules of the same animal. The individual segments are narrow at
+the proximal end, expand greatly at the sides, and have a concave
+distal profile. A limb reminds one of a stipe of _Diplograptus_.
+Both branches are spiniferous.
+
+No appendages are actually present on the abdomen, but each segment
+has a pair of scars showing the points of attachment. From the small
+size of these, it is inferred that the limbs were poorly developed.
+
+This species is described in so much detail because, if it is a
+primitive copepod, it has a very important bearing on the ancestry of
+that group and is the only related form that has been found fossil.
+
+The non-parasitic copepods have typically ten (eleven) free segments,
+including the telson, and the four abdominal segments are much more
+slender than the six in front of them. In this respect the agreement
+is striking, and the presence of five pairs of appendages in the head
+and six free segments in the thorax is a more primitive condition than
+in modern forms where the first two thoracic segments are apparently
+fused (Calman, 1909, p. 73).
+
+The large compound eyes of this animal are of course not present in
+the copepods, but as vestiges of eyes have been found in the young of
+_Calanus_, it is possible that the ancestral forms had eyes.
+
+The greatest difficulty is in finding a satisfactory explanation of
+the appendages. The general condition is somewhat more primitive than
+in the copepods, for all the appendages are biramous, while in the
+modern forms the maxillipeds are uniramous and the sixth pair of
+thoracic appendages are usually modified in the male as copulatory
+organs. In the copepods the modification is in the direction of
+reduction, both endopodites and exopodites usually possessing fewer
+segments than the corresponding branches in the trilobites. The
+endopodite of _Euthycarcinus_, on the contrary, possesses, if
+Handlirsch's interpretation is correct, twice as many segments as the
+endopodite of a trilobite. If the Copepoda are descended from the
+trilobites, as everything tends to indicate, then _Euthycarcinus_ is
+certainly not a connecting link. The only truly copepodan
+characteristic of this genus is the agreement in number and
+disposition of free segments. The division into three regions instead
+of two, the compound eyes, and the structure of the appendages are all
+foreign to that group.
+
+With the Limulava fresh in mind, one is tempted to compare
+_Euthycarcinus_ with that ancient type. The short head and large
+marginal eyes recall _Sidneyia_, and the grouping of the appendages
+about the mouth also suggests that genus and _Emeraldella_. In the
+Limulava likewise there is a contraction of the posterior segments,
+although it is behind the ninth instead of the sixth. There is no
+likeness in detail between the appendages of the Limulava and those of
+_Euthycarcinus_, but the composite claws of _Sidneyia_ show that in
+this group there was a tendency toward the formation of extra
+segments.
+
+If this fossil had been found in the Cambrian instead of the Triassic,
+it would probably have been referred to the Limulava, and is not
+at all impossible that it is a descendant from that group. As a
+connecting link between the Trilobita and Copepoda it is, however,
+quite unsatisfactory.
+
+
+OSTRACODA.
+
+The bivalved shell of the Ostracoda gives to this group of animals an
+external appearance very different from that of the trilobites, but
+the few appendages, though highly modified, are directly comparable.
+The development, although modified by the early appearance of the
+bivalved shell within which the nauplius lies, is direct. Imperfect
+compound eyes are present in one family.
+
+The antennules are short and much modified by functioning as swimming,
+creeping, or digging organs. They consist of eight or less segments.
+The antennas are also locomotor organs, and in most orders are
+biramous. The mandibles are biramous and usually with, but sometimes
+without, a gnathobase. The maxillulæ are likewise biramous but much
+modified.
+
+The homology of the third post-oral limb is in question, some
+considering it a maxilla and others a maxilliped. It has various forms
+in different genera. It is always much modified, but exopodite and
+endopodite are generally represented at least by rudiments. The fourth
+post-oral limb is a lobed plate, usually not distinctly segmented, and
+the fifth a uniramous pediform leg. The sixth, if present at all, is
+vestigial.
+
+Very little comparison can be made between the Ostracoda and
+Trilobita, other than in the ground-plan of the limbs, but the
+presence of biramous antennæ is a primitive characteristic.
+
+
+CIRRIPEDIA.
+
+Like the ostracod, the adult cirriped bears little external
+resemblance to the trilobite. The form of the nauplius is somewhat
+peculiar, but it has the typical three pairs of appendages, to which
+are added in the later metanauplius stages the maxillæ and six pairs
+of thoracic appendages. In the adult, the antennules, which serve for
+attachment of the larva, usually persist in a functionless condition,
+while the antennas disappear. The mandibles, maxillulæ, and maxillæ
+are simple and much modified to form mouth parts, and the six pairs of
+thoracic appendages are developed into long, multisegmented, biramous
+appendages bearing numerous setæ which serve for catching prey. Paired
+eyes are present in later metanauplius stages, but lost early in the
+development. The relationship to the trilobite evidently is not close.
+
+
+MALACOSTRACA.
+
+_1. Phyllocarida._
+
+The oldest malacostracans whose appendages are known are species of
+_Hymenocaris_. One, described as long ago as 1866 by Salter, has what
+seem to be a pair of antennæ and a pair of jaw-like mouth-parts.
+Another more completely known species has recently been reported by
+Walcott (1912 A, p. 183, pl. 31, figs. 1-6). This latter form is
+described as having five pairs of cephalic appendages: a pair of
+minute antennules beside the small pedunculated eyes, a pair of large
+uniramous antennæ, slender mandibles and maxillulæ, and large maxillæ
+composed of short stout segments. There are eight pairs of biramous
+thoracic limbs, the exopodites setiferous, the endopodites composed of
+short wide segments and ending in terminal claw-like spines. These
+appendages are like those of trilobites.
+
+_Hymcnocaris_ belongs to the great group of extinct ceratocarid
+Crustacea which are admitted to the lowest of the malacostracan
+orders, Phyllocarida, because of their resemblance to _Nebalia_,
+_Paranebalia_, _Nebaliopsis_, and _Nebaliella_, the four genera which
+are at present living. The general form of the recent and fossil
+representatives of the order is strikingly similar. The chief outward
+difference is that in many of the fossils the telson is accompanied by
+two furcal rami, while in the modern genera it is simple. It now
+becomes possible to make some comparison between the appendages of
+_Hymcnocaris_ of the Middle Cambrian and the Nebaliidæ of modern seas.
+
+In both there are five pairs of cephalic and eight of thoracic
+appendages, while those of the abdomen of Hymenocaris are not known.
+
+In both, the antennules are less developed than the antennæ. In the
+Nebaliidæ the antennules show evidence of having been originally
+double (they are obviously so in the embryo), while they are single in
+_Hymcnocaris_. In both, the antennæ are simple. The remaining cephalic
+organs are too little shown by the specimen from the Middle Cambrian
+to allow detailed comparison. The mandibles, maxillulæ, and maxillæ of
+_Nebalia_ are, however, of types which could be derived from the
+trilobite.
+
+In three of the genera of the Nebaliidæ, the eight pairs of thoracic
+limbs are all similar to one another, though those of the genera
+differ. All are biramous. The limbs of _Hymcnocaris_ can apparently be
+most closely correlated with those of _Nebalia antarctica_, in which
+the endopodite consists of short flattened segments, and the exopodite
+is a long setiferous plate. Epipodites are present in both _Nebalia_
+and _Hymcnocaris_.
+
+So far as the appendages of _Hymenocaris_ are known, they agree very
+well with those of the Nebaliidæ, and since they are of the trilobite
+type, it may safely be stated that the Trilobita and Malacostraca are
+closely related.
+
+_2. Syncarida._
+
+Walcott (1918, p. 170) has compared the limbs of _Neolenus_ with those
+of the syncarid genera _Anaspides_ and _Koonunga_. These are primitive
+Malacostraca without a carapace, but as they have a compressed test
+and _Anaspides_ has stalked eyes, their gross anatomy does not suggest
+the trilobite. The thoracic appendages are very trilobite-like, since
+the endopodite has six segments (in _Anaspides_) and a multisegmented
+setiferous exopodite. The coxopodites, except of the first thoracic
+segment, do not, however, show endobases, and those which are
+present are peculiar articulated ones. The cephalic appendages are
+specialized, and the antennules double as in most of the Malacostraca.
+External epipodites are very numerous on the anterior limbs.
+
+This group extends back as far as the Pennsylvanian and had then
+probably already become adapted to fresh-water life. It may be
+significant that the Palæozoic syncarids appear to have lacked
+epipodites. While differing very considerably from the Trilobita, the
+Syncarida could have been derived from them.
+
+_3. Isopoda._
+
+Since the earliest times there has been a constant temptation to
+compare the depressed shields of the trilobites with the similar ones
+of isopods. Indeed, when _Scrolls_ with its Lichadian body was first
+discovered about a hundred years ago, it was thought that living
+trilobites had been found at last. The trilobate body, cephalic
+shield, sessile eyes, abdominal shield, and pleural extensions make a
+wonderful parallel. This similarity is, however, somewhat superficial.
+The appendages are very definitely segregated in groups on the various
+regions of the body, and while the pleopods are biramous, the thoracic
+legs are without exopodites (except in very early stages of
+development of one genus). The Isopoda arose just at the time of the
+disappearance of the Trilobita, and there seems a possibility of a
+direct derivation of the one group from the other. It should be
+pointed out that while the differences of Isopoda from Trilobita are
+important, they are all of a kind which could have been produced by
+the development from a trilobite-like stock. For example:
+
+Isopoda have a definite number of segments. There is less variation in
+the number of segments among the later than the earlier trilobites.
+
+Isopoda have no facial suture. In at least three genera of trilobites
+the cheeks become fused to the cranidium and the sutures obliterated.
+
+Isopoda have one or two segments of the thorax annexed to the head.
+While this is not known to occur in trilobites, it is possible that it
+did.
+
+Most Isopoda have a fairly stiff ventral test. The ventral membrane of
+trilobites would probably have become stiffened by impregnation of
+lime if the habit of enrollment had been given up.
+
+In Isopoda the antennæ are practically uniramous sensory organs. The
+second cephalic appendages of trilobites are capable of such
+development through reduction of the exopodite.
+
+In the Isopoda the coxopodites are usually fused with the body,
+remaining as free, movably articulated segments only in a part of the
+thoracic legs of one suborder, the Asellota. Endobases are entirely
+absent. This is of course entirely unlike the condition in Trilobita,
+but a probable modification.
+
+In Isopoda there is a distinct grouping of the appendages, with
+specialization of function. The trilobites show a beginning of
+tagmata, and such development would be expected if evolution were
+progressive.
+
+In both groups, development from the embryo is direct. Rudiments of
+exopodites of thoracic legs have been seen in the young of one genus.
+
+The oldest known isopod is _Oxyuropoda ligioides_ Carpenter and
+Swain (Proc. Royal Irish Acad., vol. 27, sect. B, 1908, p. 63,
+fig. 1), found in the Upper Devonian of County Kilkenny, Ireland. The
+appendages are not known, but the test is in some ways like that of a
+trilobite. The thorax, abdomen, and pygidium are especially like those
+of certain trilobites, and there is no greater differentiation between
+thorax and abdomen than there is between the regions before and behind
+the fifteenth segment of a _Pædeumias_ or _Mesonacis_. The anal
+segment is directly comparable to the pygidium of a _Ceraurus_, the
+stiff unsegmented uropods being like the great lateral spines of that
+genus.
+
+The interpretation of the head offered by Carpenter and Swain is very
+difficult to understand, as their description and figure do not seem
+to agree. What they consider the first thoracic segment (fused with
+the head) seems to me to be the posterior part of the cephalon and it
+shows at the back a narrow transverse area which is at least analogous
+to the nuchal segment of the trilobite. If this interpretation can be
+sustained, _Oxyuropoda_ would be a very primitive isopod in which the
+first thoracic segment (second of Carpenter and Swain) is still free.
+According to the interpretation of the original authors, the species
+is more specialized than recent Isopoda, as they claim that two
+thoracic segments are fused in the head. The second interpretation was
+perhaps made on the basis of the number of segments (nineteen) in a
+recent isopod.
+
+
+=Marrella splendens= WALCOTT.
+
+ Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 192,
+ pls. 25, 26.
+
+Among the most wonderful of the specimens described by Doctor
+Walcott is the "lace crab." While the systematic position was not
+satisfactorily determined by the describer, it has been aptly compared
+to a trilobite. The great nuchal and genal spines and the large
+marginal sessile eyes, coupled with the almost total lack of thoracic
+and abdominal test, give it a bizarre appearance which may obscure its
+real relationships.
+
+The cephalon appears to bear five pairs of appendages, antennules, and
+antennæ, both tactile organs with numerous short segments, mandibles,
+and first and second maxillæ. The last three pairs are elongate, very
+spinose limbs, of peculiar appearance. They seem to have seven
+segments, but are not well preserved. These organs are attached near
+the posterior end of the labrum.
+
+There are twenty-four pairs of biramous thoracic appendages, which
+lack endobases. The endopodites are long and slender, with numerous
+spines; the exopodites have narrow, thin shafts, with long, forward
+pointed setæ. The anal segment consists of a single plate.
+
+Further information about this fossil will be eagerly awaited. None of
+the illustrations so far published shows biramous appendages on the
+cephalon. This, coupled with the presence of tactile antennæ, makes
+its reference to the Trilobita impossible, but the present
+interpretation indicates that it was closely allied to them.
+
+[Illustration: Fig. 32. _Marrella splendens_ Walcott. Restoration of
+the ventral surface, based upon the photographs and descriptions
+published by Walcott. Although all the limbs of the trunk appear to be
+biramous, only endopodites are placed on one side and exopodites on
+the other, for the sake of greater clearness in the illustration.
+Drawn by Doctor Elvira Wood, under the supervision of the writer.
+× about 6.]
+
+
+
+_Restoration of Marrella._
+
+(Text fig. 32.)
+
+The accompanying restoration of the ventral surface of _Marrella_ is a
+tentative one, based on Doctor Walcott's description and figures. The
+outline is taken from his plate 26, figure 1; the appendages of the
+head from plate 26, figures 1-3, 5, and plate 25, figures 2, 3; the
+endopodites, shown on the left side only, from figures 3 and 6, plate
+25. I have not studied actual specimens, and the original description
+is very incomplete. The restoration is therefore subject to revision
+as the species becomes better known.
+
+
+
+
+Arachnida.
+
+
+No attempt will be made to pass in review all of the subclasses of the
+arachnids. Some of the Merostomata are so obviously trilobite-like
+that it would seem that their relationship could easily be proved. The
+task has not yet been satisfactorily accomplished, however, and new
+information seems only to add to the difficulties.
+
+So far as I know, the Araneæ have not previously been compared
+directly with trilobites, although such treatment consists merely in
+calling attention to their crustacean affinities, as has often been
+done.
+
+Carpenter's excellent summary (1903, p. 347) of the relationship of
+the Arachnida to the trilobites may well be quoted at this point:
+
+ The discussion in a former section of this essay on the
+ relationship between the various orders of Arachnida led to the
+ conclusion that the primitive arachnids were aquatic animals,
+ breathing by means of appendicular gills. Naturally, therefore, we
+ compare the arachnids with the Crustacea rather than with the
+ Insecta. The immediate progenitors of the Arachnida appear to have
+ possessed a head with four pairs of limbs, a thorax with three
+ segments, and an abdomen with thirteen segments and' a telson, only
+ six of which can be clearly shown by comparative morphology to have
+ carried appendicular gills. But embryological evidence enables us
+ to postulate with confidence still more remote ancestors in which
+ the head carried well developed compound eyes and five pairs of
+ appendages, while it may be supposed that all the abdominal
+ segments, except the anal, bore limbs. In these very ancient
+ arthropods, all the limbs, except the feelers, had ambulatory and
+ branchial branches; and one important feature in the evolution of
+ the Arachnida must have been the division of labour between the
+ anterior and posterior limbs, the former becoming specialized for
+ locomotion, the latter for breathing. Another was the loss of
+ feelers and the degeneration of the compound eyes. Thus we are led
+ to trace the Arachnida (including the Merostomata and Xiphosura)
+ back to ancestors which can not be regarded as arachnids, but which
+ were identical with the primitive trilobites, and near the
+ ancestral stock of the whole crustacean class.
+
+
+TRILOBITES NOT ARACHNIDA.
+
+While no one having any real knowledge of the Trilobita has adopted
+Lankester's scheme of the inclusion of the group as the primitive
+grade in the Arachnida, reference to it may not be amiss. This theory
+is best set forth in the Encyclopædia Britannica, Eleventh Edition,
+under the article on Arachnida. It is there pointed out that the
+primitive arachnid, like the primitive crustacean, should be an animal
+without a fixed number of somites, and without definitely grouped
+tagmata. As Lankester words it, they should be anomomeristic and
+anomotagmatic. The trilobites are such animals, and he considers them
+Arachnida and not Crustacea for the following reasons:
+
+Firstly and chiefly, because they have only one pair (apart from the
+eyes) of pre-oral appendages. "This fact renders their association
+with the Crustacea impossible, if classification is to be the
+expression of genetic affinity inferred from structural coincidence."
+
+Secondly, the lateral eyes resemble no known eyes so closely as the
+lateral eyes of _Limulus_.
+
+Thirdly, the trilobation of the head and body, due to the expansion
+and flattening of the sides or pleura, is like that of _Limulus_, but
+"no crustacean exhibits this trilobite form."
+
+Fourthly, there is a tendency to form a pygidial or telsonic shield,
+"a fusion of the posterior somites of the body, which is precisely
+identical in character with the metasomatic carapace of _Limulus_." No
+crustacean shows metasomatic fusion of segments.
+
+Fifthly, a large post-anal spine is developed "in some trilobites" (he
+refers to a figure of _Dalmanites_).
+
+Sixthly, there are frequently lateral spines on the pleura as in
+_Limulus_. No crustacean has lateral pleural spines.
+
+These points may be taken up in order.
+
+1. If trilobites have one appendage-bearing segment in front of the
+mouth, they are Arachnida; if two, Crustacea. This is based on the
+idea that in the course of evolution of the Arthropoda, the mouth has
+shifted backward from a terminal position, and that as a pair of
+appendages is passed, they lose their function as mouth-parts and
+eventually become simple tactile organs. Thus arise the cheliceræ of
+most arachnids, and the two pairs of tactile antennæ of most
+Crustacea. This theory is excellent, and the rule holds well for
+modern forms, but as shown by the varying length of the hypostoma in
+different trilobites, the position of the mouth had not become fixed
+in that group. In some trilobites, like _Triarthrus_, the gnathobases
+of the second pair of appendages still function, but in all, so far as
+known, the mouth was back of the points of attachment of at least two
+pairs of appendages, and in some at least, back of the points of
+attachment of four pairs. As pointed out in the case of _Calymene_ and
+_Ceraurus_, the trilobites show a tendency toward the degeneration of
+the first and second pairs of biramous appendages, particularly of the
+gnathobases. They are in just that stage of the backward movement of
+the mouth when the function of the antennæ as mandibles has not yet
+been lost. If the presence of functional gnathobases back of the
+mouth, rather than the points of attachment in front of the mouth, is
+to be the guide, then Triarthrus might be classed as an arachnid and
+_Calymene_ and _Isotelus_ as crustaceans. In other words, the rule
+breaks down in this primitive group.
+
+2. Superficially, the eyes of some trilobites do look like those of
+_Limulus_, but how close the similarity really was it is impossible to
+say. The schizochroal eyes were certainly very different, and Watase
+and Exner both found the structure of the eye of the trilobite unlike
+that of _Limulus_.
+
+3. The importance of the trilobate form of the trilobite is very much
+overestimated. It and the pygidium are due solely to functional
+requirements. The axial lobe contained practically all the vital
+organs and the side lobes were mechanical in origin and secondarily
+protective. That the crustacean is not trilobate is frequently
+asserted by zoologists, yet every text-book contains a picture of a
+segment of a lobster with its axial and pleural lobes. It is a
+fundamental structure among the Crustacea, obscured because most of
+them are compressed rather than depressed.
+
+4. The pygidium of trilobites is compared with the metasomatic shield
+of _Limulus_. No homology, if homology is intended, could be more
+erroneous. The metasomatic shield of _Limulus_ is, as shown by
+ontogeny and phylogeny, formed by the fusion of segments formerly
+free, and includes the segments between the cephalic and anal shields,
+or what would be known as the thorax of a trilobite. No trilobite
+has a metasomatic shield. The pygidium of a trilobite, as shown by
+ontogeny, is built up by growth in front of the anal region, and since
+the segments were never free, it can not strictly be said to be
+composed of fused segments. Some Crustacea do form a pygidial shield,
+as in certain orders of the Isopoda.
+
+5. The post-anal spine of Dalmanites and some other trilobites is
+similar to that of _Limulus_, but this seems a point of no especial
+significance. That a similar spine has not been developed in the
+Crustacea is probably due to the fact that they do not have the broad
+depressed shape which makes it so difficult for a _Limulus_ to right
+itself when once turned on its back. Relatively few trilobites have
+it, and it is probably correlated with some special adaptation.
+
+6. There is nothing among the trilobites comparable to the movable
+lateral spines of the metasoma of _Limulus_.
+
+While, as classifications are made up, the Trilobita must be placed in
+the Crustacea rather than the Arachnida, there is no reason why both
+the modern Crustacea and the Arachnida should not be derived from the
+trilobites.
+
+
+
+MEROSTOMATA.
+
+It has been a custom of long standing to compare the trilobite with
+_Limulus_. Packard (1872) gave great vitality to the theory of
+the close affinity of the two when he described the so-called
+trilobite-stage in the development of _Limulus polyphemus_. His
+influence on Walcott's ideas (1881) is obvious. Lankester has gone
+still further, and associated the Trilobita with the Merostomata in
+the Arachnida.
+
+The absence of antennules at any stage in development allies _Limulus_
+so closely with the Arachnida and separates it so far from the
+Trilobita that in recent years there has been a tendency to give up
+the attempt to prove a relationship between the merostomes and
+trilobites, especially since Clarke and Ruedemann, in their extensive
+study of the Eurypterida, found nothing to indicate the crustacean
+nature of that group. A new point of view is, however, presented by
+the curious _Sidneyia inexpectans_ and _Emeraldella brocki_ described
+by Walcott from the Middle Cambrian.
+
+
+=Sidneyia inexpectans= Walcott.
+
+ Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1911, p. 21,
+ pl. 2, fig. 1 (not figs. 2, 3); pls. 3-5; pl. 6, fig. 3; pl. 7,
+ fig. 1.
+
+The body of this animal is elongate, somewhat eurypterid-like, but
+with a broad telson supplied with lateral swimmerets. The cephalon is
+short, with lateral compound eyes. The trunk consists of eleven
+segments, the anterior nine of which are conspicuously wider than the
+two behind them, and the telson consists of a single elongate plate.
+
+On the ventral side of the head there is a large hypostoma and five,
+pairs of appendages. The first pair are multisegmented antennules. The
+second pair have not been adequately described. The third are large,
+complex claws, and the fourth and fifth suggest broad, stocky
+endopodites. Broad gnathobases are attached to the coxopodites of the
+third to fifth pairs of appendages and form very strong jaws.
+
+The first nine segments of the thorax have one pair each of broad
+filiform branchial appendages, suggestive of the exopodites of
+trilobites, but no endopodites have been seen. The tenth and eleventh
+segments seem to lack appendages entirely.
+
+
+=Emeraldella brocki= Walcott.
+
+ Illustrated: _Sidneyia inexpectans_ Walcott _partim_, Smithson.
+ Misc. Coll., vol. 57, 1911, pl. 2, figs. 2, 3 (not fig. 1);--Ibid.,
+ 1912, p. 206, text fig. 10.
+
+ _Emeraldella brocki_ Walcott, Ibid., 1912, p. 203, pl. 30, fig. 2;
+ text fig. 8;--Ibid., vol. 67, 1918, p. 118 (correction).
+
+_Emeraldella_ has much the same shape as _Sidneyia_ and the same
+number of segments, but instead of a broad flat telson, it has a long
+_Limulus_-like spine. The cephalon is about as wide as long, and eyes
+have not yet been seen. The body consists of eleven segments and a
+telson (Walcott says twelve and a telson but shows only eleven in the
+figures). Nine of the segments, as in _Sidneyia_, are broad, the next
+two narrow.
+
+The ventral side of the cephalon has a long hypostoma, and five pairs
+of appendages. The first pair are very long multi segmented antennules
+and the next four pairs seem to be rather slender, spiniferous,
+jointed endopodites. Whether or not gnathobases were present is not
+shown by the figures, but owing to the long hypostoma the appendages
+are grouped about the mouth. All the segments of the body, unless it
+were the telson, seem to have borne appendages. On the anterior end,
+they were clearly biramous (1912, p. 206, text fig. 10), and that they
+were present along the body is shown by figure 2, plate 30, 1912.
+
+The present state of knowledge of both these peculiar animals leaves
+much to be desired. The indications are that the cephalic appendages
+are not biramous, and that only one pair of antennæ, the first, are
+developed as tactile organs. The thoracic appendages of _Emeraldella_
+are biramous, and also possibly those of _Sidneyia_. In the latter,
+the last two abdominal segments seem to have been without appendages,
+while in _Emeraldella_ at least one branch of each appendage, and
+possibly both, is retained.
+
+These animals, which may be looked upon as the last survivors of an
+order of pre-Cambrian arthropods, have the appearance of an
+eurypterid, but their dominant characteristics are crustacean. The
+features which suggest the Eurypterida are: elongate, obovate,
+non-trilobate, tapering body; telson-like posterior segment; marginal,
+compound, sessile eyes; claw-like third cephalic appendages; and, more
+particularly, the general resemblance of the test to that of an
+eurypterid like _Strabops_. In form, _Sidneyia_ agrees with the
+theoretical prototype of the Eurypterida reconstructed by Clarke and
+Ruedemann (Mem. 14, N. Y. State Mus., vol. 1, 1912, p. 124) in its
+short wide head with marginal eyes, and its undifferentiated body.
+There is, moreover, no differentiation of the postcephalic appendages.
+
+The crustacean characteristics are seen in the presence of five,
+instead of six, pairs of appendages on the head, the first of which
+are multisegmented antennules, and in the biramous appendages on the
+body of _Emeraldella_. It should be noted that these latter are
+typically trilobitic, each consisting of an endopodite with six
+segments and a setiferous exopodite.
+
+Clarke and Ruedemann (1912, p. 406) have discussed _Sidneyia_ briefly,
+and conclude:
+
+ It seems to us probable that the Limulava [_Sidneyia_ and
+ _Amiella_] as described are not eurypterids but constitute a
+ primitive order, though exhibiting some remarkable adaptive
+ features. This order possibly belongs to the Merostomata, but is
+ distinctly allied to the crustaceans in such important characters
+ as the structure of the legs and telson, and is therefore much
+ generalized.
+
+The specialization of _Sidneyia_ consists in the remarkable
+development of a highly complex claw on each of the third cephalic
+appendages, and in the compound tail-fin, built up of the last segment
+and one or more pairs of swimmerets. These two characteristics seem to
+preclude the possibility of deriving the eurypterids from _Sidneyia_
+itself, but it seems entirely within reason that they may have been
+derived from another slightly less specialized member of the same
+order.
+
+That _Sidneyia_ is descended from any known trilobite seems highly
+improbable, but that it was descended from the same ancestral stock as
+the trilobites is, I believe, indicated by the presence of five pairs
+of appendages on the cephalon and trilobitic legs on the abdomen.
+
+=Molaria= and =Habelia.=
+
+Other so-called Merostomata found by Walcott in the Middle Cambrian
+are the genera _Molaria_ and _Habelia_, both referred to the Cambrian
+family Aglaspidæ. These genera seem to conform with _Aglaspis_ of the
+Upper Cambrian in having a trilobite-like cephalon without facial
+sutures, a trilobite-like thorax of a small but variable (7-12) number
+of segments, and a _Limulus_-like telson. Neither of them has yet been
+fully described or figured, but (Walcott 1912 A, p. 202) _Habelia_
+appears to have five pairs of cephalic appendages, the first two pairs
+of which are multisegmented antennæ. The thoracic appendages are
+likewise none too well known, but they appear to have been biramous.
+The endopodites are better preserved than the exopodites, but in at
+least one specimen of _Molaria_ the exopodites are conspicuous.
+
+If these genera are properly described and figured, their appendages
+are typically crustacean, and fundamentally in agreement with those of
+_Marrella_. The relation to the Trilobita is evidently close, the
+principal differences being the absence of facial sutures and the
+presence of true antennæ. I am therefore transferring the Aglaspidæ
+from the Merostomata to a new subclass under the Crustacea.
+
+
+ARANEÆ.
+
+The spiders have the head and thorax fused, the abdomen unsegmented
+except in the most primitive suborder, and so appear even less
+trilobite-like than the insects. The appendages likewise are highly
+specialized. The cephalothorax bears six pairs of appendages, the
+first of which are the pre-oral cheliceræ, while behind the mouth are
+the pedipalpi and four pairs of ambulatory legs. The posterior pairs
+of walking legs belong to the thorax, but the anterior ones are to be
+homologized with the maxillæ of Crustacea, so that the spiders are
+like the trilobites in having functional walking legs on the head.
+
+The chief likenesses are, however, seen in the very young. On the germ
+band there appear a pair of buds in front of the rudiments of the
+cheliceræ which later unite to form the rostrum of the adult. At the
+time these buds appear, the cheliceræ are post-oral, but afterward
+move forward so that both rostrum and cheliceræ are in front of the
+mouth. The rostrum is therefore the product of the union of the
+antennules, and the cheliceræ are to be homologized with the antennæ.
+There seems to be some doubt about the homology of the pedipalps with
+the mandibles, as at least one investigator claims to have found
+rudiments of a segment between the one bearing the cheliceræ and that
+with the pedipalps.
+
+Jaworowski (Zool. Anzeiger, 1891, p. 173, fig. 4) has figured the
+pedipalp from the germ band of _Trochosa singoriensis_, and called
+attention to the fact that it consists of a coxopodite and two
+segmented branches which may be interpreted as exopodite and
+endopodite. He designated as exopodite the longer branch which
+persists in the adult, but since the ambulatory legs of Crustacea are
+endopodites, that would seem a more likely interpretation. As the
+figure is drawn, the so-called endopodite would appear to spring from
+the proximal segment of the "exopodite." If the two terms were
+interchanged, the homology with the limb of the trilobite or other
+crustacean would be quite perfect.
+
+In the young, the abdomen is segmented and the anterior segments
+develop limb-buds, the first pair of which become the lung books and
+the last two pairs the spinnerets of the adult. There seems to be some
+question about the number of segments. Montgomery (Jour. Morphology,
+vol. 20, 1909, p. 337). reviewing the literature, finds that from
+eight to twelve have been seen in front of the anal segment. The
+number seem to vary with the species studied. This of course suggests
+connection with the anomomeristic trilobites.
+
+The oldest true spiders are found in the Pennsylvanian, and several
+genera are now known. The head and thorax are fused completely, but
+the abdomen is distinctly segmented. Some of the Anthracomarti
+resemble the trilobites more closely than do the Araneæ, as they lack
+the constriction between the cephalothorax and abdomen. The spiders of
+the Pennsylvanian have this constriction less perfectly developed than
+do modern Araneæ, and occupy an intermediate position in this respect.
+In the Anthracomarti, the pedipalpi are simple, pediform, and all the
+appendages have very much the appearance of the coxopodites and
+endopodites of trilobites. Cheliceræ are not known, and pleural lobes
+are well developed in this group. Anthracomarti have not yet been
+found in strata older than the Pennsylvanian, but they seem to be to a
+certain extent intermediate between true spiders and the marine
+arachnid.
+
+
+
+
+Insecta.
+
+
+Handlirsch (in several papers, most of which are collected in "Die
+Fossilen Insekten," 1908) has attempted to show that all the
+Arthropoda can be derived from the Trilobita, and has advocated the
+view that the Insecta sprang directly from that group, without the
+intervention of other tracheate stock. At first sight, this
+transformation seems almost an impossibility, and the view does not
+seem to have gained any great headway among entomologists in the
+fourteen years since it was first promulgated. If an adult trilobite
+be compared with an adult modern insect, few likenesses will be seen,
+but when the trilobite is stripped of its specializations and compared
+with the germ-band of a primitive insect, the theory begins to seem
+more possible.
+
+Handlirsch really presented very little specific evidence in favor of
+his theory. In fact, one gets the impression that he has insisted on
+only two points. Firstly, that the most ancient known insects, the
+Palæodictyoptera, were amphibious, and their larvæ, which lived in
+water, were very like the adult. Secondly, that the wings of the
+Palæodictyoptera probably worked vertically only, and the two main
+wings were homologous with rudimentary wing-like outgrowths on each
+segment of the body. These outgrowths have the appearance of, and
+might have been derived from, the pleural lobes of trilobites.
+
+He figured (1908, p. 1305, fig. 7) a reconstructed larva of a
+palæodictyopterid as having biramous limbs on each segment, but so far
+as I can find, this figure is purely schematic, for there seems to be
+no illustration or description of any such larva in the body of his
+work.
+
+That the insects arose directly from aquatic animals is of course
+possible, and Handlirsch's first argument has considerable force. It
+may, however, be purely a chance that the oldest insects now known to
+us happen to be an amphibious tribe. The Palæodictyoptera are not yet
+known to antedate the Pennsylvanian, but there can be no doubt that,
+insects existed long before that time, and the fact that their remains
+have not been found is good evidence that the pre-Pennsylvanian
+insects were not aquatic. Comstock, who has recently investigated the
+matter, does not believe that the Palæodictyoptera were amphibious
+(The Wings of Insects, Ithaca, N. Y., 1918, p. 91).
+
+The second argument, that wings arose from the pleural lobes of
+trilobites, is exceedingly weak. Where most fully set forth (1907, p.
+157), he suggests that trilobites may occasionally have left the
+water, climbed a steep bank or a plant, and then glided back into
+their native element, taking advantage of the broad flat shape to make
+a comfortable and gentle descent! This sport apparently became so
+engaging that the animal tried experiments with flexible wing tips,
+eventually got the whole of the pleural lobes in a flexible condition,
+and selected those of the second and third thoracic segments for
+preservation, while discarding the remainder. The pleural lobes of
+trilobites are not only too firmly joined to the axial portion of the
+test to be easily transformed into movable organs, but they are
+structurally too unlike the veined wings of insects to make the
+suggestion of this derivation even worthy of consideration.
+
+Tothill (1916) has recently reinvestigated the possible connection
+between insects, chilopods, and trilobites, and, from the early
+appearance of the spiracles in the young, came to the conclusion that
+the insects were derived from terrestrial animals. He suggested that
+they may have come through the chilopods from the trilobites. The
+hypothetical ancestor of the insects, as restored by Tothill from the
+evidence of embryology and comparative anatomy, is an animal more
+easily derived from the Chilopoda than from the Trilobita. Five pairs
+of appendages are present on the head, and the trunk is made up of
+fourteen similar segments, each with a pair of walking limbs and a
+pair of spiracles.
+
+Only the maxillæ and maxillulæ are represented as biramous. If the
+ancestor of the Insecta was, as seems possible, tracheate, this fact
+alone would rule out the trilobites. Among tracheates, the Chilopoda
+are certainly more closely allied to the Insecta than are any other
+wingless forms. If the ancestors of the insects were not actually
+chilopods, they may have been chilopod-like, and there can be little
+doubt that both groups trace to the same stock.
+
+As to the ancestry of the Chilopoda, it is probable that they had the
+same origin as the other Arthropoda. Tothill has pointed out that in
+the embryo of some chilopods there are rudiments of two pairs of
+antennæ and that the two pairs of maxillæ and the maxillipeds are
+biramous. This would point rather to the Haplopoda than directly to
+the trilobites as possible ancestors, and may explain why the former
+vanish so suddenly from the geological record after their brief
+appearance in the Middle Cambrian. They may have gone on to the land.
+
+There seem to be no insuperable obstacles to prevent the derivation,
+indirectly, of the insects from some trilobite with numerous free
+segments, and small pygidium. The antennules and pleural lobes must be
+lost, the antennas and trunk limbs modified by loss of exopodites.
+Wings and tracheæ must be acquired.
+
+Handlirsch places the date of origin of the Insecta rather late, just
+at the end of the Devonian and during the "Carboniferous." By that
+time most families of trilobites had died out, so that the
+possibilities of origin of new stocks were much diminished. If the
+haplopod-chilopod-insect line is a better approximation to the truth,
+then the divergence began in the Cambrian.
+
+
+
+
+Chilopoda.
+
+
+The adult chilopod lacks the antennules, and all of the other
+appendages, with the exception of the maxillulæ, are uniramous. The
+walking legs are similar to the endopodites of trilobites, and usually
+have six or seven segments. The appendages are therefore such as could
+be derived by modification of those of trilobites by the almost
+complete loss of the exopodites and shortening of the endopodites of
+the head. The position of the post-oral appendages, the posterior ones
+outside those closest the mouth, is perhaps foreshadowed in the
+arrangement of those of Triarthrus.
+
+The Chilopoda differ from the Hexapoda in developing the antennæ
+instead of the antennules as tactile organs, but this can not be used
+with any great effect as an argument that the latter did not arise
+from the ancestors of the former, since it is entirely possible that
+in early Palæozoic times the pre-Chilopoda possessed two pairs of
+antennæ. The first pair are still recognizable in the embryo of
+certain species.
+
+The oldest chilopods are species described by Scudder (Mem. Boston
+Soc. Nat. Hist., vol. 4, 1890, p. 417, pl. 38) from the Pennsylvania!!
+at Mazon Creek, Grundy County, Illinois. Only one of these, _Latzelia
+primordialis_ Scudder (pl. 38 fig. 3), is at all well preserved. This
+little animal, less than an inch long, had a depressed body, with a
+median carina, exceedingly long slender legs, and about nineteen
+segments. The head is very nearly obliterated.
+
+
+
+
+Diplopoda.
+
+
+The diplopods, especially the polydesmids with their lateral
+outgrowths, often have a general appearance somewhat like that of a
+trilobite, but on closer examination few likenesses are seen. The most
+striking single feature of the group, the possession by each segment
+of two pairs of appendages, is not in any way foreshadowed in the
+trilobites, none of which shows any tendency toward a fusion of pairs
+of adjacent segments. The antennules are short, antennæ absent,
+mandibles and maxillulæ much modified, the latter possibly biramous,
+and the maxillæ absent. The trunk appendages are very similar to those
+of chilopods, and could readily be derived from the endopodites of
+trilobites.
+
+The oldest diplopods are found in the Silurian (Ludlow) and Devonian
+(Lower Old Red) of Scotland, and three species belonging to two genera
+are known. The oldest is _Archidesmus loganensis_ Peach (1889, p. 123,
+pl. 4, fig. 4), and the Devonian species are _Archidesmus macnicoli_
+Peach and _Kampecaris forfarensis_ Page (Peach 1882, p. 182, pl. 2,
+fig. 2, 2a, and p. 179, pl. 2, figs. 1-1g). All of these species show
+lateral expansions like the recent Polydesmidæ, and these of course
+suggest the pleural lobes of trilobites. All three of the species are
+simpler than any modern diplopod, for there is only a single pair of
+appendages on each segment. No _foramina repugnatoria_ were observed,
+and the eyes of _Kampecaris forfarensis_ as described are singularly
+like those of a phacopid.
+
+Peach says: "The eye itself is made up of numerous facets which are
+arranged in oblique rows, the posterior end of each row being inclined
+downwards and outwards, the facets being so numerous and so close
+together that the eye simulates a compound one." There is also a
+protecting ridge which somewhat resembles a palpebral lobe (1882, pl.
+7, fig. la). Peach comments on the strength of the test, and from his
+description it appears that it must have been preserved in the same
+manner as the test of trilobites. It was punctate, and granules and
+spines were also present. The presence of the lateral outgrowths in
+these ancient specimens would seem to indicate that they are primitive
+features, and may have been inherited. While possibly not homologous
+with the pleural extensions of trilobites, they may be vestiges of
+these structures.
+
+The limbs are made up of seven segments which are circular in section
+and expand at the distal end. The distal one bears one or two minute
+spines. They are most readily compared with the endopodites of
+_Isotelus_. The resemblance is, in fact, rather close. The sternal
+plates are wider and the limbs of opposite sides further apart than in
+modern diplopods. Except for one pair of antennæ, no cephalic
+appendages are preserved.
+
+While these specimens do not serve to connect the Diplopoda with the
+Trilobita, they do show that most of the specializations of the former
+originated since Lower Devonian times, and lead one to suspect that
+the derivation from marine ancestors took place very early, perhaps in
+the Cambrian. If no very close connection with the trilobites is
+indicated, there is also nothing to show that the diplopods could not
+have been derived from that group.
+
+
+
+
+Primitive Characteristics of Trilobites.
+
+
+TRILOBITES THE MOST PRIMITIVE ARTHROPODS.
+
+The Arthropoda, to make the simplest possible definition, are
+invertebrate animals with segmented body and appendages. The most
+primitive arthropod would appear to be one composed of exactly similar
+segments bearing exactly similar appendages, the segments of the
+appendages themselves all similar to one another. It is highly
+improbable that this most primitive arthropod imaginable will ever be
+found, but after a survey of the whole phylum, it appears that the
+simpler trilobites approximate it most closely.
+
+That the trilobites are primitive is evidenced by the facts that they
+have been placed at the bottom of the Crustacea by all authors and
+claimed as the ancestors of that group by some; that Lankester derived
+the Arachnida from them; and that Handlirsch has considered them the
+progenitors of the whole arthropodan phylum.
+
+Specializations among the Arthropoda, even among the free-living
+forms, are so numerous that it would be difficult to make a complete
+list of them. In discussing the principal groups, I have tried to show
+that the essential structures can be explained as inherited from the
+Trilobita, changed in form by explainable modifications, and that new
+structures, not' present in the Trilobita, are of such a nature that
+they might be acquired independently in even unrelated groups.
+
+The chief objections to the derivation of the remainder of the
+Crustacea from the trilobites have been: first, that the trilobites
+had broad pleural extensions; second, that they had a large pygidium;
+and lastly, that they had only one pair of tactile antennæ.
+
+It has now been pointed out that many modern Crustacea have pleural
+extensions, but that they usually bend down at the sides of the body,
+and also that in the trilobites and more especially in _Marrella_,
+there was a tendency toward the degeneration of the pleural lobes. A
+glance at the Mesonacidæ or Paradoxidæ should be convincing proof that
+in some trilobites the pygidium is reduced to a very small plate.
+
+In regard to the second antennæ standard text-books contain statements
+which are actually surprising. A compilation shows that the antennæ
+are entirely uniramous in but a very few suborders, chiefly among the
+Malacostraca; that they are biramous with both exopodite and
+endopodite well developed in most Copepoda, Ostracoda, and
+Branchiopoda; and that the exopodite, although reduced in size, still
+has a function in some suborders of the Malacostraca. The Crustacea
+could not possibly be derived from an ancestor with two pairs of
+uniramous antennæ.
+
+Although I have defended the trilobites, perhaps with some warmth,
+from the imputation that they were Arachnida, my argument does not
+apply in the opposite direction, and I believe Lankester was right in
+deriving the Arachnida from them. If the number of appendages in front
+of the mouth is fundamental, then the trilobites were generalized,
+primitive, and capable of giving rise to both' Crustacea and
+Arachnida. As shown on a previous page (p. 119), the "connecting
+links" so far found tend to disprove rather than to prove the thesis,
+but the present finds should be looked upon as only the harbingers of
+the greater ones which are sure to come.
+
+
+LIMBS OF TRILOBITES PRIMITIVE.
+
+The general presence, in an adult or larva, of some sort of biramous
+limbs throughout the whole class Crustacea has led most zoologists to
+expect such a limb in the most primitive crustaceans, and apparently
+the appendage of the trilobite satisfies the expectation. It is well,
+perhaps, as a test, to consider whether by modification this limb
+could produce the various types of limbs seen in other members of the
+class. In the first place, it is necessary to have clearly in mind the
+peculiarities of the appendage to be discussed.
+
+It should first of all be remembered that the limb is articulated with
+the dorsal skeleton in a manner which is very peculiar for a
+crustacean. The coxopodite swings on a sort of ball-and-socket joint,
+and at the outer end both the exopodite and the basipodite articulate
+with it. Since the exopodite articulates with the basipodite as well
+as with the coxopodite, the two branches are closely connected with
+one another and there is little individual freedom of movement. This
+is, of course, a necessary consequence of their articulation with a
+segment which is itself too freely movable to provide a solid base for
+attachment of muscles. The relation of the appendifer, coxopodite, and
+two rami is here shown diagrammatically (fig. 33), the exopodite
+branching off from the proximal end of the basipodite at the junction
+with the coxopodite.
+
+In all trilobites the endopodite consists of six segments, and the
+coxopodite of a single segment the inner end of which is prolonged as
+an endobase. There does not seem to be any variation from this plan in
+the subclass, although individual segments are variously modified. The
+exopodites are more variable, but all consist of a flattened shaft
+with setæ on one margin. No other organs such as accessory gills,
+swimming plates, or brood pouches have yet been found attached to the
+appendages, the evidence for the existence of the various epipodites
+and exites described by Walcott being unsatisfactory (see p. 23).
+
+[Illustration: Fig. 33.--Diagrammatic representation of an appendage
+of the anterior end of the thorax of _Triarthrus becki_ Green, to show
+relation of exopodite and endopodite to each other and to the
+coxopodite. Much enlarged.]
+
+In the Ostracoda the appendages are highly variable, but it is easily
+seen that they are modifications of a limb which is fundamentally
+biramous. In most species, both exopodite and endopodite suffer
+reduction. The exopodite springs from the basipodite and that segment
+is closely joined to the coxopodite, producing a protopodite. In some
+cases the original segments of the endopodites fuse to form a stiff
+rod. While highly diversified, these appendages are very
+trilobite-like, and some Ostracoda even have biramous antennæ.
+
+The non-parasitic Copepoda have limbs exceedingly like those of
+trilobites. Many of them are biramous, the endopodites sometimes
+retaining the primitive six segments. Coxopodite and basipodite are
+generally united, and endopodite and exopodite variously modified.
+Like some of the Ostracoda, the more primitive Copepoda have biramous
+antennæ.
+
+As would be expected, the appendages of the Cirripedia are much
+modified, although those of the nauplius are typical. The thoracic
+appendages of many are biramous, but both branches are multisegmented.
+
+In the modern Malacostraca the ground plan of the appendages is
+biramous, but in most orders they are much modified. In many, however,
+the appendages of some part of the body are biramous, and in many the
+endopodites show the typical six segments. From the coxopodites arise
+epipodites, some of which assist in swimming, and some in respiration.
+Because of the many instances in which such extra growths arise, and
+because of the form of the appendages of the Branchiopoda, it has
+been suggested that the primitive crustacean leg must have been more
+complex than that of the trilobite. In looking over the Malacostraca,
+however, one is struck by the fact that epipodites generally arise
+where the exopodites have become aborted or are poorly developed, and
+seem largely to replace them. The coxopodite and basipodite are
+usually fused to form a protopodite, and a third segment is sometimes
+present in the proximal part of the appendage.
+
+In the Branchiopoda are found the most complex crustacean limbs, and
+the ones most difficult to homologize with those of trilobites. In
+recent years, Lankester's homologies of the parts of the limbs of
+_Apus_ with those of the Malacostraca have been quite generally
+accepted, and the appendages of the former considered primitive.
+Now that it is known that the Branchiopoda of the Middle Cambrian
+(_Burgessia_ _et at._) had simple trilobite-like appendages, it
+becomes necessary to exactly reverse the opinion in this matter. The
+same homologies stand, but the thoracic limbs of _Apus_ must be looked
+upon as highly specialized instead of primitive.
+
+[Illustration: Fig. 34.--One of the appendages of the anterior part of
+the trunk of _Apus_, showing the endites (beneath) and exites (above).
+The proximal endite forms a gnathobase which is not homologous with
+the gnathobase (or endobase) of the trilobite. Copied from Lankester.
+Much enlarged.]
+
+Lankester (Jour. Micros. Sci., vol. 21, 1881) pointed out that the
+axial part of the thoracic limb of _Apus_ (fig. 34) is homologous with
+the protopodite in the higher Crustacea, that the two terminal endites
+corresponded to the exopodite and endopodite, and that the other
+endites and exites were outgrowths from the protopodite analogous
+to the epipodites of Malacostraca. There seems to be no objection
+to retaining this interpretation, but with the meaning that both
+endopodite and exopodite are much reduced, and their functions
+transferred to numerous outgrowths of the protopodite. One of the
+endites grows inward to form an endobase, the whole limb showing an
+attempt to return to the ancestral condition of the trilobite. The
+limbs of some other branchiopods are not so easy to understand, but
+students of the Crustacea seem to have worked out a fairly
+satisfactory comparison between them and _Apus_.
+
+The discovery that the ancestral Branchiopoda had simple biramous
+appendages instead of the rather complex phyllopodan type is another
+case in which the theory of "recapitulation" has proved to hold. It
+had already been observed that in ontogeny the biramous limb preceded
+the phyllopodan, but so strong has been the belief in the primitive
+character of the Apodidæ that the obvious suggestion has been ignored.
+Even in such highly specialized Malacostraca as the hermit crabs the
+development of certain of the limbs illustrates the change from the
+schizopodal to the phyllopodan type, and Thompson (Proc. Boston Soc.
+Nat. Hist., vol. 31, 1903, pl. 5, fig. 12) has published an especially
+good series of drawings showing the first maxilliped. In the first to
+fourth zoeæ the limb is biramous but in the glaucothoe a pair of broad
+processes grow out from the protopodite, while the exopodite and
+particularly the endopodite become greatly reduced. In the adult the
+endopodite is a mere vestige, while the flat outgrowths from the
+protopodite have become very large and bear setæ.
+
+_Summary._
+
+The limbs of most Crustacea are readily explained as modifications of
+a simple biramous type. These modifications usually take the form of
+reduction by the loss or fusion of segments and quite generally either
+the entire endopodite or exopodite is lacking. Modification by
+addition frequently occurs in the growth of epipodites, "endites," and
+"exites" from the coxopodite, basipodite, or both. A protopodite is
+generally formed by the fusion of coxopodite and basipodite,
+accompanied by a transference of the proximal end of the exopodite to
+the distal end of the basipodite. A new segment, not known in the
+trilobites (precoxal), is sometimes added at the inner end.
+
+Among modern Crustacea, the anterior cephalic appendages and thoracic
+appendages of the Copepoda and the thoracic appendages of certain
+Malacostraca, Syncarida especially, are most nearly like those of the
+trilobite. The exact homology, segment for segment, between the
+walking legs of the trilobite and those of many of the Malacostraca,
+even the Decapoda, is a striking instance of retention of primitive
+characteristics in a specialized group, comparable to the retention of
+primitive appendages in man.
+
+
+NUMBER OF SEGMENTS IN THE TRUNK.
+
+Various attempts have been made to show that despite the great
+variability, trilobites do show a tendency toward a definite number of
+segments in the body.
+
+Emmrich (1839), noting that those trilobites which had a long thorax
+usually had a short pygidium, and that the reverse also held true,
+formulated the law that the number of segments in the trunk was
+constant (20 + 1) Very numerous exceptions to this law were, however,
+soon discovered, and while the condition of those with less than
+twenty-one segments was easily explained, the increasing number of
+those with more than twenty-one soon brought the idea into total
+disrepute.
+
+Quenstedt (1837) had considered the number of segments of at least
+specific importance, and both he and Burmeister (1843) considered that
+the number of segments in the thorax must be the same for all members
+of a genus. As first shown by Barrande (1852. p. 191 et seq.), there
+are very many genera in which there is considerable variation in the
+number of thoracic segments, and a few examples can be cited in which
+there is variation within a species, or at least in very closely
+related species.
+
+Carpenter (1903, p. 333) has tabulated the number of trunk segments of
+such trilobites as were listed by Zittel in 1887 and finds a steady
+increase throughout the Palæozoic. His table, which follows, is,
+however, based upon very few genera.
+
+ Period No. of Genera Average No. of
+ body-segments
+ ===============================================
+ Cambrian 12 17.66
+ Ordovician 23 18.58
+ Silurian 16 19.34
+ Devonian 10 20.70
+ Carboniferous 2 20.75
+
+Due chiefly to the efforts of Walcott, an increasingly large number of
+Cambrian genera are now represented by entire specimens, and since
+these most ancient genera are of greatest importance, a few comments
+on them may be offered.
+
+The total number of segments can be fairly accurately determined in at
+least nineteen genera of trilobites from the Lower Cambrian. These
+include eight genera of the Mesonacidæ (_Olenellus_ was excluded)
+and _Eodiscus_, _Goniodiscus_, _Protypus_, _Bathynotus_, _Atops_,
+_Olenopsis_, _Crepicephalus_, _Vanuxemella_, _Corynexochus_,
+_Bathyuriscus_, and _Poliella_. The extremes of range in total
+segments of the trunk is seen in _Eodiscus_ (9) and _Pædeumias_ (45+),
+and these same genera show the extremes in the number of thoracic
+segments, there being 3 in the one and 44+ in the other. _Pædeumias_
+probably shows the greatest variation of any one genus of trilobites,
+various species showing from 19 to 44+ thoracic segments. The average
+for the nineteen genera is 13.9 segments in the thorax, 3.7 segments
+in the pygidium, or a total average of 17.6 segments in the trunk.
+_Crepicephalus_ with 12-14 segments in the thorax and 4-6 in the
+pygidium, and _Protypus_, with 13 in the thorax and 4-6 in the
+pygidium, are the only genera which approach the average. All of the
+Mesonacidæ, except one, _Olenelloides_, have far more thoracic and
+fewer pygidial segments than the average, while the reverse is true of
+the Eodiscidæ, _Vanuxemella_, _Corynexochus_, _Bathyuriscus_, and
+Poliella.
+
+The eight genera of the Mesonacidæ, _Nevadia_, _Mesonacis_,
+_Elliptocephala_, _Callavia_, _Holmia_, _Wanneria_, _Pædeumias_, and
+_Olenelloides_, have an average of 20.25 segments in the thorax and
+1.5 in the pygidium, a total of 21.75. If, however, the curious little
+_Olenelloides_ be omitted, the average for the thorax rises to 22.14
+and the total to 23.84. _Olenelloides_ is, in fact, very probably the
+young of an _Olenellus_. Specimens are only 4.5 to 11 mm. long, and
+occur in the same strata with _Olenellus_ (see Beecher 1897 A, p.
+191).
+
+Thirty-three genera from the Middle Cambrian afford data as to the
+number of segments, the Agnostidæ being excluded. The extreme of
+variation there is smaller than in the Lower Cambrian. The number of
+thoracic segments varies from 2 in Pagetia to 25 in _Acrocephalites_,
+and these same genera show the greatest range in total number of trunk
+segments, 8 and 29 respectively.
+
+The average of thoracic segments for the entire thirty-three genera is
+10.5, of pygidial segments 5.9, a total average of 16.4. It will be
+noted that the thorax shows on the average less and the pygidium more
+segments than in the Lower Cambrian. If the Agnostidæ could be
+included, this result would doubtless be still more striking. Of the
+genera considered, _Asaphiscus_ with 7-11 thoracic and 5-8 pygidial
+segments, _Blainia_ with 9 thoracic and 6-11 pygidial, _Zacanthoides_
+with 9 thoracic and 5 pygidial, and _Anomocare_ with 11 thoracic
+and 7-8 pygidial segments came nearest to the average. Only a few
+departed widely from it. The genera tabulated were _Acrocephalites_,
+_Alokistocare_, _Crepicephalus_, _Karlia_, _Hamburgia_,
+_Corynexochus_, _Bathyuriscus_, Poliella, _Agraulos_,
+_Dolichometopus_, _Ogygopsis_, _Orria_, _Asaphiscus_, _Neolenus_,
+_Burlingia_, _Blainia_, _Blountia_, _Marjumia_, _Pagetia_, _Eodiscus_,
+_Goniodiscus_, _Albertella_, _Oryctocara_, _Zacanthoides_,
+_Anomocare_, _Anomocarella_, _Coosia_, _Conocoryphe_, _Ctenocephalus_,
+_Paradoxides_, _Ptychoparia_, _Sao_, and _Ellipsocephalus_.
+
+Enough genera of Upper Cambrian trilobites are not known from entire
+specimens to furnish satisfactory data. Excluding from the list the
+Proparia recently described by Walcott, the average total trunk
+segments in ten genera is 18, but as most of the genera are Olenidæ or
+olenid-like, not much weight can be attached to these figures.
+
+For the Cambrian as a whole, the average for sixty-two genera is
+between 17 and 18 trunk segments, which is surprisingly like the
+result obtained by Carpenter from only twelve genera, and tends to
+indicate that it must be somewhere near the real average. If the 5 or
+6 segments of the head be added, it appears that the "average" number
+of segments is very close to the malacostracan number 21. Genera with
+16 to 18 trunk segments are Callavia, _Protypus_, _Bathynotus_,
+_Crepicephalus_, _Bathyuriscus_, _Ogygopsis_, _Burlingia_, _Orria_,
+_Asaphiscus_, _Blainia_, _Zacanthoides_, _Neolenus_, _Anomocare_,
+_Conocoryphe_, _Saukia_, _Olenus_, and _Eurycare_.
+
+The order Proparia originated in the Cambrian, and Walcott has
+described four genera, one from the Middle, and three from the Upper.
+The number of segments in these genera is of interest. _Burlingia_,
+the oldest, has 14 segments in the thorax and 1 in the pygidium. Of
+the three genera in the Upper Cambrian, _Norwoodia_ has 8-9 segments
+in the thorax and 3-4 in the pygidium; _Millardia_ 23 in thorax and
+3-4 in pygidium; and _Menomonia_ 42 in thorax and 3-4 in pygidium. It
+is of considerable interest and importance to note that the very
+elongate ones are not from the Middle but from the Upper Cambrian.
+
+Forty genera of Ordovician trilobites known from entire specimens were
+tabulated, and it was found that the range in the number of segments
+in the thorax and pygidium was surprisingly large. _Agnostus_, which
+was not included in the table, has the fewest, and _Eoharpes_, with
+29, the most. While the range in number of segments in the thorax is 2
+to 29, the range of the number in the pygidium, 2 to 26, is almost as
+great. A species of _Dionide_ has 26 in the pygidium, while
+_Remopleurides_ and _Glaphurus_ have evidence of only 2. The average
+number of segments in the thorax for the forty genera was 10.15, in
+the pygidium 8.81, and the average number for the trunk 19.
+
+Genera with just 19 segments in the trunk appear to be rare in the
+Ordovician, a species of _Ampyx_ being the only one I have happened to
+notice. _Calymene_, _Tretaspis_, _Triarthrus_, _Asaphus_, _Ogygites_,
+and _Goldius_ come with the range of 18 to 20. _Goldius_, with 10
+segments in the thorax and (apparently) 8 in the pygidium, comes
+nearest to the averages for these two parts of the trunk. _Goldius_,
+_Amphilichas_, _Bumastus_, _Acidaspis_, _Actinopeltis_, and
+_Sphærexochus_ are among the genera having 10 segments in the thorax,
+and there are many genera which have only one or two segments more or
+less than 10.
+
+In most Ordovician genera, thirty-five out of the forty tabulated, the
+number of segments in the thorax is fixed, and the variation is in any
+case small. In four of the five genera where it was not fixed, there
+was a variation of only one segment, and the greatest variation was in
+_Pliomerops_, where the number is from 15 to 19. This of course
+indicates that the number of segments in the thorax tends to become
+fixed in Ordovician time. The variation in the number of segments in
+the pygidium is, however, considerable. It is difficult in many cases
+to tell how many segments are actually present in this shield, as it
+is more or less smooth in a considerable number of genera. Extreme
+cases of variation within a genus are found in _Encrinurus_, species
+of which have from 7 to 22 segments in the pygidium, _Cybeloides_ with
+10 to 20, and _Dionide_ with 10 to 26. As the number in the thorax
+became settled, the number in the pygidium became more unstable, so
+that not even in the Ordovician can the total number of segments in
+the trunk be said to show any tendency to become fixed.
+
+The genera used in this tabulation were: _Eoharpes_, _Cryptolithus_,
+_Tretaspis_, _Trinucleus_, _Dionide_, _Raphiophorus_, _Ampyx_,
+_Endymionia_, _Anisonotus_, _Triarthrus_, _Remopleurides_,
+_Bathyurus_, _Bathyurellus_, _Ogygiocaris_, _Asaphus_, _Ogygites_,
+_Isotelus_, _Goldius_, _Cyclopyge_, _Amphilichas_, _Odontopleura_,
+_Acidaspis_, _Glaphurus_, _Encrinurus_, _Cybele_, _Cybeloides_,
+_Ectenonotus_, _Calymene_, _Ceraurus_, _Pliomera_, _Pliomerops_,
+_Pterygometopus_, _Chasmops_, _Eccoptochile_, _Actinopeltis_,
+_Sphærexochus_, _Placoparia_, _Pilekia_, _Selenopeltis_, and
+_Calocalymene_.
+
+Only sixteen genera of Devonian trilobites were available for
+tabulation, and it is not always possible to ascertain the exact
+number of segments in the pygidium, although genera with smooth caudal
+shields had nearly all disappeared. The number of segments in the
+thorax had become pretty well fixed by the beginning of the Devonian,
+_Cyphaspis_ with a range of from 10 to 17 furnishing the only notable
+exception. The range for the sixteen genera is from 8 to 17, the
+average 11, the number exhibited by the Phacopidæ which form so large
+a part of the trilobites of the Devonian. The greater part of the
+species have large pygidia, and while the range is from 3 to 23, the
+average is 11.2. _Probolium_, with 11 in the thorax and 11-13 in the
+pygidium, and _Phacops_, with 11 in the thorax and 9-12 in the
+pygidium, approach very closely to the "average" trilobite, and
+various species of other genera of the Phacopidæ have the same number
+of segments as the norm. In every genus, however, the number of
+segments in the pygidium is variable, the greatest variation being in
+_Dalmanites_, with a range of from 9 to 23. The number of segments in
+the pygidium was therefore not fixed and was on the average higher
+than in earlier periods.
+
+The genera used in the tabulation were: _Calymene_, _Dipleura_,
+_Goldius_, _Proëtus_, _Cyphaspis_, _Acidaspis_, _Phacops_,
+_Hausmania_, _Coronura_, _Odontochile_, _Pleuracanthus_, _Calmonia_,
+_Pennaia_, _Dalmanites_, _Probolium_, and _Cordania_.
+
+The trilobites of the late Palæozoic (Mississippian to Permian)
+belong, with two possible exceptions, to the Pröetidæ, and only three
+genera, _Proëtus_, _Phillipsia_, and _Griffithides_, appear to be
+known from all the parts. I am, however, assuming that both
+_Brachymetopus_ and _Anisopyge_ have 9 segments in the thorax, and so
+have tabulated five genera. The range in the number of segments in the
+pygidium is large, from 10 in some species of _Proëtus_ to 30 in
+_Anisopyge_, and the average, 17.3, is high, as is the average for
+total number in the trunk, 26.3. _Anisopyge_, a late Permian trilobite
+described by Girty from Texas, is perhaps the last survivor of the
+group. It seems to have had 39 segments in the trunk, making it, next
+to the Cambrian _Pædeumias_ and _Menomonia_, the most numerously
+segmented of all the trilobites.
+
+The above data may be summarized in the following table:
+
+ Period No. of Av. No. of Av. No. of Av. No.
+ genera segments in segments in of trunk
+ thorax pygidium segments
+ ==========================================================
+ Lower Cambrian 19 13.9 3.7 17.6
+ Middle Cambrian 33 10.5 5.9 16.4
+ Entire Cambrian 62 ... ... 17-19
+ Ordovician 40 10.15 8.81 18.96
+ Devonian 16 11 11.2 22.2
+ Late Palæozoic 5 9 17.3 26.3
+
+
+This table confirms that made up by Carpenter, and shows even more
+strikingly the progressive increase in the average number of segments
+in the trunk throughout the Palæozoic.
+
+While the two trilobites with the greatest number of segments are
+Cambrian, yet on the average, the last of the trilobites had the more
+numerously segmented bodies. The multisegmented trilobites are:
+
+ Period Genus Av. No. of Av. No. of Av. No.
+ segments in segments in of trunk
+ thorax pygidium segments
+ ================================================================
+ Lower Cambrian _Pædeumias_ 44+ 1 45+
+ Upper Cambrian _Menomonia_ 42 4 46
+ _Ectenonotus_ 12 22 34
+ Ordovician _Encrinurus_ 11 22 33
+ _Dionide_ 6 26 32
+ Silurian _Harpes_ 29 3 32
+ Devonian _Coronura_ 11 23 34
+ _Dalmanites_ 11 23 34
+ Permian _Anisopyge_ 7+(9?) 30 39?
+
+
+_Anisopyge_, the last of the trilobites, stands third on the list of
+those having great numbers of segments, and in each period there are a
+few which have considerably more than the average number. It may be of
+some significance that of these nine genera only _Pædeumias_ and
+_Anisopyge_ belong to the Opisthoparia, the great central group, and
+that five are members of the Proparia, the latest and most specialized
+order.
+
+
+FORM OF THE SIMPLEST PROTASPIS.
+
+It would naturally be expected that the young of the Cambrian
+trilobites should be more primitive than the young of species from
+later formations, and Beecher (1895 C) has shown that this is the
+case. He had reference, however, chiefly to the eyes, free cheeks, and
+spines, and by comparison of ontogeny and phylogeny, demonstrated the
+greater simplicity of the protaspis which lacked these organs. It
+remains to inquire which among the other characteristics are most
+fundamental.
+
+Among the trilobites of the Lower Cambrian, no very young have been
+seen except of Mesonacidæ. Of these, the ontogeny of _Elliptocephala
+asaphoides_ Emmons is best known, thanks to Ford, Walcott, and
+Beecher, but, as the last-named has pointed out, the actual protaspis
+or earliest shield has not yet been found. The youngest specimen is
+the one roughly figured by Beecher (1895 C, p. 175, fig. 6). It lacks
+the pygidium, but if completed by a line which is the counterpart of
+the outline of the cephalon, it would have been 0.766 mm. long. The
+pygidium would have been 0.183 mm. long, or 23 per cent of the whole
+length. The axial lobe was narrow, of uniform width along the
+cephalon, showed a neck-ring and four indistinct annulations, but did
+not reach quite to the anterior end, there being a margin in front of
+the glabella about 0.1 mm. wide. The greatest width of the cephalon
+was 0.66 mm., and of the glabella 0.233 mm., or practically 35 per
+cent of the total width. Other young _Elliptocephala_ up to a length
+of 1 mm., and young _Pædeumias_, _Mesonacis_, and _Holmia_ (see Kiær,
+Videnskaps, Skrifter, 1 Mat.-Naturv. Klasse, 1917, No. 10) show about
+the same characteristics, but all these have large compound eyes on
+the dorsal surface and specimens in still younger stages are expected.
+It may be pointed out, however, that in these specimens the pygidium
+is proportionately larger than in the adult. Walcott cites one adult
+126 mm. long in which the pygidium is 6 mm. long, or between 4 and 5
+per cent of the total length, while in the incomplete specimen
+described above, it was apparently 23 per cent. In a specimen 1 mm.
+long figured by Walcott, the pygidium is 0.15 mm. long, or 15 per cent
+of the whole length.
+
+The development of several species of trilobites from the Middle
+Cambrian is known. Barrande (1852) described the protaspis of _Sao
+hirsuta_, _Peronopsis integer_, _Phalacroma bibullatum_, _P. nudum_,
+and _Condylopyge rex_. Broegger figured that of a _Liostracus_ (Geol.
+For. Förhandl., 1875, pl. 25, figs. 1-3) and Lindstroem (1901, p. 21)
+has reproduced the same. Matthew (Trans. Roy. Soc. Canada, vol. 5,
+1888, pl. 4, pls. 1, 2) has described the protaspis of a _Liostracus_,
+_Ptychoparia linnarssoni_ Broegger, and _Solenopleura robbi_ Hartt.
+Beecher (1895 C, pl. 8) has figured the protaspis of _Ptychoparia
+kingi_ Meek, and the writer that of a Paradoxides (Bull. Mus. Comp.
+Zool., vol. 58, No. 4, 1914, pl. i).
+
+_Sao_, _Liostracus_, _Ptychoparia_, and _Solenopleura_ all have the
+same sort of protaspis. In all, the axial lobe reaches the anterior
+margin and is somewhat expanded at that end; in all, the glabella
+shows but slight trace of segmentation; and in all, the pygidium
+occupies from one fifth to one fourth the total length. There is
+considerable variation in the width of the axial lobe. It is narrowest
+in _Ptychoparia_, where in the middle it is only 14 per cent of the
+whole width, and widest in _Solenopleura_, where it is 28 per cent. In
+_Ptychoparia_ the pygidium of the protaspis occupies from 18 to 22 per
+cent of the whole length. In the adult it occupies 10 to 12 per cent.
+In _Solenopleura_ it makes up about 26 per cent of the protaspis, and
+in the adult about 8 per cent.
+
+In the youngest stages of all these trilobites, the pygidium is
+incompletely separated from the cephalon. The first sign of
+segmentation is a transverse crack which begins to separate the
+cephalon and pygidium, and by the time this has extended across the
+full width the neck segment has become rather well defined. In this
+stage the animal is prepared to swim by means of the pygidium, and
+first becomes active. The coincident development of the free pygidium
+and the neck-ring strongly suggests that the dorsal longitudinal
+muscles are attached beneath the neck-fur row.
+
+The single protaspis of _Paradoxides_ now known, while only 1 mm.
+long, is not in the youngest stage of development. It is like the
+protaspis of _Olenellus_ in having large eyes on the dorsal surface
+and a narrow brim in front of the glabella. The glabella is narrower
+than in the adult.
+
+The initial test of no agnostid has probably as yet been seen, as
+all the young now known show the cephalon and pygidium distinctly
+separated. _Phalacroma bibullatum_ and _P. nudum_ are both practically
+smooth and isopygous when 1.5 mm. long. _P. bibullatum_ shows no axial
+lobe at this stage, but a wide glabella and median tubercle develop
+later, and when the glabella first appears, it extends to the anterior
+margin. In _Peronopsis integer_ and _Condylopyge rex_, the axial lobe
+is outlined on each of the equal shields in specimens about 1 mm.
+long, but is without furrows and reaches neither anterior nor
+posterior margin.
+
+From the foregoing brief description it appears that the pygidium of
+the protaspis varies in different groups from as little as 15 per cent
+of the total length in the Mesonacidæ to as much as 50 per cent in the
+Agnostidæ; that the axial lobe varies from as little as 14 per cent of
+the total width in one _Ptychoparia_ to as much as 50 per cent in
+_Phalacroma nudum_; that the glabella reaches the anterior margin in
+the Olenidæ, Solenopleuridæ, and _Phalacroma bibullatum_, while there
+is a brim in front of it in the Olenellidæ, Paradoxidæ, and three of
+the species of the Agnostidæ. The decision as to which of these
+conditions are primitive may be settled quite satisfactorily by study
+of the ontogeny of the various species.
+
+
+ORIGIN OF THE PYGIDIUM.
+
+Taking first the pygidium, it has already been pointed out that in
+each case the pygidium of the adult is proportionally considerably
+smaller than the pygidium of the protaspis. The stages in the growth
+of the pygidium are better known in Sao hirsuta than in any other
+trilobite, and a review of Barrande's description will be
+advantageous.
+
+Barrande recognized twenty stages in the development of this species,
+but there was evidently a still simpler protaspis in his hands than
+the smallest he figured, for he says, after describing the specimen in
+the first stage: "We possess one specimen on which the head extends
+from one border to the other of the disk, but as this individual is
+unique we have not thought it sufficient to establish a separate
+stage." This specimen is important as indicating a stage in which
+there was not even a suggestion of division between cephalon and
+pygidium.
+
+In the first stage described by Barrande, the form is circular, the
+length is about 0.66 mm., and the glabella is narrow with parallel
+sides and no indications of lateral furrows. The neck segment is
+indicated by a slight prominence on the axial lobe, and back of it a
+constriction divides the axial lobe of the pygidium into two nodes,
+but does not cross the pleural lobes. The position of the nuchal
+segment permits a measurement of the part which is to form the
+pygidium, and shows that that shield made up 30 per cent of the entire
+length.
+
+In the second stage, when the test is 0.75 mm. long, the cephalon and
+pygidium become distinctly separated, and the latter shield shows
+three annulations on the axial and two pairs of ribs on the pleural
+lobes. It now occupies 33-1/3 per cent of the total length.
+
+In the third stage, when the total length is about 1 mm., the pygidium
+has continued to grow. It now shows five annulations on the axial
+lobe, and is 46 per cent of the total length.
+
+In the fourth stage, two segments of the axial lobe have been set free
+from the front of the pygidium. The length is now 1.5 mm. and the
+pygidium makes up 32 per cent of the whole. From this time the
+pygidium continues to decrease in size in proportion to the total
+length, as shown in the following table.
+
+ Stage Length in Percentage Segments in Segments in
+ mm. of pygidium thorax pygidium
+ ========================================================
+ 1 0.66 30 0 2
+ 2 0.75 33-1/3 0 3
+ 3 1.00 46 0 5
+ 4 1.50 32 2 5-6
+ 5 1.50 25 3 4
+ 6 1.75 23 4 4
+ 7 1.80 21 5 3
+ 8 2.00 17 6 3
+ 9 2.50 13 7 3
+ 10 3.00 12 8 3
+ 11 3.50 11 9 3-4
+ 12 4.00 11 10 3-4
+ 13 5.00 10 11 3
+ 14 5.50 9 12 2-4
+ 15 6.00 8 13 3-4
+ 16 6.50 8 14 3
+ 17 7.00 7 15 3
+ 18 7.50 7 16 3
+ 19 7.50 6 17 2
+ 20 10.25 6 17 2
+
+This table shows the rapid increase in the length of the pygidium till
+the time when the thorax began to be freed, the very rapid decrease
+during the earlier part of its formation until six segments had been
+set free, and then a more gradual decrease until the entire seventeen
+segments had been acquired, after which time the relative length
+remained constant. From an initial proportion of 30 per cent, it rose
+to nearly one half the whole length, and then dwindled to a mere 6 per
+cent, showing conclusively that the thorax grew at the expense of the
+pygidium.
+
+If this conclusion can be sustained by other trilobites, it indicates
+that the large pygidium is a more primitive characteristic of a
+protaspis than is a small one. I have already shown that the pygidium
+is proportionately larger in the protaspis in the Mesonacidæ,
+Solenopleuridæ, and Olenidæ, and a glance at Barrande's figures of
+_"Hydrocephalus" carens_ and _"H." saturnoides_, both young of
+_Paradoxides_ will show that the same process of development goes
+on in that genus as in _Sao_. There is first an enlargement of the
+pygidium to a maximum, a rise from 20 per cent to 33 per cent in
+the case of _H. carens_ and then, with the introduction of thoracic
+segments, a very rapid falling off. All of these are, however,
+trilobites with small pygidia, and it has been a sort of axiom among
+palæontologists that large pygidia were made up of a number of
+coalesced segments. While not definitely so stated, it has generally
+been taken to mean the joining together of segments once free. The
+asaphid, for instance, has been thought of as descended from some
+trilobite with rich segmentation, and a body-form like that of a
+_Mesonacis_ or _Paradoxides_.
+
+The appeal to the ontogeny does not give as full an answer to this
+question as could be wished, for the complete life-history of no
+trilobite with a large pygidium is yet known. While the answer is not
+complete, enough can be gained from the study of the ontogeny of
+_Dalmanites_ and _Cyclopyge_ to show that in these genera also the
+thorax grows by the breaking down of the pygidium and that no segment
+is ever added from the thorax to the pygidium. The case of _Dalmanites
+socialis_ as described by Barrande (1852, p. 552, pl. 26) will be
+taken up first, as the more complete. The youngest specimen of this
+species yet found is 0.75 mm. long, the pygidium is distinctly
+separated from the cephalon, and makes up 25 per cent of the length.
+This is probably not the form of the shell as it leaves the egg. At
+this stage there are two segments in the pygidium, but they increase
+to four when the test is 1 mm. long. The cephalon has also increased
+in length, however, so that the proportional length is the same. The
+subjoined table, which is that compiled by Barrande with the
+proportional length of the pygidium added, is not as complete as could
+be desired, but affords a very interesting history of the growth of
+the caudal shield. The maximum proportional length is reached before
+the introduction of thoracic segments, and during the appearance of
+the first five segments the size of the pygidium drops from 25 to 15
+per cent. Several stages are missing at the critical time between
+stages 8 and 9 when the pygidium had added three segments to itself
+and has supplied only one to the thorax. This would appear to have
+been a sort of resting or recuperative stage for the pygidium, for it
+increased its own length to 20 per cent, but from this stage up to
+stage 12 it continued to give up segments to the thorax and lose in
+length itself. After stage 12, when the specimens were 8 mm. long, no
+more thoracic segments were added, but new ones were introduced into
+the pygidium, until it reached a size equal to one fifth the entire
+length, as compared with one fourth in the protaspis.
+
+ Stage Length Percentage Segments in Segments in
+ in mm. of pygidium thorax pygidium
+ ====================================================
+ 1 0.75 25 0 2
+ 2 0.75 25 0 3
+ 3 1.00 25 0 4
+ 4 1.00 22 1 3
+ 5 1.25 20 2 3
+ 6 1.25 18 3 3
+ 7 1.60 15 4 3
+ 8 1.60 15 5 3
+ 9 3.00 20 6 6
+ 10 3.50 20 7 6
+ 11 8.00 18 9 7
+ 12 8.00 16 11 5
+ 13 12.00 16 11 7
+ 14 19.00 18 11 9
+ 15 95.00 20 11 11
+
+
+Since the above was written, Troedsson (1918, p. 57) has described the
+development of _Dalmanites eucentrus_, a species found in the
+Brachiopod shales (Upper Ordovician) of southern Sweden. This species
+follows a course similar to that of _D. socialis_, so that the full
+series of stages need not be described. The pygidium is, however, of
+especial interest, for there is a stage in which it shows two more
+segments than in the adult. Troedsson figures a pygidium 1.28 mm. long
+which has eight pairs of pleural ribs, while the adult has only six
+pairs. The ends of all these ribs are free spines, and were the
+development not known one would say that this was a case of incipient
+fusion, while as a matter of fact, it is incipient freedom.
+
+A further interest attaches to this case, because of the close
+relationship between _D. eucentrus_ and _D. mucronatus_. The latter
+species appears first in the _Staurocephalus_ beds which underlie the
+Brachiopod shales, so that in its first appearance it is somewhat the
+older. The pygidium of the adult _D. mucronatus_ is larger than that
+of _D. eucentrus_, having eight pairs of pleural ribs, the same number
+as in the young of the latter. In short, _D. eucentrus_ is probably
+descended from _D. mucronatus_, and in its youth passes through a
+stage in which it has a large pygidium like that species. Once more it
+appears that the small pygidium is more specialized than the large
+one.
+
+The full ontogeny of _Cyclopyge_ is not known, but young specimens
+show conclusively that segments are not transferred from the thorax to
+the pygidium, but that the opposite occurs. As shown by Barrande
+(1852) and corroborated by specimens in the Museum of Comparative
+Zoology, the process is as follows: The third segment of the adult of
+this species, that is, the fourth from the pygidium, bears a pair of
+conspicuous cavities on the axial portion. In a young specimen, 7 mm.
+long, the second segment bears these cavities, but as the thorax has
+only four segments, this segment is also the second instead of the
+fourth ahead of the pygidium. The pygidium itself, instead of being
+entirely smooth, as in the adult state, is smooth on the posterior
+half, but on the anterior portion has two well formed but still
+connected segments, the anterior one being more perfect than the
+other. These are evidently the two missing segments of the thorax, and
+instead of being in the process of being incorporated in the pygidium,
+they are in fact about to be cast off from it to become free thoracic
+segments. In other words, the thorax grows through the degeneration
+of the pygidium. That the thorax grows at actual expense to the
+pygidium is shown by the proportions of this specimen. In an adult of
+this species the pygidium, thorax, and cephalon are to each other as
+9:11:13. In the young specimen they are as 10:6:12, the pygidium being
+longer in proportion both to the thorax and to the cephalon than it
+would be in the adult.
+
+This conception of the breaking down of the pygidium to form the
+thorax will be very helpful in explaining many things which have
+hitherto seemed anomalous. For instance, it indicates that the
+Agnostidæ, whose subequal shields in early stages have been a puzzle,
+are really primitive forms whose pygidia do not degenerate; likewise
+the Eodiscidæ, which, however, show within the family a tendency to
+free some of the segments. The annelidan Mesonacidæ may not be so
+primitive after all, and their specialized cephala may be more truly
+indicative of their status than has previously been supposed.
+
+The facts of ontogeny of trilobites with both small and large pygidia
+do show that there is a reduction of the relative size of the caudal
+shield during the growth-stages, and therefore that the large pygidium
+in the protaspis is probably primitive. The same study also shows that
+the large pygidium is made up of "coalesced segments" only to the
+extent that they are potentially free, and not in the sense of fused
+segments.
+
+
+WIDTH OF THE AXIAL LOBE.
+
+That the narrow type of axial lobe is more primitive than the wide one
+has already been demonstrated by the ontogeny of various species, and
+space need not be taken here to discuss the question. Most Cambrian
+trilobites have narrow axial lobes even in the adult so that their
+development does not bring this out very strikingly, though it can be
+seen in Sao, Ptychoparia, etc., but in Ordovician trilobites such as
+Triarthrus and especially Isotelus, it is a conspicuous feature.
+
+
+PRESENCE OR ABSENCE OF A "BRIM."
+
+That the extension of the glabella to the front of the cephalon is a
+primitive feature is well shown by the development of Sao (Barrande,
+1852, pl. 7), Ptychoparia (Beecher, 1895 C, pl. 8), and Paradoxides
+(Raymond, Bull. Mus. Comp. Zool., vol. 57, 1914), although in the last
+genus the protaspis has a very narrow brim, the larva during the
+stages of introduction of new segments a fairly wide one, and most
+adults a narrow one.
+
+The brim of Sao seems to be formed partly by new growth and partly at
+the expense of the frontal lobe, for that lobe is proportionately
+shorter in the adult than in the protaspis. In _Cryptolithus_ and
+probably in _Harpes_, _Harpides_, etc., the brim is quite obviously new
+growth and has nothing to do with the vital organs. Its presence or
+absence may not have any great significance, but when the glabella
+extends to the frontal margin, it certainly suggests a more anterior
+position of certain organs. In _Sao_, the only trilobite in which
+anything is known of the position of the hypostoma in the young, the
+posterior end is considerably further forward in a specimen a. 5 mm.
+long than in one 4 mm. long, thus indicating a backward movement of
+the mouth during growth, comparable to the backward movement of the
+eyes.
+
+
+SEGMENTATION OF THE GLABELLA.
+
+The very smallest specimens of _Sao_ show a simple, unsegmented axial
+lobe, and the same simplicity has been noted in the young of other
+genera. Beecher considered this as due to imperfect preservation of
+the exceedingly small shells, which practically always occur as moulds
+or casts in soft shale. There is, however, a very general increase in
+the strength of glabellar segmentation in the early part of the
+ontogeny of all trilobites whose life history is known, and in some
+genera, like the Agnostidæ, there is no question of the comparatively
+late acquisition of glabellar furrows. Even in _Paradoxides_, the
+furrows appear late in the ontogeny.
+
+_Summary._
+
+If absence of eyes on the dorsal surface be primitive, as Beecher
+has shown, and if the large pygidium, narrow axial lobe, and long
+unsegmented glabella be primitive, then the known protaspis of the
+Mesonacidæ and Paradoxidæ is not primitive, that of the Olenidæ is
+very primitive, and that of the Agnostidæ is primitive except that in
+one group the axial lobe, when it appears, is rather wide, and in the
+other a brim is present.
+
+[Illustration: Fig. 35.--A specimen of _Weymouthia nobilis_ (Ford),
+collected by Mr. Thomas H. Clark at North Weymouth, Mass. Note the
+broad smooth shields of this Lower Cambrian eodiscid. × 6.]
+
+Subsequent development from the simple unsegmented protaspis would
+appear to show, first, an adaptation to swimming by the use of the
+pygidium; next, the invagination of the appendifers as shown in the
+segmentation of the axial lobe indicates the functioning of the
+appendages as swimming legs; then with the introduction of thoracic
+segments the assumption of a bottom-crawling habit is indicated. Some
+trilobites were fully adapted for bottom life, and the pygidium became
+reduced to a mere vestige in the production of a worm-like body. Other
+trilobites retained their swimming habits, coupled with the crawling
+mode of life, and kept or even increased (_Isotelus_) the large
+pygidium.
+
+
+
+
+The Simplest Trilobite.
+
+
+In the discussion above I have placed great emphasis on the large size
+of the primitive pygidium, because, although there is nothing new in
+the idea, its significance seems to have been overlooked.
+
+If the large pygidium is primitive, then multisegmentation in
+trilobites can not be primitive but is the result of adaptation to a
+crawling life. It is annelid-like, but is not in itself to be relied
+upon as showing relationship to the Chætopoda. Simple trilobites with
+few segments, like the Agnostidæ, Eodiscidæ etc., were, therefore,
+properly placed by Beecher at the base of his classification, and
+there is now less chance than ever that they can be called degenerate
+animals.
+
+From the phylogeny of certain groups, such as the Asaphidæ, it is
+learned that the geologically older members of the family have more
+strongly segmented anterior and posterior shields than the later ones.
+That there has been a "smoothing out" is demonstrated by a study of
+the ontogeny of the later forms. From such examples it has come to
+be thought that all smooth trilobites are specialized and occupy a
+terminal position in their genealogical line. This has caused some
+wonder that smooth agnostids like _Phalacroma bibullatum_ and _P.
+nudum_ should be found in strata so old as the Middle Cambrian, and
+was a source of great perplexity to me in the case of _Weymouthia_
+(Ottawa Nat., vol. 27, 1913) (fig. 35). This is a smooth member of the
+Eodiscidæ, and, in fact, one of the simplest trilobites known, for
+while it has three thoracic segments, it shows almost no trace of
+dorsal furrows or segmentation on cephalon or pygidium, and, of
+course, no eyes. Following the general rule, I took this to be a
+smooth-out eodiscid, and was surprised that it should come from the
+Lower Cambrian, where it is associated with _Elliptocephala_ at Troy,
+New York, and with _Callavia_ at North Weymouth, Massachusetts, and
+where it has lately been found by Kiær associated with _Holmia_ and
+_Kjerulfia_ at Tømten, Norway. It now appears it is really in its
+proper zone, and instead of being the most specialized, is the
+simplest of the Eodiscidæ.
+
+What appears to be a still simpler trilobite is the form described by
+Walcott as Naraoia.
+
+
+=Naraoia compacta= Walcott.
+
+(Text fig. 36.)
+
+ Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 175,
+ pl. 28, figs. 3, 4.--Cleland, Geology, Physical and Historical, New
+ York, 1916, p. 412, fig. 382 F (somewhat restored).
+
+This very imperfectly known form is referred by Walcott to the
+Notostraca on what appear to be wholly inadequate grounds, and while I
+do not insist on my interpretation, I can not refrain from calling
+attention to the fact that it _can_ be explained as the most primitive
+of all trilobites. It consists of two subequal shields, the anterior
+of which shows slight, and the posterior considerable evidence of
+segmentation. It has no eyes, no glabella, and no thorax, and is
+directly comparable to a very young _Phalacroma bibullatum_ (see
+Barrande 1852, pl. 49, figs. a, b). Walcott states that there is
+nothing to show how many segments there are in the cephalic shield,
+but that on one specimen fourteen were faintly indicated on the
+abdominal covering. The appendages are imperfectly unknown, as no
+specimen showing the ventral side has yet been described. The possible
+presence of antennas and three other appendages belonging to the
+cephalic shield is mentioned, and there are tips of fourteen legs
+projecting from beneath the side of one specimen. As figured, some of
+the appendages have the form of exopodites, others of endopodites,
+indicating that they were biramous.
+
+_Naraoia_ is, so far as now known, possessed of no characteristics
+which would prevent its reference to the Trilobita, while the
+presence of a large abdominal as well as a cephalic shield would make
+it difficult to place in even so highly variable a group as the
+Branchiopoda. On the other hand, its only exceptional feature as a
+trilobite is the lack of thorax, and all study of the ontogeny of the
+group has led us to expect just that sort of a trilobite to be found
+some day in the most ancient fossiliferous rocks. _Naraoia_ can, I
+think, be best explained as a trilobite which grew to the adult state
+without losing its protaspian form. It was found in the Middle
+Cambrian of British Columbia.
+
+Even if _Naraoia_ should eventually prove to possess characteristics
+which preclude the possibility of its being a primitive trilobite, it
+at least represents what I should expect a pre-Cambrian trilobite to
+look like. What the ancestry of the nektonic primitive trilobite may
+have been is not yet clear, but all the evidence from the morphology
+of cephalon, pygidium, and appendages indicates that it was a
+descendant of a swimming and not a crawling organism.
+
+Since the above was written, the Museum of Comparative Zoology has
+purchased a specimen of this species obtained from the original
+locality. The shields are subequal, the posterior one slightly the
+larger, and the axial lobes are definitely outlined on both. The
+glabella is about one third the total width, nearly parallel-sided,
+somewhat pointed at the front. There are no traces of glabellar
+furrows. The axial lobe of the pygidium is also about one third the
+total width, extends nearly to the posterior margin, and has a rounded
+posterior end. The measurements are as follows: Length, 33 mm.; length
+of cephalon, 16 mm., width, 15 mm.; length of glabella, 11.5 mm.,
+width, 5.5 mm.; length of pygidium, 17 mm., width, 15 mm.; length of
+axial lobe, 14 mm., width, 5.5 mm.
+
+The species is decidedly _Agnostus_-like in both cephalon and
+pygidium, and were it not so large, might be taken for the young of
+such a trilobite. The pointed glabella is comparable to the axial
+lobes of the so-called pygidia of the young of _Condylopyge rex_ and
+_Peronopsis integer_ (Barrande, Syst. Sil., vol. 1, pl. 49).
+
+
+
+
+The Ancestor of the Trilobites, and the Descent of the Arthropoda.
+
+
+The "annelid" theory of the origin of the Crustacea and therefore of
+the trilobites, originating with Hatschek (1877) and so ably
+championed by Bernard (1892), has now been a fundamental working
+hypothesis for some years, and has had a profound influence in
+shaping thought about trilobites. This hypothesis has, however,
+its weak points, the principal one being its total inhibition of
+the workings of that great talisman of the palæontologist, the law of
+recapitulation. Its acceptance has forced the zoologist to look upon
+the nauplius as a specially adapted larva, and has caused more than
+one forced explanation of the protaspis of the trilobite. When so keen
+a student as Calman says that the nauplius must point in some way to
+the ancestor of the Crustacea (1909, p. 26), it is time to reëxamine
+some of the fundamentals. This has been done in the preceding pages
+and evidence adduced to show that the primitive features of a
+trilobite indicate a swimming animal, and that the adaptations are
+those which enabled it to assume a crawling mode of existence. It has
+also been pointed out that in Naraoia there is preserved down to
+Middle Cambrian times an animal like that to which ontogeny points as
+a possible ancestor of the trilobites. _Naraoia_ is not the simplest
+conceivable animal of its own type, however, for it has built up a
+pygidium of fourteen or fifteen somites. One would expect to find in
+Proterozoic sediments remains of similar animals with pygidia composed
+of only one or two somites, with five pairs of appendages on the
+cephalon, one or two pairs on the pygidium, a ventral mouth, and a
+short hypostoma. Anything simpler than this could not, in my opinion,
+be classed as a trilobite.
+
+What the ancestor of this animal was is mere surmise. It probably had
+no test, and it may be noted in this connection that _Naraoia_ had a
+very thin shell, as shown by its state of preservation, and was in
+that respect intermediate between the trilobite and the theoretical
+ancestor. Every analysis of the cephalon of the trilobite shows that
+it is made up of several segments, certainly five, probably six,
+possibly seven. Every study of the trilobite, whether of adult, young,
+or protaspis, indicates the primitiveness of the lateral extensions or
+pleural lobes. The same studies indicate as clearly the location of
+the vital organs along the median lobe. These suggestions all point to
+a soft-bodied, depressed animal composed of few segments, probably
+with simple marginal eyes, a mouth beneath the anterior margin,
+tactile organs at one or both ends, with an oval shape, and a straight
+narrow gut running from anterior mouth to terminal anus. The broad
+flat shape gives great buoyancy and is frequently developed in the
+plankton. Inherited by the trilobites, it proved of great use to the
+swimmers among them.
+
+The known animal which most nearly approaches the form which I should
+expect the remote ancestor of the trilobites to have had is _Amiskwia
+sagittiformis_ Walcott (Smithson. Misc. Coll., vol. 57, 1911, p. 112,
+pl. 22, figs. 3, 4). This "worm" from the Middle Cambrian is similar
+in outline to the recent _Spadella_, and is referred by Walcott to the
+Chætognatha. It has a pair of lateral expansions and a flattened
+caudal fin, a narrow median alimentary canal, and a pair of rather
+long simple tentacles. With the exception of a thin septum back of the
+head, no traces of segmentation are shown.
+
+Some time in the late pre-Cambrian, the pre-trilobite, which probably
+swam by rhythmic undulations of the body, began to come into
+occasional contact with a substratum, and two things happened:
+symmetrically placed, i. e., paired, appendages began to develop on
+the contact surface, and a test on the dorsal side. The first use of
+the appendages may have been in pushing food forward to the mouth,
+and for the greater convenience in catching such material, a fold
+in front of the mouth may have elongated to form the prototype of the
+hypostoma. At this time the substratum may not have been the ocean
+bottom at all, but the animals, still free swimmers, may have alighted
+at feeding time on floating algæ from the surface of which they
+collected their food. While the dorsal test was originally jointed at
+every segment, the undulatory mode of swimming seems to have given way
+to the method of sculling by means of the posterior end only, or by
+the use of the appendages, and the anterior segments early became
+fused together.
+
+The result of the hardening of the dorsal test was of course to reduce
+to that extent the area available for respiration, and this function
+was now transferred in part to the limbs, which bifurcated, one branch
+continuing the food-gathering process and the other becoming a gill.
+The next step may have been the "discovery" of the ocean bottom and
+the tapping of an hitherto unexploited supply of food. Upon this,
+there set in those adaptations to a crawling mode of existence which
+are so well shown in the trilobite. The crawling legs became
+lengthened and took on a hardened test, the hypostoma was greatly
+elongated, pushing the mouth backward, and new segments were added to
+produce a long worm-like form which could adapt itself to the
+inequalities of the bottom. That the test of the appendages became
+hardened later than that of the body is shown by the specimens of
+Neolenus, in which the dorsal shell as preserved in the shale is thick
+and solid, while the test of the appendages is a mere film.
+
+The late Proterozoic or very earliest Cambrian was probably the time
+of the great splitting up into groups. The first development seems to
+have been among the trilobites themselves, the Hypoparia giving rise
+to two groups with compound eyes, first the Opisthoparia and later the
+Proparia. About this same time the Copepoda may have split off from
+the Hypoparia, continuing in the pelagic habitat. At first, most of
+the trilobites seem to have led a crawling existence, but about Middle
+Cambrian time they began to go back partially to the ancestral
+swimming habits, and retained some of the trunk segments to form a
+larger pygidium. The functional importance of the pygidium explains
+why it can not be used successfully in making major divisions in
+classification. Nearly related trilobites may be adapted to diverse
+methods of life.
+
+
+EVOLUTION WITHIN THE CRUSTACEA.
+
+The question naturally arises as to whether the higher Crustacea were
+derived from some one trilobite, or whether the different groups have
+been developed independently from different stocks. The opinion that
+all other crustaceans could have been derived from an _Apus_-like form
+has been rather generally held in recent years, but Carpenter (1903,
+p. 334) has shown that the leptostracan, _Nebalia_, is really a more
+primitive animal than _Apus_. He has pointed out that in Leptostraca
+the thorax bears eight pairs of simple limbs with lamelliform
+exopodites and segmented endopodites, while the abdomen of eight
+segments has six pairs of pleopods and a pair of furcal processes,
+so that only one segment is limbless. Contrasted with this are the
+crowded and complicated limbs of the anterior part of the trunk of
+_Apus_, and the appendage-less condition of the hinder portion.
+Further, a comparison between the appendages of the head of _Nebalia_
+and those of _Apus_ shows that the former are the more primitive. The
+antennules of Nebalia are elongate, those of _Apus_ greatly reduced;
+the mandible of _Nebalia_ has a long endopodite, and Carpenter points
+out that from it either the malacostracan mandible with a reduced
+endopodite or the branchiopodan mandible with none could be derived,
+but that the former could not have arisen from the latter. The maxillæ
+of _Apus_ are also much the more specialized and reduced.
+
+_Nebalia_ being in all else more primitive than _Apus_, it follows
+that the numerous abdominal segments of the latter may well have
+arisen by the multiplication of an originally moderate number, and the
+last trace of primitiveness disappears.
+
+It is now possible to add to the results obtained from comparative
+morphology the testimony of palæontology, already outlined above, and
+since the two are in agreement, it must be admitted that the modern
+Branchiopoda are really highly specialized.
+
+As has already been pointed out, _Hymenocaris_, the leptostracan of
+the Middle Cambrian, has very much the same sort of appendages as the
+Branchiopoda of the same age, both being of the trilobite type. Which
+is the more primitive, and was one derived from the other?
+
+The Branchiopoda were much more abundant and much more highly
+diversified in Cambrian times than were the Leptostraca, and,
+therefore, are probably older. Some of the Cambrian branchiopods were
+without a carapace, and some were sessile-eyed. These were more
+trilobite-like than Hymenocaris. Many of the Cambrian branchiopods had
+developed a bivalved carapace, though not so large a one as that of
+the primitive Leptostraca. The present indications are, therefore,
+that the Branchiopoda are really older than the Leptostraca, and also
+that the latter were derived from them. It seems very generally agreed
+that the Malacostraca are descended from the Leptostraca, and the
+fossils of the Pennsylvanian supply a number of links in the chain of
+descent. Thus, _Pygocephalus cooperi_, with its brood pouches, is
+believed by Calman (1909, p. 181) to stand at the base of the
+Peracaridan series of orders, and _Uronectes_, _Palæocaris_, and
+the like are Palæozoic representatives of the Syncarida. Others
+of the Pennsylvanian species appear to tend in the direction of
+the Stomatopoda, whose true representatives have been found in the
+Jurassic. The Isopoda seem to be the only group of Malacostraca not
+readily connected up with the Leptostraca. Their depressed form, their
+sessile-eyes, and their antiquity all combine to indicate a separate
+origin for the group, and it has already been pointed out how readily
+they can be derived directly from the trilobite.
+
+While the Copepoda seem to have been derived directly from the
+Hypoparia, the remainder of the Crustacea apparently branched off
+after the compound eyes became fully developed, unless, as seems
+entirely possible, compound eyes have been developed independently in
+various groups. Most Crustacea were derived from crawling trilobites
+(Lower Cambrian or pre-Cambrian Opisthoparia), for they lost the large
+pygidium, and also the major part of the pleural lobes. In all
+Crustacea, too, other than the Copepoda and Ostracoda, there is a
+tendency to lose the exopodites of the antennæ.
+
+These modifications, which produced a considerable difference in the
+general appearance of the animal, are easily understood. As has been
+shown in previous pages, the trilobites themselves exhibit the
+degenerative effect on the anterior appendages of the backward
+movement of the mouth, and the transformation of a biramous appendage
+with an endobase into a uniramous antenna is a simple result of such
+a process. The feeding habits of the trilobites were peculiar and
+specialized, and it is natural that some members of the group should
+have broken away from them. In any progressive mode of browsing
+the hypostoma was a hindrance, so was soon gotten rid of, and the
+endobases not grouped around the mouth likewise became functionless.
+The chief factor in the development of the higher Crustacea seems to
+have been the pinching claw, by means of which food could be conveyed
+to the mouth. It had the same place in crustacean development that the
+opposable thumb is believed to have had in that of man.
+
+An intermediate stage between the Trilobita and the higher Crustacea
+is at last exhibited to us by the wonderful, but unfortunately rather
+specialized _Marrella_, already described. It retains the hypostoma
+and the undifferentiated biramous appendages of the trilobite, but has
+uniramous antennæ, there are no endobases on the coxopodites of the
+thoracic appendages, the pygidium is reduced to a single segment, and
+the lateral lobes of the thorax are also much reduced. _Marrella_ is
+far from being the simplest of its group, but is the only example
+which survived even down to Middle Cambrian times of what was probably
+once an important series of species transitional between the
+trilobites and the higher Crustacea.
+
+In this theory of the origin of the Crustacea from the Trilobita, the
+nauplius becomes explicable and points very definitely to the
+ancestor. According to Calman (1909, p. 23):
+
+ The typical nauplius has an oval unsegmented body and three pairs
+ of limbs, corresponding to the antennules, antennas, and mandibles
+ of the adult. The antennules are uniramous, the others biramous,
+ and all three pairs are used in swimming. The antennæ may have a
+ spiniform or hooked masticatory process at the base, and share with
+ the mandibles which have a similar process, the function of seizing
+ and masticating the food. The mouth is overhung by a large labrum
+ or upper lip and the integument of the dorsal surface of the body
+ forms a more or less definite dorsal shield. The paired eyes are as
+ yet wanting, but the median eye is large and conspicuous.
+
+The large labrum or hypostoma, the biramous character of the
+appendages, especially of the antennæ, the functional gnathobases on
+the second and third appendages, and the oval unsegmented shield are
+all characteristics of the trilobites, and it is interesting to note
+that all nauplii have the free-swimming habit.
+
+The effect of inheritance and modification through millions of
+generations is also shown in the nauplius, but rather less than would
+be expected. The most important modification is the temporary
+suppression of the posterior pairs of appendages of the head, so that
+they are generally developed later than the thoracic limbs. The median
+or nauplius eye has not yet been found in trilobites, and if it is, as
+it appears to be, a specialized eye, it has probably arisen since the
+later Crustacea passed the trilobite stage in their phylogeny.
+
+The oldest Crustacea, other than trilobites, so far known are the
+Branchiopoda and Phyllocarida described by Walcott and discussed
+above. It is important to note that while the former have already
+achieved such modified characteristics that they have been referred to
+modern orders, they retain the trilobite-like limbs and some of them
+still have well developed pleural lobes.
+
+Calman (1909, p. 101) says of the Copepoda:
+
+ On the hypothesis that the nauplius represents the ancestral type
+ of the Crustacea, the Eucopepoda would be regarded as the most
+ primitive existing members of the class, retaining as they do,
+ naupliar characters in the form of the first three pairs of
+ appendages and in the absence of paired eyes and of a shell-fold.
+ As already indicated, however, it is much more probable that they
+ are to be regarded as a specialized and in some respects degenerate
+ group which, while retaining, in some cases, a very primitive
+ structure of the cephalic appendages, has diverged from the
+ ancestral stock in the reduction of the number of somites, the loss
+ of the paired eyes and the shell-fold, and the simplified form of
+ the trunk-limbs.
+
+If the Eucopepoda be viewed in the light of the theory of descent here
+suggested, it is at once seen that while they are modified and
+specialized, they more nearly approximate the hypothetical ancestor
+than any other living Crustacea. Compound eyes are absent, and it can
+not be proved that they were ever present, although Grobben is said to
+have observed rudiments of them in the development of _Calanus_. The
+"simplified limbs" are the simple limbs of the trilobite, somewhat
+modified. The absence of the shell-fold and carapace is certainly a
+primitive characteristic. Add to this the direct development of the
+small number of segments, and the infolded pleural lobes, and it must
+be admitted that the group presents more trilobite-like
+characteristics than any other. It seems very likely that the
+primitive features were retained because of the pelagic habitat of a
+large part of the group.
+
+Ruedemann (Proc. Nat. Acad. Sci., vol. 4, 1918, p. 382, pl.) has
+recently outlined a possible method of derivation of the acorn
+barnacles from the phyllocarids. Starting from a recent _Balanus_ with
+rostrum and carina separated by two pairs of lateralia, he traces back
+through _Calophragmus_ with three pairs of lateralia to _Protobalanus_
+of the Devonian with five pairs. Still older is the newly discovered
+_Eobalanus_ of the upper Ordovician, which also has five pairs of
+lateralia but the middle pair is reversed, so that when the lateralia
+of each side are fitted together, they form a pair of shields like
+those of _Rhinocaris_, separated by the rostrum and carina, which are
+supposed to be homologous with the rostrum and dorsal plate of the
+Phyllocarida. Ruedemann suggests that the ancestral phyllocarid
+attached itself by the head, dorsal side downward, and the lateralia
+were developed from the two valves of the carapace during its upward
+migration, to protect the ventral side exposed in the new position.
+
+This theory is very ingenious, but has not been fully published at the
+time of writing, and it seems very doubtful if it can be sustained.
+
+_Summary._
+
+The salient points in the preceding discussion should be disentangled
+from their setting and put forward in a brief summary.
+
+It is argued that the ancestral arthropod was a short and wide pelagic
+animal of few segments, which so far changed its habits as to settle
+upon a substratum. As a result of change in feeding habits, appendages
+were developed, and, due perhaps to physiological change induced by
+changed food, a shell was secreted on the dorsal surface, covering
+the whole body. Such a shell need not have been segmented, and, in
+fact, the stiffer the shell, the more reason for development of the
+appendages. Activity as a swimming and crawling animal tended to break
+up the dorsal test into segments corresponding to those of the soft
+parts, and, by adaptation, a floating animal became a crawling one,
+with consequent change from a form like that of _Naraoia_ to one like
+_Pædeumias_. (See figs. 36-40.) A continuation of this line of
+development by breaking up and loss of the dorsal test led through
+forms similar to _Marrella_ to the Branchiopoda of the Cambrian, in
+which not only is there great reduction in the test, but also loss of
+appendages. The origin of the carapace is still obscure, but Bernard
+(1892, p. 214, fig. 48) has already pointed out that some trilobites,
+Acidaspidæ particularly, have backward projecting spines on the
+posterior margin of the cephalon, which suggest the possibility of the
+production of such a shield, and in _Marrella_ such spines are so
+extravagantly developed as almost to confirm the probability of such
+origin. In this line of development two pairs of tactile antennæ were
+produced, while the anomomeristic character of the trilobite was
+retained. From similar opisthoparian ancestors there were, however,
+derived primitive Malacostraca retaining biramous antennæ, but with a
+carapace and reduced pleural lobes and pygidium. From this offshoot
+were probably derived the Ostracoda, the Cirripedia, and the various
+orders of the Malacostraca, with the possible exception of the
+Isopoda. I have suggested independent origins of the Copepoda and
+Isopoda, but realize the weighty arguments which can be adduced
+against such an interpretation.
+
+[Illustration: Fig. 36.--_Naraoia compacta_ Walcott. An outline of
+the test, after Walcott. Natural size.]
+
+[Illustration: Fig. 37.--_Pagetia clytia_ Walcott. An eodiscid with
+compound eyes. After Walcott. × 5.]
+
+[Illustration: Fig. 38.--_Asaphiscus wheeleri_ Meek. A representative
+trilobite of the Middle Cambrian of the Pacific province. After Meek.
+× 1/2.]
+
+[Illustration: Fig. 39.--_Pædeumias robsonensis_ Burling. Restored
+from a photograph published by Burling. × 1/4.]
+
+[Illustration: Fig. 40.--_Robergia_ sp. Restored from fragments found
+in the Athens shale (Lower Middle Ordovician), at Saltville, Va.
+Natural size.]
+
+It is customary to speak of the Crustacea and Trilobita as having had
+a common ancestry, rather than the former being in direct line of
+descent from the latter, but when it can be shown that the higher
+Crustacea are all derivable from the Trilobita, and that they possess
+no characteristics which need have been inherited from any other
+source than that group, it seems needless to postulate the evolution
+of the same organs along two lines of development.
+
+I can not go into the question of which are more primitive, sessile or
+stalked eyes, but considering the various types found among the
+trilobites, one can but feel that the stalked eyes are not the most
+simple. While no trilobite had movable stalked eyes, it is possible to
+homologize free cheeks with such structures. They always bear the
+visual surface, and, in certain trilobites (_Cyclopyge_), the entire
+cheek is broken up into lenses. Since a free cheek is a separate
+entity, it is conceivable that it might lie modified into a movable
+organ.
+
+
+EVOLUTION OF THE MEROSTOMATA.
+
+It has been pointed out above that the Limulava (_Sidneyia_,
+_Amiella_, _Emeraldella_) have certain characteristics in common with
+the trilobites on the one hand and the Eurypterida on the other. These
+relationships have been emphasized by Walcott, who derives the
+Eurypterida through the Limulava and the Aglaspina from the Trilobita.
+The Limulava may be derived from the Trilobita, but indicate a line
+somewhat different from that of the remainder of the Crustacea. In
+this line the second cephalic appendages do not become antennæ and
+the axial lobe seems to broaden out, so that the pleural lobes become
+an integral part of the body. As in the modern Crustacea, the pygidium
+is reduced to the anal plate, and this grows out into a spine-like
+telson.
+
+From the Limulava to the Eurypterida is a long leap, and before it can
+be made without danger, many intermediate steps must be placed in
+position. The direct ancestor of the Eurypterida is certainly not to
+be seen in the highly specialized _Sidneyia_, and probably not in
+_Emeraldella_, but it might be sought in a related form with a few
+more segments. The few species now known do suggest the beginning of a
+grouping of appendages about the mouth, a suppression of appendages on
+the abdomen, and a development of gills on the thorax only. Further
+than that the route is uncertain.
+
+Clarke and Ruedemann, whose recent extensive studies give their
+opinion much weight, seem fully convinced that the Merostomata could
+not have been derived from the Trilobita, but are rather inclined to
+agree with Bernard that the arachnids and the crustaceans were derived
+independently from similar chætopod annelids (1912, p. 148).
+
+The greater part of their work was, however, finished before 1910, and
+although they refer to Walcott's description of the Limulava (1911),
+they did not have the advantage of studying the wonderful series of
+Crustacea described by him in 1912. While the evidence is far from
+clear, it would appear that the discovery of animals with the form of
+Limiting and the eurypterids and the appendages of trilobites means
+something more than descent from similar ancestors. Biramous limbs of
+the type found in the trilobites would probably not be evolved
+independently on two lines, even if the ancestral stocks were of the
+same blood.
+
+The Aglaspidæ, as represented by _Molaria_ and _Habelia_ in the Middle
+Cambrian, are quite obvious closely related to the trilobites easily
+derived from them, and retain numerous of their characteristics. That
+they are not trilobites is, however, shown by the presence of two
+pairs of antennæ, the absence of facial sutures, and the possession of
+a spine-like telson.
+
+The Aglaspidæ have always been placed in the Merostomata, and nearer
+the Limulidæ than the Eurypterida. The discovery of appendages does
+not at all tend to strengthen that view, but indicates rather that
+they are true Crustacea which have not given rise to any group now
+known. The exterior form is, however, _Limulus_-like, and since it is
+known from ontogeny that the ancestor of that genus was an animal with
+free body segments, there is still a temptation to try to see in the
+Aglaspidæ the progenitors of the limulids.
+
+The oldest known _Limulus_-like animal other than the Aglaspidæ is
+_Neolimulus falcatus_ Woodward (Geol. Mag., dec. 1, vol. 5, 1868,
+p. 1, pl. 1, fig. 1). The structure of the head of this animal is
+typically limuloid, with simple and compound eyes and even the
+ophthalmic ridges. Yet, curiously enough, it shows what in a trilobite
+would be considered the posterior half of the facial suture, running
+from the eye to the genal angle. The body is composed of eight free
+segments with the posterior end missing. _Belinurus_, from the
+Mississippian and Pennsylvanian, has a sort of pygidium, the posterior
+three segments being fused together, and _Prestwichia_ of the
+Pennsylvanian has all the segments of the abdomen fused together. So
+far as form goes, a very good series of stages can be selected, from
+the Aglaspidæ of the Cambrian through _Neolimulus_ to the Belinuridæ
+of the late Palæozoic and the Limulidæ of the Mesozoic to recent.
+Without much more knowledge of the appendages than is now available,
+it would be quite impossible to defend such a line. It is, however,
+suggestive.
+
+
+EVOLUTION OF THE "TRACHEATA."
+
+The trilobites were such abundant and highly variable animals,
+adapting themselves to various methods of life in the sea, that it
+appears highly probably that some of them may have become adapted to
+life on the land. The ancestors of the Chilopoda, Diplopoda, and
+Insecta appear to have been air-breathing animals as early as the
+Cambrian, or at latest, the Ordovician. Since absolutely nothing is
+yet known of the land or even of the fresh-water life of those
+periods, nothing can now be proved.
+
+In discussing the relationship of the trilobites to the various
+tracheate animals, I have pointed out such palæontologic evidence
+as I have been able to gather. Studies in the field of comparative
+morphology do not fall within my province. I only hope to have made
+the structure of the trilobite a little more accessible to the student
+of phylogenies.
+
+
+SUMMARY ON LINES OF DESCENT.
+
+In order to put into graphic and concise form the suggestions made
+above, it is necessary to define and give names to some of the groups
+outlined. The hypothetical ancestor need not be included in the
+classification and for reasons of convenience may be referred to
+merely as the Protostracean.
+
+The group of free-swimming trilobites without thoracic segments was
+probably a large one, and within it there were doubtless considerable
+variations and numerous adaptations. While the only known animal which
+could possibly be referred to this group, _Naraoia_, is blind, it is
+entirely possible that other species had eyes, and that the cephala
+and pygidia were variously modified. For this reason and because of
+the lack of all thoracic segments, it seems better to erect a new
+order rather than merely a family for the group, and _Nektaspia_
+(swimming shields) may be suggested. The only known family is Naraoidæ
+Walcott, which must be redefined.
+
+_Marrella_ and _Habelia_ are types of Crustacea which can neither be
+placed in the Trilobita nor in any of the established subclasses of
+the Eucrustacea. They represent a transitional group, the members of
+which are, so far as known, adapted to the crawling mode of life,
+though it may prove that there are also swimmers which can be
+classified with them. To this subclass the name _Haplopoda_ may be
+applied, the feet being simple.
+
+The two known families, Marrellidæ Walcott and Aglaspidæ Clarke,
+belong to different orders, the second having already the name
+Aglaspina Walcott. The name _Marrellina_ may therefore be used for the
+other.
+
+For _Sidneyia_, Walcott proposed the new subordinal name Limulava,
+placing it under the Eurypterida. While _Sidneyia_, _Emeraldella_, and
+_Amiella_ may belong to the group that gave rise to the Eurypterida,
+they are themselves Crustacea, and a place must be found for them in
+that group. The possession of only one pair of antennæ prevents their
+reception by the Haplopoda, and allies them to the Trilobita, but the
+modifications of the trunk and its appendages keep them out of that
+subclass, and a new one has to be erected for them. This may be known
+as the _Xenopoda_, in allusion to the strange appendages of
+_Sidneyia_.
+
+
+_Synopsis._
+
+Class Crustacea.
+
+Subclass Trilobita Walch.
+
+Crustacea with one pair of uniramous antennæ, and possessing facial
+sutures.
+
+Order Nektaspia nov.
+
+Trilobita without thoracic segments. Cephala and pygidia simple.
+
+Family Naraoidæ Walcott.
+
+Cephalon and pygidium large, both shields nearly smooth. Eyes absent.
+A single species: _Naraoia compacta_ Walcott, Middle Cambrian, British
+Columbia.
+
+Subclass Haplopoda nov.
+
+Crustacea with trilobate form, two pairs of uniramous antennæ, no
+facial sutures, sessile compound eyes present or absent, pygidium and
+pleural lobes generally reduced, large labrum present, appendages of
+the trunk biramous.
+
+Order Marrellina nov.
+
+Form trilobite-like, pleural lobes reduced, endobases absent from
+coxopodites of body, pygidium a small plate.
+
+Family Marrellidæ Walcott.
+
+Cephalon with long genal and nuchal spines. Eyes marginal. A single
+species: _Marrella splendens_ Walcott, Middle Cambrian, British
+Columbia.
+
+Order Aglaspina Walcott.
+
+Body trilobite-like, with few thoracic segments, and a spine-like
+telson. Appendages biramous.
+
+Family Aglaspidæ Clarke.
+
+Cephalon trilobate, with or without compound eyes, seven or eight
+segments in the thorax.
+
+Genus _Aglaspis_ Hall.
+
+Compound eyes present, seven segments in thorax. Upper Cambrian,
+Wisconsin.
+
+Genus _Molaria_ Walcott.
+
+Compound eyes absent, eight segments in thorax. Middle Cambrian,
+British Columbia.
+
+Genus _Habelia_ Walcott.
+
+Compound eyes absent. Not yet fully described. Middle Cambrian,
+British Columbia.
+
+Subclass Xenopoda nov.
+
+Crustacea with more or less eurypterid-like form, one pair of
+uniramous antennæ, biramous appendages on anterior part of trunk,
+modified endopodites on cephalon.
+
+Order Limulava Walcott.
+
+Cephalon with lateral or marginal eyes and large epistoma. Body with
+eleven free segments and a telson. Cephalic appendages grouped about
+the mouth.
+
+Family Sidneyidæ Walcott.
+
+Trunk probably with exopodites only, and without appendages on the
+last two segments. Telson with a pair of lateral swimmerets.
+
+Genus _Sidneyia_ Walcott.
+
+Third cephalic appendage a large compound claw. Gnathobases forming
+strong jaws. Middle Cambrian, British Columbia.
+
+Genus _Amiella_ Walcott.
+
+Middle Cambrian, British Columbia.
+
+Family Emeraldellidæ nov.
+
+Trunk with biramous appendages in anterior part, and appendages on all
+segments except possibly the spine-like telson.
+
+Genus _Emeraldella_ Walcott.
+
+Cephalic appendages simple spiniferous endopodites. Eyes unknown.
+Middle Cambrian, British Columbia.
+
+[Illustration: Fig. 41.--A diagram showing possible lines of descent
+of the other Arthropoda from the Trilobita. The three recognized
+orders of the latter are shown separately. The known geological range
+is indicated in solid black, the hypothetical range and connections
+stippled. The short branch beside the Opisthoparia represents the
+range of the Haplopoda. The term Arachnida is used for all arachnids
+other than Merostomata, merely as a convenient inclusive name for the
+groups not especially studied.]
+
+
+
+
+Final Summary.
+
+
+It is generally believed that the Arthropoda constitute a natural,
+monophyletic group. The data assembled in the preceding pages indicate
+that the other Arthropoda were derived directly or indirectly from the
+Trilobita because:
+
+(1) the trilobites are the oldest known arthropods;
+
+(2) the trilobites of all formations show great variation in the
+number of trunk segments, but with a tendency for the number to become
+fixed in each genus;
+
+(3) the trilobites have a constant number of segments in the head;
+
+(4) the position of the mouth is variable, so that either the
+Crustacea or the Arachnida could be derived from the trilobites;
+
+(5) the trilobite type of appendage is found, in vestigial form at
+least, throughout the Arthropoda;
+
+(6) the appendages of all other Arthropoda are of forms which could
+have been derived from those of trilobites;
+
+(7) the appendages of trilobites are the simplest known among the
+Arthropoda;
+
+(8) the trilobites show practically all known kinds of sessile
+arthropodan eyes, simple, compound, and aggregate;
+
+(9) the apparent specializations of trilobites, large pleural lobes
+and pygidia, are primitive, and both suffer reduction within the
+group.
+
+The ancestor of the trilobite is believed to have been a soft-bodied,
+free-swimming, flat, blind or nearly blind animal of few segments,
+because:
+
+(a) the form of both adult and embryo is of a type more adapted for
+floating than crawling;
+
+(b) the large pygidium is shown by ontogeny to be primitive, and the
+elongate worm-like form secondary;
+
+(c) the history of the trilobites shows a considerable increase in the
+average number of segments in successive periods from the Cambrian to
+the Permian;
+
+(d) the simplest trilobites are nearly or quite blind.
+
+
+
+
+PART IV.
+
+DESCRIPTION OF THE APPENDAGES OF INDIVIDUAL SPECIMENS.
+
+
+
+
+Triarthrus becki Green.
+
+
+In order to make easily available the evidence on which the present
+knowledge of the appendages of Triarthrus and _Cryptolithus_ rests, it
+has seemed wise to publish brief descriptions and photographic figures
+of some of the better specimens preserved in the Yale University
+Museum. These specimens are pyritic replacements, and while they do
+not as yet show any signs of decomposition, it should be realized that
+it is only a matter of time when either they will be self-destroyed
+through oxidation, or else embedded for safe keeping in such a fashion
+that they will not be readily available for study. It is therefore
+essential to keep a photographic record of the more important
+individuals.
+
+
+Specimen No. 220 (pl. 3, fig. 2).
+
+ Illustrated: Amer. Geol., vol. 15, 1895, pl. 4 (drawing);
+ Amer. Jour. Sci., vol. 13, 1902, pl. 3 (photograph).
+
+This is one of the largest specimens showing appendages, and is
+developed from the ventral side. It shows some appendages on all parts
+of the body, but its special features are the exhibition of the shafts
+on the proximal ends of the antennules, the rather well preserved
+appendages of the cephalon and anterior part of the thorax, and the
+preservation of the anal opening. In the drawing in the American
+Geologist, the right and left sides are reversed as in a mirror, a
+point which should be borne in mind when comparing that figure with
+a photograph or description.
+
+The shaft of the left antennule is best preserved and is short,
+cylindrical, somewhat enlarged and ball-shaped at the proximal end. It
+is 1.5 mm. long. The posterior part of the hypostoma is present, but
+crushed, and the metastoma is not visible, the pieces so indicated
+in Beecher's figure being the rim of the hypostoma. Back of the
+hypostoma may be seen four (not three as in Beecher's figure) pairs
+of gnathites, the first three pairs broad and greatly overlapping, the
+fourth pair more slender, but poorly preserved. The inner edges of the
+gnathites on the right side are distinctly nodulose, and roughened for
+mastication.
+
+The outer ends of one endopodite and three exopodites project beyond
+the margin on the right side. The dactylopodite of the endopodite is
+especially well preserved. It is cylindrical, the end rounded but not
+enlarged or pointed, and bears three small sharp spines, all in a
+horizontal plane, one anterior, one central, and one posterior. The
+outer ends of the exopodites show about ten segments each (in 2.5 mm.)
+beyond the margin of the test, and from three to five setæ attached to
+the posterior side of each segment. These hairs are attached in a
+groove, well shown in this specimen. On the anterior margin of the
+exopodite there is a minute spine at each joint.
+
+_Measurements:_ Length, 38 mm.; width at back of cephalon, 19 mm.
+
+
+Specimen No. 210 (pl. 2, fig. 3).
+
+ Illustrated: Amer. Jour. Sci., vol. 46, 1893, p. 469, fig. 1
+ (head and right side); Amer. Geol., vol. 13, 1894, pl. 3, fig. 7
+ (same figure as the last); Amer. Jour. Sci., vol. 13, 1902, pl. 2,
+ fig. 1 (photograph).
+
+This individual supplied the main basis for Professor Beecher's first
+figure showing the appendages of the thorax, the head and appendages
+of the right side having been taken from it, and the appendages of
+the left side from No. 206. Such of the endopodites as are well
+preserved show from three to four segments projecting beyond the test,
+and the dactylopodites have one or two terminal spines. The antennules
+are unusually well preserved and have about forty segments each in
+front of the cephalon, or an average of five to one millimeter.
+
+Specimens 209 and 210 are on a slab about 7 × 5.5 inches, and with
+them are twelve other more or less well preserved individuals, all but
+one of which are smaller than these. Two of the fourteen are ventral
+side up on the slab, which means dorsal side up in the rock. Nine are
+oriented in one direction, two at exactly right angles to this, and
+three at an angle of 45 with the others. If the majority of the
+specimens are considered to be headed northward, then seven are so
+oriented, two northeast, one east, two south, one southwest, and one
+west.
+
+Nine of the specimens show antennules. Five of these are specimens
+headed north, and in all of them the antennules are in or very near
+the normal position. The antennules of two, one headed east and the
+other west, are imperfectly preserved, but the parts remaining diverge
+much more than do the antennules of those in the normal position. The
+individual headed southwest has one antennule broken off, while the
+other is curved back so that its tip is directed northward. Another
+one, headed south, has the antennules in the normal position. These
+observations indicate that the specimens were oriented by currents of
+water, rather than in life attitudes, and that the distal portions of
+the antennules were relatively flexible.
+
+_Measurements:_ The specimen (No. 210) is 20 mm. long, 9.5 mm. wide at
+the back of the cephalon, and the antennules project 8 mm. in front of
+the head. The smallest specimen on the slab is 6.5 mm. long. A
+specimen 7.5 mm. long has antennules which project 2.5 mm. in front of
+the cephalon.
+
+
+Specimen No. 201 (pl. 2, fig. 1; pl. 3, fig. 4).
+
+ Illustrated: Amer. Jour. Sci., vol. 46, 1893, p. 469, figs. 2, 3;
+ Amer. Geol., vol. 13, 1894, pl. 3, figs. 8, 9.
+
+An entire specimen 17 mm. long, exposed from the dorsal side. It shows
+only traces of the appendages of the head, but displays well those of
+the anterior part of the thorax, and a number of appendages emerge
+from under the abdominal shield. This specimen is of particular
+interest as it is the subject of the first of Professor Beecher's
+papers on appendages of trilobites. On the right side the pleura have
+been removed, so as to expose the appendages of the second, third, and
+fourth segments from above. The first two of the appendages on the
+right are best preserved, and these are the ones figured. They belong
+to the second and third segments. The endopodites of each are ahead of
+the exopodites, and the proximal portion of each exopodite overlies
+portions of the first two segments (second and third) of the
+corresponding endopodite. The coxopodites are not visible, but very
+nearly the full length of the first segment of the endopodite (the
+basipodite) is exposed. The first two visible segments (the first and
+second) extend just to the margin of the pleural lobe, while the other
+four extend beyond the dorsal cover. The segments decrease in length
+outward, but not regularly, the meropodite being generally longer than
+the ischiopodite or the carpopodite. The terminal segment
+(dactylopodite) is short and bears short sharp hair-like spines which
+articulate in sockets at the distal end. On this specimen the anterior
+limb on the right side shows one terminal spine, the second endopodite
+on that side has two, and two of the endopodites on the left-hand side
+preserve two each. The segments of the limbs are nearly cylindrical,
+but the ischiopodites and meropodites of several of the endopodites
+show rather deep longitudinal grooves which appear to be rather the
+result of the shrinkage of the thin test than natural conformations.
+
+The endopodites on the left-hand side have a number of short, sharp,
+movable, hair-like spines, and cup-shaped depressions which are the
+points of insertion of others. On the distal end of the carpopodite of
+the first thoracic segment there seems to have been a spine, whose
+place is now shown by a pit. This same endopodite shows, rather
+indistinctly, three pits in the groove of the carpopodite, and the
+propodite has two. On the endopodite of the second appendage on this
+side, both the carpopodite and propodite possess a fine hair-like
+articulated spine at the distal end, that of the propodite arising on
+the dorsal and that of the carpopodite on the posterior side. On the
+dorsal side of the carpopodite there are three pits for the
+articulation of spines, and on the propodite, one.
+
+The exopodites belonging to the thoracic segments are of equal length
+with the endopodites, and while the proximal portion of each is
+stouter than that of the corresponding endopodite, the exopodites
+taper to a hair-like termination, while the endopodites remain fairly
+stout to the distal segment. Most of the setæ of the exopodites have
+been removed, so that each remains as a curving, many-segmented organ,
+transversely striated, with a continuous groove along the posterior
+side. The setæ appear to be set in this groove, one for each of the
+transverse ridges on the shaft.
+
+A good deal of the test has been cut away on the left-hand side from
+the thorax and pygidium, and the appendages exposed from above. Enough
+of the dorsal shell has been cut away so that the anal opening is
+exposed, and directly behind the pygidium, on the median line, is a
+bilaterally symmetrical plate with serrated edges which appears to be
+the appendage of the anal segment. (See pl. 3, fig. 4.)
+
+_Measurements:_ The specimen is 17 mm. long, and 8 mm. in greatest
+width (at the back of the cephalon). From the median tubercle to the
+outer edge of the pleuron of the second thoracic segment the distance
+is 3.7 mm. From the point of articulation to the distal end of the
+spines on the dactylopodite of the second endopodite on the right-hand
+side is 4.3 mm. The basipodite of this appendage is 1.5 mm. long, the
+ischiopodite 1 mm. long, the meropodite 1.2 mm. long, the carpopodite
+0.5 mm. long, the propodite 0.35 long, and the dactylopodite 0.15 mm.
+long. On the left-hand side the endopodite of the first segment
+projects 3 mm. beyond the pleuron, the second, 3.2 mm. At the back the
+appendages extend a maximum distance of 2.5 mm. behind the pygidium.
+The median spinose process of the anal segment extends 0.75 mm. behind
+the pygidium, and is 1.6 mm. in greatest width.
+
+
+Specimen No. 204 (pl. 3, fig. 1; pl. 4, fig. 6; text fig. 42).
+
+ Illustrated: Amer. Jour. Sci., vol. 13, 1902, pl. 2, figs. 4, 5
+ (reproduced from photographs).
+
+This specimen, which is developed from the dorsal surface, shows
+especially well nine appendages of the left side. The first represent
+the last segment of the cephalon; the remainder belong to the thorax.
+As is usual, the exopodites of these appendages overlie and curve
+behind the endopodites. All the exopodites have lost their setæ and
+the segments of the endopodites are flattened by crushing. The
+endopodites, while retaining only one or two of the movable spines,
+have the cup-like bases of from two to four on each of the visible
+segments, namely, the meropodite, carpopodite, propodite, and, in one
+case, the dactylopodite. These appendages, although really marvellous
+in preservation, are of such small size and react so badly to light
+that their study is very difficult, and Professor Beecher, who had
+observed hundreds of specimens through all stages of the laborious
+process of cleaning the matrix from them, undoubtedly was much better
+equipped to interpret them than any other person.
+
+The drawing is made on the assumption that the appendages are
+displaced and all moved uniformly outward so that the distal ends of
+the coxopodites emerge from under the pleural lobe, whereas these ends
+would normally be under the dorsal furrow, and the distal end of the
+ischiopodite should reach the margin of the pleural lobe. While it
+seems very remarkable that it should happen, that all the appendages
+should be so moved that they would lie symmetrically a few millimeters
+from their normal position, nevertheless it is found on measuring that
+they bear the same proportion to the length and width that the
+appendages of other specimens do, thus indicating that Professor
+Beecher's interpretation of them was correct. I am unable, however, to
+see the coxopodites which he has drawn as articulating with the two
+branches of the limb.
+
+[Illustration: Fig. 42.--_Triarthrus becki_ Green. Appendages of
+specimen 204. Inked in by Miss Wood from the original tracing. × 10.]
+
+This individual shows, better than any other, the connection of the
+exopodite with the endopodite. Even though the coxopodites are gone,
+the two branches of each appendage remain together, showing that the
+basipodite as well as the coxopodite is involved in the articulation
+with the exopodite. Just what the connection is can not be observed,
+but there seems to be a firm union between the upper surface of the
+basipodite and the lower side of the proximal end of the exopodite, as
+indicated diagrammatically in text figure 33.
+
+_Measurements:_ The specimen is 20 mm. long and 9 mm. wide at the back
+of the cephalon. From the tubercle on the middle of the first segment
+of the thorax to the tip of the corresponding appendage the distance
+is 8 mm. The entire length of the exopodite of the first thoracic
+segment is 4.6 mm. The exopodite of the appendage belonging to the
+seventh segment is only 3.5 mm. long. The pleural lobe is 2.5 mm. wide
+at the front of the thorax.
+
+
+Specimen No. 205 (pl. 2, fig. 4).
+
+ Illustrated: Amer. Jour. Sci., vol. 13, 1902, pl. 5, figs. 2, 3
+ (photographs).
+
+This is a small imperfect specimen, developed from the ventral side.
+It retains the best preserved metastoma in the collection, but was
+used by Professor Beecher especially to illustrate the convergent
+ridges on the inside of the ventral membrane in the axial region of
+the thorax. These ridges are very low, and on each segment of the
+thorax there is a central one, outside of which is a pair which are
+convergent forward, making angles of 35 to 45 with the axis.
+
+The metastoma is shaped much like the hypostoma of an _Illænus_. It is
+convex, nearly semicircular, with the straight side forward, and there
+is a continuous raised border around the curved sides and back. This
+border is separated from the central convex body by a deep linear
+depression.
+
+The hypostoma is also rather well preserved and has a narrow, slightly
+elevated border at the sides and back.
+
+_Measurements:_ The incomplete specimen, from which only a very small
+portion of the length is missing, is 9 mm. long. The metastoma is 0.45
+mm. long and 0.58 mm. wide.
+
+
+Specimen No. 214 (pl. 1, fig. 2; pl. 3, fig. 6).
+
+This is a large specimen, developed from the ventral side. It shows
+the antennules and some other appendages of the head, but derives its
+special interest from the excellent preservation of a few of the
+exopodites, which are turned back parallel to the axis of the body and
+lie within the axial lobe.
+
+The shaft of the exopodite is made up of numerous short segments which
+at their anterior outer angles are produced into spines, and which
+also bear movable spines along the anterior border. As shown in
+several other specimens, the exopodite ends in a more or less long
+spoon-shaped segment bearing on its lower surface a broad groove. No
+setæ appear to be attached to this, but both anterior and posterior
+margins bear numerous small, apparently movable spines. From the
+groove along the ventral side of the remainder of the exopodite arise
+numerous long slender filaments which become progressively shorter
+toward the tip. This specimen shows that they are not cylindrical, but
+are flattened along opposite faces, at least at their distal ends.
+While no connection can be seen between adjacent setæ, they seem to
+stay together like the barbs on a feather.
+
+_Measurements:_ Length, 33 mm., width at back of cephalon, 16 mm.;
+from front of cephalon to back of hypostoma, 6 mm.
+
+
+Specimen No. 219 (pl. 2, fig. 6; pl. 4, fig. 4).
+
+ Illustrated: Amer. Jour. Sci., vol. 13, 1902, pl. 4, fig. 1, pl. 5,
+ fig. 4 (photograph and drawing).
+
+The endopodites of most of the appendages of the thorax are well
+shown, and occasional portions of exopodites. The coxopodites are
+long, flattened, and do not taper much. The anterior and posterior
+edges of the basipodites of the endopodites of the first two segments
+are approximately parallel, but on the succeeding endopodites the
+basipodites and ischiopodites are triangular in form, with the apex
+backward. In successive endopodites toward the posterior end, the
+angle made by the backward-directed sides of the basipodites becomes
+increasingly acute, so that in some of the posterior appendages this
+segment is wider than long. The ischiopodite shows a similar increase
+of width and angularity on successive segments, and the meropodites
+and carpopodites also become wider on the posterior segments, and even
+triangular in outline toward the back of the thorax and on the
+pygidium.
+
+Along the median portion of the axial lobe the specimen has been
+cleaned until the inner side of the ventral membrane was reached. Here
+the test shows on the inner surface at each segment of the thorax a
+series of low ridges which are roughly parallel to the axial line, but
+which really converge in an anterior direction. Between the ridges
+are shallow canoe-shaped depressions, which have the appearance of
+areas for the insertion of muscles.
+
+_Measurements:_ Length, 31 mm.; width at back of head, 15 mm.;
+distance, in a straight line, from point of insertion of the right
+antennule to its tip, 14.25 mm.; it projects 12 mm. beyond the
+cephalon.
+
+
+Specimen No. 218 (pl. 6, fig. 3; text fig. 43).
+
+This specimen is a large one, developed from the lower side, but
+retains only the endopodites of a few appendages. The cephalon and
+anterior portion of the thorax are missing.
+
+Professor Beecher had a drawing made to show the appendages on the
+right-hand side of the last two segments of the thorax, seen of course
+from the ventral side. This drawing shows well the broadening of the
+basipodite, ischiopodite, and meropodite, while the coxopodite is
+thick and heavy, and the inner end of the gnathobase somewhat rugose.
+Almost every segment of the endopodites has one or more pits for
+insertion of spines, these being along the anterior or posterior
+margins. The exopodites lack the setæ, but show no unusual features.
+
+[Illustration: Fig. 43.--_Triarthrus becki_ Green. Drawing to
+represent the writer's interpretation of the appendages of specimen
+218. Drawn by Miss Wood. × 10.]
+
+
+Specimen No. 222 (pl. 4, fig. 5).
+
+ Illustrated: Amer. Jour. Sci., vol. 47, 1894, pl. 7, fig. 3
+ (drawing).
+
+A small specimen, developed from the lower side, and used by Professor
+Beecher to illustrate the form of the segments of the endopodites of
+the pygidium. In addition to this, it shows very well the form of the
+endopodites of the thorax. All of the appendages on the specimen are
+shifted to the left of their normal position. This specimen differs
+from most of the others in that the segments of the endopodites do not
+lie with their greatest width in the horizontal plane, but were
+embedded vertically, with the posterior edge downward. From this
+circumstance they retain their natural shape, and it is seen that they
+are naturally flattened, with about the same thickness in proportion
+to length and width as in some of the modern isopods (Serolis, for
+instance). In even the most anterior of these endopodites (that of the
+second segment) the ischiopodite, meropodite, and carpopodite are
+triangular in shape, with the point backward, but in all the
+endopodites at the anterior end of the thorax, the triangle has a very
+obtuse angle at the apex, and the base is much longer than the
+perpendicular. On the other hand, those of the pygidium, which were
+figured by Beecher, have a number of short wide segments, all wider
+than long, and, excepting the dactylopodites, triangular in form.
+
+_Measurements:_ Length, 8.75 mm.; width at back of cephalon,
+about 4 mm.
+
+
+Specimen No. 230 (pl. 5, fig. 3; text fig. 44).
+
+ Illustrated: Amer. Jour. Sci., vol. 47, 1894, pl. 7, fig. 2
+ (drawing); Ibid., vol. 13, 1902, pl. 2, fig. 2.
+
+[Illustration: Fig. 44.--_Triarthrus becki_ Green. Appendages of the
+posterior part of the thorax and pygidium of specimen 230. Inked by
+Miss Wood from a tracing made under the direction of Professor
+Beecher.]
+
+An entire specimen of medium size, developed from the ventral side. It
+seems to have been the first one to yield to Professor Beecher any
+satisfactory knowledge of the appendages of the pygidium. There are
+five endopodites, all on one side, which appear to belong here. The
+segments in this region are characterized by their short, wide,
+triangular form. At the apex of each is a small tuft of spines or
+short hairs, and the ventral surfaces of some of the endopodites show
+pits for the insertion of spines.
+
+_Measurements:_ Length, 21 mm.; width at back of cephalon, 10 mm.
+
+
+
+
+Cryptolithus tessellatus Green.
+
+
+Specimen No. 233 (pl. 7, fig. 1; text fig. 45).
+
+This is the best preserved entire specimen. It is developed from the
+lower side, and shows the hypostoma, antennules, and a few fragmentary
+appendages of the cephalon, the outer portions of the exopodites of
+thorax and pygidium on both sides, and the endopodites on the left
+side.
+
+The hypostoma is imperfectly preserved and is turned completely
+around, so that the anterior margin is directed backward, and the
+posterior one is so much in the shadow that it does not show well in
+any of the photographs. The form is, however, essentially like that
+of _Trinucleoides reussi_ (Barrande), the only other trinucleid of
+which the hypostoma is known, except that the border does not extend
+so far forward along the sides, and it is much smaller.
+
+The antennules are not inserted close to the hypostoma, as in
+Triarthrus, but at some distance from it, and, as nearly as can be
+determined, directly beneath the antennal pits which are seen near the
+front of the glabella in many species of trinucleids.
+
+[Illustration: Fig. 45.--_Cryptolithus tessellatus_ Green. Drawing of
+specimen 233, made by Professor Beecher. × 9. Below are parts of two
+of the endopodites of specimen 236, showing the interarticular
+membranes. × 41.]
+
+The antennules are long, and are composed of far fewer and longer
+segments than those of Triarthrus. In this specimen they converge
+backward, cross each other and at the distal end are more or less
+intertwined.
+
+As is shown in the drawing and photograph, very little can be learned
+from this individual about the other appendages of the cephalon. A few
+fragments of exopodites on either side suggest that these members
+pointed forward and were much like those in Triarthrus, but nothing
+conclusive is shown.
+
+The exopodites and endopodites of the left side of the thorax are best
+preserved. The exopodites are above the endopodites, and only that
+portion exposed from the ventral side which projects beyond the line
+at which the endopodites bend backward. The endopodite on the left
+side of the first thoracic segment is the best preserved. It shows
+seven segments, the outer ones best. The coxopodite is short and
+narrow, the basipodite somewhat heavier and longer, while the
+carpopodite and propodite are the widest and strongest segments. The
+propodite is triangular and flattened, like the segments on the middle
+and posterior part of the thorax of Triarthrus. At the inner end of
+the ischiopodite and meropodite are tufts of spines pointing inward
+and backward. These are not shown on any of the photographs, but may
+be seen with the light striking the specimen at the proper angle.
+
+It is not possible to count the exact number of limbs, but one gets
+the impression that on the left side of this specimen there are
+twenty-one sets of appendages, six of which of course belong to the
+thorax. On the thorax and anterior part of the pygidium, successive
+endopodites show the propodites and dactylopodites becoming
+progressively more slender and shorter, while the ischiopodites,
+meropodites and carpopodites become shorter and more triangular, and
+with increasingly large numbers of short spines on their posterior
+borders. Back of the fourth endopodite on the pygidium it is not
+possible to make out the detail, but the appearance is of an
+endopodite consisting of short broad segments fringed at the back with
+short spines, the ones at the very posterior end appearing to be
+exceedingly short and rudimentary.
+
+The exopodites are not so well shown as in some others but the setæ
+are flattened and blade-shaped, and often bear numerous small spines.
+
+_Measurements:_ Length (lacking most of the fringe), 10.5 mm. Width of
+thorax, 10.5 mm. Length of hypostome, 1.41 mm., width at front, 1.46
+mm. The distance from back of fringe to end of antennules is 5.4 mm.
+If straightened out, the left antennule would be about 6.1 mm. long.
+In the first 3.1 mm., there are only ten segments, so that the average
+length of a segment is 0.31 mm. The distance from the inner end of the
+endobase of the first segment of the thorax to the outer end of the
+meropodite is 2.43 mm., and from that point to the end of the
+dactylopodite 2.47 mm. making the total length 4.90 mm. These
+measurements are taken from the photograph. Measurements taken from
+Professor Beecher's drawing, which was made with the camera-lucida,
+give a total length of 4.57 mm., the distance to the outer end of the
+meropodite being 2.3 mm. and thence to the tip of the dactylopodite
+2.27 mm. Detailed measurements of the segments, on the photograph, are
+as follows: coxopodite, 0.321 mm.; basipodite, 0.78 mm.; ischiopodite,
+0.68 mm.; meropodite, 0.642 mm.; carpopodite, 0.642 mm.; propodite,
+1.01 mm., dactylopodite, 0.825 mm.
+
+
+Specimen No. 235 (pl. 7, fig. 2; pl. 8, fig. 3; pl. 9, figs. 1, 2).
+
+ Illustrated: Amer. Jour. Sci., vol. 49, 1895, pl. 3, figs. 5, 6.
+
+Specimens 235 and 236 were originally parts of an entire
+_Cryptolithus_, but, as Professor Beecher has explained, the specimen
+was cut in two longitudinally on the median line, and the halves
+transversely just back of the cephalon, so that each now represents
+one half of a thorax and pygidium. Both halves have been cleaned from
+both upper and lower side, a perfectly marvelous piece of work, for
+the thickness is no greater than that of a thin sheet of paper, and
+the soft shale of the matrix has a very slight cohesive power.
+
+Both sides of specimen 235 were figured, but the dorsal side was
+apparently then somewhat less fully developed than at present. On
+plate 9 are two figures in which specimens 235 and 236 are brought
+together again, and both dorsal and ventral sides illustrated.
+
+On the dorsal side, specimen 235 shows portions of three exopodites
+which lie in a direction roughly parallel to the outer portions of the
+endopodites on the lower side, that is, their direction if projected
+would reach the axis in an acute angle back of the end of the
+pygidium. The setæ stand at right angles to the shaft, and on a
+portion of it 0.5 mm. long there are seven of them. This is a fragment
+of an exopodite near the front of the thorax, and the setæ, which are
+flattened, are about 1.63 mm. long.
+
+On the ventral side this same specimen shows incomplete endopodites
+and exopodites of about seventeen segments, six of which would belong
+to the thorax and the remainder to the pygidium. The greater part of
+the appendages belonging to the pygidium are exceedingly small (about
+0.15 mm. long) and so incompletely exposed that the structure can not
+be definitely made out.
+
+The endopodites of the thoracic segments all lack the greater part of
+their proximal segments and are all of practically the same form. They
+turn abruptly backward at the outer end of the meropodite, and the
+carpopodite of each is greatly widened, projects inward and is armed
+with tufts of spines. The propodite and dactylopodite are wide,
+flattened, and taper but slightly outward, the dactylopodite bearing
+on its distal end a tuft of spines. On several of the endopodites, the
+meropodites are visible and they bear on their inner ends fringes of
+spines pointing inward. Behind these well preserved appendages the
+proximal segments of several endopodites are visible, and a regular
+succession of flattened, oval bodies armed with numerous
+forward-pointing spines. These latter bodies Professor Beecher took to
+be leaf-like exopodites, which they certainly resemble, and as they
+lie beyond the line of endopodites they probably do belong to the
+outer halves of the appendages.
+
+The exopodites under the thorax are long, the shaft shows numerous
+short segments, and is in each case bent backward, though not through
+a right angle. They extend considerably beyond the endopodites. The
+setæ do not diverge from the shaft at a right angle as on the dorsal
+side of this same specimen, but at an acute angle, indicating that
+they were not rigid. The individual hairs are broad and blade-shaped,
+frequently with a linear depression along the median line, perhaps due
+to collapse of the internal tube.
+
+_Measurements:_ The greatest length of the fragment in its present
+state is 5 mm. The dactylopodite of the second endopodite (without
+terminal spines) is 0.18 mm. long, the propodite 0.23 mm. long and
+0.15 mm. wide; the carpopodite is 0.24 mm. long and 0.38 mm. wide. All
+measurements were made on the photographs.
+
+
+Specimen No. 236 (pl. 7, figs. 3-5; pl. 9, figs. 1, 2; text fig. 45).
+
+The right half of the same thorax and pygidium as specimen No. 235.
+
+The specimen is cleaned from both upper and lower sides and, the
+dorsal test being removed, reveals the long blade-like setæ of the
+exopodites, each blade being concave along its median line. They are
+long on the exopodites of the thoracic segments, but become shorter,
+without, however, any visible change of form on the pygidium. Although
+the posterior end is not well preserved, one gets no suggestion from a
+study of this side of the specimens that the exopodites of the
+posterior end are in any striking way different from those of segments
+further forward. The tips of some of the setæ show minute spines, one
+to each blade.
+
+On the ventral side are a number of endopodites, but they are more
+fragmentary than those of the other half of the specimen. Some of the
+exopodites are well shown, the blades being in all cases broken from
+the shaft. Two of the endopodites of this specimen are of especial
+interest, as they have interarticular membranes between the last three
+segments. Professor Beecher made a drawing of one of these which he
+placed under his pen drawing (text fig. 45).
+
+_Measurements:_ The specimen is 5 mm. long from the front of the
+second thoracic segment to the end of the pygidium. The setæ on the
+exopodites of the anterior thoracic segments are 1.7 mm. long, as
+exposed from the dorsal side. Some of those on the posterior part of
+the pygidium, only incompletely exposed, are 0.31 mm. long.
+
+[Illustration: Fig. 46. _Cryptolithus tessellatus_ Green. A part of a
+thorax and pygidium, showing appendages. Drawn by Professor Beecher.
+Specimen 238. × 10.]
+
+The dactylopodite of the first endopodite showing the articular
+membranes is 0.23 mm. long and 0.13 mm. wide. The propodite is of the
+same length and 0.17 mm. wide. The interarticular membrane between
+them is 0.066 mm. thick. The spines on the dactylopodite of this
+appendage are 0.15 mm. long. All measurements were made on
+photographs.
+
+
+Specimen No. 238 (pl. 8, fig. 4; text fig. 46).
+
+A triangular specimen consisting of the greater part of a pygidium and
+parts of all the thoracic segments. Under the thorax the specimen has
+been so cleaned that the outer portions of the endopodites are well
+shown, while under the pygidium the greater part of the endopodites
+seem to have been removed, disclosing the setæ of the exopodites. As
+in other specimens, the endopodites of the thorax turn backward at the
+distal end of the carpopodite, which is broad and curved, and bears a
+tuft of spines on the posterior margin. The dactylopodites seem to
+preserve their natural shape, and are very nearly cylindrical in form.
+Under the pygidium are several sets of overlapping fringes of setæ of
+exopodites, and along the edge of the dorsal furrow, a number of
+fragments of segments of what may be coxopodites while with them are a
+number of fragmentary shaft of exopodites.
+
+_Measurements:_ The pygidium is 3.3 mm. long, the thorax 3 mm.
+
+
+
+
+BIBLIOGRAPHY.
+
+
+Agassiz, L.
+
+ 1873.--Discovery of the basal joint of legs of trilobites. Amer. Nat.,
+ vol. 7, pp. 741-742.
+
+
+Angelina N. P.
+
+ 1854.--Palæontologia Scandinavica, pars 1, Crustacea formationis
+ transitionis.
+
+
+Audouin, J. V.
+
+ 1821.--Recherches sur les rapports naturels qui existent entre les
+ trilobites et les animaux articulés. Ann. Gen. Sci. Phys. Nat.
+ Bruxelles, vol. 8, p. 233, pl. 26. 1822. Isis (Encycl. Zeitung),
+ Oken., vol. 10, p. 87, pl. 1, No. 4, figs. 1-5.
+
+
+Barrande, J.
+
+ 1852.--Systême Silurien du centre de la Bohême, vol. 1, pp. 226-230,
+ and 629, pl. 30, figs. 38, 39.
+
+ 1872.--Ibid., vol. 1, Suppl., p. 180, pl. 4.
+
+
+Barth, Hermann von.
+
+ 1875.--Die Stellung der Trilobiten in zoologischen Systeme. Das
+ Ausland, 26. Jahrg., p. 2 5.
+
+
+Beecher, C. E.
+
+ 1893.--On the thoracic legs of _Triarthrus_. Amer. Jour. Sci. (3),
+ vol. 46, pp. 367-370, 467-470, text figs. 1-3.
+
+ 1894 A.--On the mode of occurrence, and the structure and development
+ of _Triarthrus becki_. Amer. Geol., vol. 13, pp. 38-43, pl. 3.
+
+ 1894 B.--The appendages of the pygidium of _Triarthrus_. Amer. Jour.
+ Sci. (3), vol. 47, pp. 298-300, pl. 7, text fig, 1.
+
+ 1895 A.--Further observations on the ventral structure of _Triarthrus_.
+ Amer. Geol., vol. 15, pp. 91-100, pls. 4-5.
+
+ 1895 B.--Structure and appendages of _Trinucleus_. Amer. Jour. Sci. (3),
+ vol. 49, pp. 307-311, pl. 3.
+
+ 1895 C.--The larval stages of trilobites. Amer. Geol., vol. 16,
+ pp. 166-197, pls. 8-10.
+
+ 1896 A.--The morphology of Triarthrus. Amer. Jour. Sci. (4), vol. 1,
+ pp. 251-256, pl. 8; Geol. Mag., dec. 4, vol. 3, pp. 193-197,
+ pl. 9.
+
+ 1896 B.--On a supposed discovery of the antennas of trilobites by
+ Linnæus in 1759. Amer. Geol., vol. 17, pp. 303-306, text figs.
+ 1-3.
+
+ 1897 A.--Outline of a natural classification of trilobites. Amer.
+ Jour. Sci. (4), vol. 3, pp. 89-106, 181-207, pl. 3.
+
+ 1897 B.--Remarks on Kingsley's "Systematic position of the
+ trilobites." Amer. Geol., vol. 20, pp. 38-40.
+
+ 1900.--Trilobita. Eastman-Zittel Text-book of Paleontology, vol. 1,
+ pp. 607-638, text figs. 1261-1331; ed. 2, 1913, p. 700. London.
+
+ 1901.--Structure and development of trilobites. In "Studies in
+ Evolution," pp. 109-225. New York and London.
+
+ 1902.--The ventral integument of trilobites. Amer. Jour. Sci. (4),
+ vol. 13, pp. 165-174, pls. 2-5, text fig. 1; Geol. Mag., dec. 4,
+ vol. 9, pp. 152-162, pls. 9-11, text figs. 1-8.
+
+
+Bernard, H. M.
+
+ 1892.--The Apodidæ.
+
+ 1893.--Trilobites with antennæ at last! Nature, vol. 48, p. 582.
+
+ 1894.--The systematic position of the trilobites. Quart. Jour. Geol.
+ Soc., London, vol. 50, pp. 411-434, text figs. 1-17.
+
+ 1895 A.--The zoological position of the trilobites. Science Prog.,
+ vol. 4, pp. 33-49.
+
+ 1895 B.--Supplementary notes on the systematic position of the
+ trilobites. Quart. Jour. Geol. Soc., London, vol. 51,
+ pp. 352-360, figs. A-C.
+
+
+Beyrich, E.
+
+ 1846.--Untersuchungen ueber Trilobiten. 2. Stück, p. 30, pl. 4,
+ fig. 1c.
+
+
+Billings, E.
+
+ 1870.--Notes on some specimens of Lower Silurian trilobites. Quart.
+ Jour. Geol. Soc., London, vol. 26, pp. 479-486, pls. 31-32.
+ Abstract in Geol. Mag., vol. 7, p 291, and Nature, vol. 2, p. 94.
+
+
+Brongniart, A.
+
+ 1822.--Histoire naturelle des crustacés fossiles. Paris.
+
+
+Brünnich, F. E.
+
+ 1781.--Beskrivelse over trilobiten, en dyreslaegt og dens arter, med
+ en ney arts aftegning. Nye Samlig of det Kong. Danske Vidensk.
+ Selskabs. Skriften, Copenhagen.
+
+
+Burling, L. D.
+
+ 1916.--Pædeumias and the Mesonacidæ, with description of a new
+ species, having at least 44 segments, from the Lower Cambrian of
+ British Columbia. Ottawa Nat., vol. 30, pp. 53-58, pl. 1.
+
+ 1917.--Was the lower Cambrian trilobite supreme? Ibid., vol. 31,
+ pp. 77-79, text figs. 1-2.
+
+
+Burmeister, H.
+
+ 1843.--Die Organisation der Trilobiten. Berlin.
+
+ 1846.--The organization of trilobites, deduced from their living
+ affinities. Eng. translation, Ray Society, London.
+
+
+Calman, W. T.
+
+ 1909.--Crustacea, in "A treatise on zoology," edited by Sir Ray
+ Lankester. London.
+
+ 1919. Dr. C. D. Walcott's researches on the appendages of trilobites.
+ Geol. Mag., dec. 6, vol. 6, pp. 359-363, pl. 8, text fig. 1.
+
+
+Carpenter, G. H.
+
+ 1903.--On the relationships between the classes of Arthropoda. Proc.
+ Roy. Irish Acad., vol. 24, pp. 320-360, pl. 6.
+
+
+Castelnau, F. DE.
+
+ 1843.--Systeme Silurien de l'Amérique Septentrionale, p. 15, pl. 2,
+ figs. 1, 4.
+
+
+Clarke, J. M.
+
+ 1888.--The structure and development of the visual area in the
+ trilobite, _Phacops rana_ Green. Jour. Morph., vol. 2, pp. 253-270,
+ pl. 1.
+
+
+Crampton, G. C.
+
+ 1916.--The phylogenetic origin and the nature of the wings of insects,
+ according to the paranotal theory. Jour. New York Entomol. Soc.,
+ vol. 24, pp. 1-39, pls. 1, 2.
+
+ 1919.--The evolution of the arthropods and their relatives, with
+ especial reference to insects. Amer. Nat, vol. 53, pp. 143-179.
+
+
+Dalman, J. W.
+
+ 1826.--Om Palæaderna eller de så kallade Trilobiterna. Stockholm,
+ Acad. Handl., pp. 113-152, 226-294.
+
+ 1828.--Ueber die Palæaden, oder die sogennanten Trilobiten. Nuremberg.
+
+
+Dana, J. D.
+
+ 1871.--On the supposed legs of the trilobite, _Asaphus platycephalus_.
+ Amer. Jour. Sci. (3), vol. 1, pp. 320-321, 386; Ibid. (3), vol.
+ 3, 1872, pp. 221-222. Also printed in Ann. Mag. Nat. Hist, vol.
+ 7, 1871, pp. 366, 451.
+
+
+Dekay, J. E.
+
+ 1824.--Observations on the structure of trilobites, and description of
+ an apparently new genus. Ann. Lye. Nat. Hist. New York, vol. I,
+ p. 174, 2 pls.; Isis (Encycl. Zeit.), Oken, 1825 and 1832.
+
+
+Dollo, L.
+
+ 1910.--La paléontologie éthologique. Bull. Soc. Beige de Geol., Pal.,
+ et d'Hydrol., vol. 23, pp. 377-421, figs. 1-13, pls. 7-11.
+
+
+Eichwald, E. VON.
+
+ 1825.--Geognostico-zoologicæ per Ingriam Marisque Baltici Provincias
+ nee non de trilobitis observationes. Section 45.
+
+ 1858.--Beiträge zur geographischen Verbreitung der fossilen Thiere
+ Russlands. Bull. Soc. Imp. des Natural, de Moscou, vol. 30,
+ 1855-1857, p. 204.
+
+ 1860.--Lethæa Rossica, pl. 21.
+
+ 1863.--Beiträge zur nähern Kenntniss der in meiner Lethæa Rossica
+ beschriebenen Ilænen. Bull. Soc. Imp. des Natural, de Moscou,
+ vol. 36, p. 408.
+
+
+Emmrich, H. F.
+
+ 1839.--De trilobitis dissertatio petrefactologica, etc. Berlin.
+
+
+Exner, S.
+
+ 1891.--Die Physiologic der facettirten Augen von Krebsen und Insecten.
+ Leipzig and Vienna. Pp. 33-35, pl. 2, figs. 18-19.
+
+
+Finch, G. E.
+
+ 1904.--Notes on the position of the individuals in a group of _Nileus
+ vigilans_ found at Elgin, Iowa. Proc. Iowa Acad. Sci. for 1903,
+ vol. 11, pp. 179-181, pl. 14.
+
+
+Gegenbaur, C.
+
+ 1878.--Elements of comparative anatomy. Eng. ed. (Bell and Lankester).
+ London.
+
+
+Goldfuss, A.
+
+ 1828.--Observation sur le place qu'occupent les trilobites dans le
+ règne animal. Ann. Sci. Nat., Zoologie, vol. 15, p. 83, pl. 2,
+ figs. 5, 7, 9, 10.
+
+
+Green, J.
+
+ 1839 A.--The inferior surface of the trilobite discovered. The Friend,
+ Philadelphia, March 16.
+
+ 1839 B.--The inferior surface of the trilobite discovered.
+ Illustrated, with colored models. Philadelphia.
+
+ 1839 C.--Remarks on the trilobites. Amer. Jour. Sci. (1), vol. 37,
+ p. 25 _et seq._
+
+ 1840.--An additional fact, illustrating the inferior surface of
+ _Calymene bufo_. Ibid., vol. 38, p. 410.
+
+
+Handlirsch, A.
+
+ 1906.--Ueber Phylogenie der Arthropoden. Verhandl. d. k. k. zool.-bot.
+ Gesell., Vienna, Jahrg. 1906, pp. 88-103.
+
+ 1907.--Functionswechsel einiger Organe bei Arthropoden. Ibid., Jahrg.
+ 1907, pp. 153-158.
+
+ 1908.--Die fossilen Insekten. Leipzig.
+
+ 1914.--Eine interessante Crustaceenform aus der Trias der Vogesen.
+ Verhandl. d. k. k. zool.-bot. Gesell., Vienna, Jahrg. 1914,
+ pp. 1-7, pls. 1, 2.
+
+
+Hawle, I., and Corda, A. J. C.
+
+ 1847.--Prodrom einer Monographie der boehmischen Trilobiten, pp. 9,
+ 24, 56, pl. 2, fig. 10; pl. 3, fig. 15; pl. 4, fig. 33b-g.
+
+
+Jaekel, O.
+
+ 1901.--Beiträge zur Beurtheilung der Trilobiten, Theil I. Zeits. d.
+ deutsch. geol. Gesell., Bd. 53, pp. 133-171. Pis. 4-6, text
+ figs. 1-30.
+
+
+Kingsley, J. S.
+
+ 1897.--The systematic position of the trilobites. Amer. Geol.,
+ vol. 20, pp. 33-38.
+
+
+Koenen, A. von.
+
+ 1872.--Ueber die Organisation der Trilobiten. Verhandl. d. naturhist.
+ Ver. d. preuss. Rheinl. u. Westphalen, vol. 29, C, pp. 93-95.
+
+ 1880.--Ueber die Unterseite der Trilobiten. Neues Jahrb. f. Min..,
+ Geol., u. Pal,, Bd. 1, pp. 430-432. pl. 8.
+
+
+Lang, A.
+
+ 1891.--Text-book of comparative anatomy, Eng. ed. (Bernard). London.
+
+
+Lankester, E. R.
+
+ 1881.--Observations and reflections on the appendages and on the
+ nervous system of _Apus cancriformis_. Quart. Jour. Micros. Soc.,
+ vol. 21, pp. 343-376.
+
+
+Laurie, M.
+
+ 1911.--A reconstructed trilobite. Nature, vol. 88, p. 26.
+
+
+Lindstroem, G.
+
+ 1901.--Researches on the visual organs of the trilobites. K. svenska
+ Vet.-Akad. Handl., new ser., vol. 34, pp. 1-86, pls. 1-6.
+
+
+Linné, K.
+
+ 1759.--Petrificatet entomolithus paradoxus sådant, som det finnes uti
+ Hans Excellence Riks. Rådets Högoälborne Herr Grefve C. G.
+ Tessins Samling. K. svenska Vet.-Akad. Handl., vol. 20, pp. 21,
+ 22, pl. 1, fig. 1.
+
+
+Matthew, W. D.
+
+ 1893.--On antennæ and other appendages of _Triarthrus becki_. Amer.
+ Jour. Sci. (3), vol. 46, pp. 121-125, pl. 1; Trans. New York
+ Acad. Sci., vol. 12, pp. 237-241, pl. a.
+
+
+McCoy, F.
+
+ 1846.--A synopsis of the Silurian fossils of Ireland, p. 42.
+
+
+Mickleborough, J.
+
+ 1883.--Locomotory appendages of trilobites. Jour. Cincinnati Soc. Nat.
+ Hist., vol. 6, pp. 200-204; Geol. Mag., dec. 3, vol. 1, 1884,
+ pp. 80-84; Amer. Jour. Sci. (3), vol. 27, 1884, p. 409. Reviewed
+ by Dames, Neues Jahrb. f. Min., Geol., u. Pal., Bd. 1, 1885,
+ p. 477.
+
+
+Miller, S. A.
+
+ 1880.--Silurian ichnolites, with definitions of new genera and
+ species. Jour. Cincinnati Soc. Nat. Hist, vol. 2, pp. 217-218,
+ fig.
+
+
+Milne-Edwards, H.
+
+ 1881.--Compte rendu des nouvelles recherches de M. Walcott relatives à
+ la structure des trilobites, suivi de quelques considérations sur
+ l'interprétation des faits ainsi constatés. Ann. Sci. Nat,
+ Zoologie, ser. 6, vol. 12, pp. 1-33, pls. 10-12. Paris.
+
+
+Moberg, J. C.
+
+ 1902.--Bidrag till Kännedomen om trilobiternas byggnad. Geol. Fören
+ Förhandl., Bd. 24, pp. 295-302; pl. 3, text fig. 1.
+
+ 1907.--Om ett gätfultt fossil frän sveriges olenidskiffer samt en kort
+ ofversigt af viktigase data rorande trilobiternas ventrala
+ skelettdelar. Ibid., Bd. 29, Heft 5, pp. 265-272, pl. 4, fig. 2;
+ pl. 5, fig. 1.
+
+
+Oehlert, D. P.
+
+ 1896.--Résumé des derniers travaux sur l'organisation et le
+ developpement des trilobites. Bull. Soc. Géol. France, ser. 3,
+ vol. 24, pp. 97-116, text figs. 1-34.
+
+
+Packard, A. H.
+
+ 1872.--On the development of _Limulus polyphemus_. Mem. Boston Soc.
+ Nat. Hist., vol. 2, pp. 155-202, pls. 3-5.
+
+ 1880.--The structure of the eye of trilobites. Amer. Nat., vol. 14,
+ pp. 503-508.
+
+ 1882.--On the homologies of the crustacean limb. Ibid., vol. 16,
+ pp. 785-799, figs. 11, 12.
+
+
+Pander, C.
+
+ 1830.--Beiträge zur Geognosie des russischen Reiches. St. Petersburg.
+
+
+Peach, B. N.
+
+ 1882.--On some fossil myriopods from the Lower Old Red Sandstone of
+ Forfarshire. Proc. Roy. Physical Soc., Edinburgh, vol. 7,
+ pp. 177-187, pl. 2.
+
+ 1899.--O some new myriopods from the Palæozoic rocks of Scotland.
+ Ibid., vol. 14, pp. 113-126, pl. 4.
+
+
+Quenstedt, A.
+
+ 1837.--Beitrag zur Kenntniss der Trilobiten, mit besonderer Rücksicht
+ auf ihre bestimmte Gliederzahl. Archiv f. Naturg., Berlin, 3.
+ Jahrg., 1 Bd., pp. 337-352.
+
+
+Raymond, P. E.
+
+ 1910.--On two new trilobites from the Chazy near Ottawa, Ontario.
+ Ottawa Nat., vol. 24, pp. 129-134, pl. 2.
+
+ 1917.--Beecher's classification of trilobites, after twenty years.
+ Amer. Jour. Sci. (4), vol. 43, pp. 196-210, text figs. 1-3.
+
+
+Raymond, P. E., and Barton, D. C.
+
+ 1913.--A revision of the American species of _Ceraurus_. Bull. Mus.
+ Comp. Zool., vol. 54, pp. 525-543. pls. 1, 2, 3 text figs. 1-3.
+
+
+Reed, F. R. C.
+
+ 1916.--The genus _Trinucleus_. Pt. 4. Geol. Mag., dec. 6, vol. 3,
+ pp. 121, 122.
+
+
+Richter, R.
+
+ 1848.--Bitrag zur Palæeontologie des Thüringer Waldes. Dresden and
+ Leipzig.
+
+
+Ringueberg, E. N. S.
+
+ 1886.--A trilobite track illustrating one mode of progression of the
+ trilobites. Proc. Amer. Assoc. Adv. Sci., vol. 35, p. 228
+ (abstract only).
+
+
+Ruedemann, R.
+
+ 1916.--The presence of a median eye in trilobites. Bull. New York
+ State Mus., No. 189. pp. 127-143, pls. 34-36.
+
+
+Schlotheim, E. F. von.
+
+ 1823.--Nachträge zur Petrefactenkunde, II. Gotha.
+
+
+Six, Achille.
+
+ 1884.--Les appendices des trilobites d'après M. Ch. D. Walcott. Ann.
+ Soc. Geol. du Nord, vol. 11, pp. 228-236.
+
+
+Spencer, W. K.
+
+ 1903.--The hypostomic eyes of trilobites. Geol. Mag., dec. 4, vol. 10,
+ pp. 489-492.
+
+
+Staff, Hans v., and Reck, Hans.
+
+ 1911.--Ueber die Lebensweise der Trilobiten. Eine
+ entwicklungsmechanische Studie. Gesell. naturforsch. Freunde,
+ Sitzb., pp. 130-146, figs. 1-20.
+
+
+Sternberg, K. M.
+
+ 1830.--Ueber die Gliederung und die Füsse der Trilobiten. Isis
+ (Encycl. Zeitung), Oken, p. 516, pl. 5, figs. 1-3.
+
+
+Stokes, C.
+
+ 1823.--On a trilobite from Lake Huron. Trans. Geol. Soc., London, ser.
+ 2, vol. 1, p. 208, pl. 27.
+
+
+Swinnerton, H. H.
+
+ 1919.--The facial suture of the trilobite. Geol. Mag., dec. 6, vol. 6,
+ pp. 103-110.
+
+
+Törnquist, S. L.
+
+ 1896 A.--On the appendages of trilobites. Ibid., dec. 4, vol. 3,
+ p. 142.
+
+ 1896 B.--Linnæus on the appendages of trilobites. Ibid., pp. 567-569.
+
+
+Tothill, J. D.
+
+ 1916.--The ancestry of insects, with particular reference to chilopods
+ and trilobites. Amer. Jour. Sci. (4), vol. 42, pp. 373-383. text
+ figs. 1-8.
+
+
+Troedsson, G. T.
+
+ 1918.--Om skanes Brachiopodskiffer. Lunds Universitets Arsskrift, n.
+ f., Avd. 2, Bd. 15, Nr. 3. pp. 57-67, pl. 1, figs. 19-24.
+
+
+Valiant, W. S.
+
+ 1901.--Appendaged trilobites. The Mineral Collector, vol. 8, No. 7,
+ pp. 105-112.
+
+
+Volborth, A. von.
+
+ 1858.--Ueber die Bewegungs-Organe der Trilobiten. Verhandl. russ. k.
+ mineral. Gesell. zu St Petersburg, 1857-1858, p. 168.
+
+ 1863.--Ueber die mit glatten Rumpfgliedern versehenen russischen
+ Trilobiten, nebst einem Anhange ueber die Bewegungs-organe und
+ ueber das Herz derselben. Mem. Acad. Imp. Sci. St. Petersburg,
+ ser. 7, vol. 6, No. 2, pp. 44-47, pl. 1, fig. 12.
+
+ 1866.--Ueber Herrn von Eichwald's Beitrag zu näheren Kenntniss der
+ Illænen. Bull. Soc. Imp. des Natural, de Moscou, vol. 39, p. 40.
+
+
+Wahlenberg, G.
+
+ 1821.--Petrificata telluris Suecana examinata a Georgio Wahlenberg.
+ Nova Acta Reg. Soc. Scient. Upsala, vol. 8.
+
+
+Walcott, C. D.
+
+ 1875.--Description of the interior surface of the dorsal shell of
+ _Ceraurus pleurexanthemus_ Green. Ann. Lye. Nat. Hist. New York,
+ vol. II, pp. 159-162, pl. 11.
+
+ 1876.--Preliminary notice of the discovery of the natatory and
+ branchial appendages of trilobites. 28th Rept. New York State
+ Mus. Nat. Hist., adv. sheets, pp. 89-92; published as full report
+ in 1879.
+
+ 1877.--Notes on some sections of trilobites. 31st Rept. New York State
+ Mus. Nat. Hist., adv. sheets, pp. 61-63, pl. 1; published as full
+ report in 1879. Reviewed by Dames, Neues Jahrb. f. Min., Geol.,
+ u. Pal., Bd. 1, 1880, p. 428.
+
+ 1879.--Notes upon the legs of trilobites. 31st Rept. New York State
+ Mus. Nat. Hist., adv. sheets, p. 64.
+
+ 1881.--The trilobite: New and old evidence relating to its
+ organization. Bull. Mus. Comp. Zool., vol. 8, pp. 192-224,
+ pls. 1-6.
+
+ 1884.--The appendages of the trilobite. Science, vol. 3, pp. 276-279,
+ figs. 1-3. Reviewed by Dames, Neues Jahrb. f. Min., Geol., u.
+ Pal.., Bd. 1, 1885, Referate, p. 102.
+
+ 1894.--Note of some appendages of the trilobites. Proc. Biol. Soc.
+ Washington, vol. 9, pp. 89-97, pl. 1; Geol. Mag., dec. 4, vol. 1,
+ pp. 246-251, pl. 8.
+
+ 1911.--Middle Cambrian Merostomata. Smithson. Misc. Coll., vol. 57,
+ No. 2, pp. 17-40, pls. 2-7.
+
+ 1912 A.--Middle Cambrian Branchiopoda, Malacostraca, Trilobita, and
+ Merostomata. Ibid., No. 6, pp. 145-228, pls. 24-34, text figs.
+ 8-10.
+
+ 1912 B.--New York Potsdam-Hoyt fauna. Ibid., No. 9, pp. 251-304,
+ pls. 37-49.
+
+ 1913.--Eastman-Zittel Text-book of Paleontology, ed. 2, vol. 1,
+ figs. 1343, 1376, 1377.
+
+ 1916.--Ann Rept., Secretary Smithsonian Inst, for 1915, pl. 9.
+
+ 1918.--Appendages of trilobites. Smithsonian Misc. Coll., vol. 67,
+ No. 4, pp. 115-226, pls. 14-42.
+
+
+Watase, S.
+
+ 1890.--On the morphology of the compound eyes of arthropods. Johns
+ Hopkins Univ., Studies from Biol. Lab., vol. 4, no. 6, p. 290
+ (footnote).
+
+
+Woodward, H.
+
+ 1870.--Note on the palpus and other appendages of _Asaphus_, from
+ the Trenton limestone, in the British Museum. Quart. Jour. Geol.
+ Soc., London, vol. 26, pp. 486-488, fig. 1. Abstract in Geol.
+ Mag., dec. 1, vol. 7, p. 292, also in Nature, vol. 2, p. 94.
+
+ 1871.--On the structure of trilobites. Geol. Mag., dec. 1, vol. 8,
+ pp. 289-294, pl. 8.
+
+ 1884.--Notes on the appendages of trilobites. Geol. Mag., dec. 3,
+ vol. 1, pp. 162-165, 2 text figs.
+
+ 1895.--Some points in the life history of the Crustacea in early
+ Palæozoic times. Quart. Jour. Geol. Soc., London, vol. 51,
+ pp. lxx-lxxxviii, 1 pl.
+
+
+
+ * * * * *
+
+
+PLATE 1.
+
+Photographs of _Triarthrus becki_, made by C. E. Beecher.
+
+Fig. 1. Specimen 213. The dorsal test has been removed from the
+glabella, revealing the outline of the posterior end of the hypostoma,
+the proximal ends of the antennules, the gnathites, and incomplete
+endopodites of some appendages, × 5.43.
+
+
+Fig. 2. Specimen 214. The head of a complete large specimen. Part of
+the thorax is shown on pl. 3, fig. 6. Note especially the form of the
+segments of the endopodites and of the anterior coxopodite on the
+right side, × 7.33.
+
+Fig. 3. Specimen 217. This specimen shows better than any other the
+form of the gnathites of the cephalon. Note also the setæ of the
+exopodites under the cheek at the right. The appearance of a hook on
+the posterior gnathite on the right may be accidental, but it does not
+show broken edges, × 6.85.
+
+Fig. 4. Specimen 215. The ventral side of the cephalon of a small
+entire specimen. Shows well the form of some of the gnathites and a
+few of the endopodites. Note the unusual position of the antennules. ×
+7.63.
+
+Fig. 5. Specimen 226. This specimen did not photograph well, but is
+important as showing the exopodites and endopodites emerging from
+under the cephalon. × about 6.
+
+
+PLATE I.
+
+
+HELIOTYPE CO. BOSTON
+
+ * * * * *
+
+
+PLATE 2.
+
+Photographs of _Triarthrus becki_, made by C. E. Beecher.
+
+Fig. 1. Specimen 201. The entire specimen, details of which are shown
+in pl. 3, fig. 4 and pl. 4, figs. 1, 2. The dorsal test has been
+removed from the anterior segments on the right side. × 4.12.
+
+Fig. 2. Specimen 206. A small individual with the endopodites, and the
+exopodites minus their setæ; well preserved on the left side. Note the
+position of the antennules. The course of the facial suture is
+unusually well shown. × 10.
+
+Fig. 3. Specimen 210. The specimen which served as the main basis for
+Professor Beecher's first figure of the appendages of the thorax,
+specimen 206 (fig. 2, this plate) having supplemented it. Note the
+"normal" position of the antennules and the extension of the
+appendages from beneath the pleural lobe. Specimens with the
+antennules in this position may possibly be males. × 4.
+
+Fig. 4. Specimen 205. A small specimen with some of the appendages
+preserved, especially toward the posterior end, but particularly
+valuable for the unusually well preserved metastoma. × 11.
+
+Fig. 5. Specimen 211. A small cephalon, cleaned from the ventral side,
+and showing well the gnathites which approach each other unusually
+closely on the median line. × 10.5.
+
+Fig. 6. Specimen 219. An entire specimen of medium size, developed
+from the ventral side. It shows particularly well the "normal"
+curvature of the antennules, the change in form of the segments of the
+endopodites from cephalon to pygidium, and, along the axial lobe, the
+apodemes of the ventral integument. See also pl. 4, fig. 4. × 3.6.
+
+
+PLATE II.
+
+
+HELIOTYPE CO. BOSTON
+
+ * * * * *
+
+
+PLATE 3.
+
+Photographs of _Triarthrus becki_, made by C. E. Beecher.
+
+Fig. 1. Specimen 204. See also text fig. 42 and pl. 4, fig. 6. The
+exopodites and endopodites of the first few segments of this specimen
+are better preserved than those of any other revealing them from the
+dorsal side, × 9.5.
+
+Fig. 2. Specimen 220. A large individual exposed from the lower side.
+It shows well the endopodites and part of the exopodites, and, rather
+better than any other specimen, the endobases of the coxopodites. ×
+2.4.
+
+Fig. 3. Specimen 216. A small entire specimen showing considerable of
+the detail of the appendages of the cephalon, and some of those of the
+remainder of the body, × 7.4.
+
+Fig. 4. Specimen 201. This figure shows the details of the appendages
+of the left side and of the pygidium. Note the plate on the median
+line back of the pygidium, the sockets for spines, and the terminal
+spines on the anterior endopodites. See also pl. 2, fig. 1 and pl. 4,
+figs. 1, 2. × 7.1.
+
+Fig. 5. Specimen 207. One half of the posterior part of the thorax and
+pygidium, showing exopodites and endopodites as seen from the dorsal
+side, × 7.6.
+
+Fig. 6. Specimen 214. The exopodites have been turned back nearly
+parallel to the axis of the shell. Notice particularly the long
+flattened setæ and the spinose spatula-shaped terminal portion of each
+shaft. See also pl. 1, fig. 2. × 7.
+
+
+PLATE III.
+
+
+HELIOTYPE CO. BOSTON
+
+ * * * * *
+
+
+PLATE 4.
+
+Photographs of _Triarthrus becki_, made by C. E. Beecher.
+
+Fig. 1. Specimen 201. Another photograph, similar to fig. 4, pl. 3,
+but showing more clearly some details of spines on the endopodites. ×
+12.66.
+
+Fig. 2. Specimen 201. Three appendages on the right side of the
+thorax. See also pl. 2, fig. 1 and pl. 3, fig. 4. × 12.66.
+
+Fig. 3. Specimen 223. A small crushed specimen which nevertheless
+shows well the appendages of the right side of the thorax, developed
+from the ventral side. Note coxopodites, exopodites, and endopodites,
+and that all appendages are moved equally laterally from their
+original position. × 11.4.
+
+Fig. 4. Specimen 219. Another photograph, with different lighting, of
+the individual shown in pl. 2, fig. 6. This print brings out better
+the coxopodites and the folds of the ventral membrane. × 3.23.
+
+Fig. 5. Specimen 222. This specimen is interesting, because it shows
+the endopodites in what is probably their natural position, that is,
+in a plane nearly vertical to the plane of the body, instead of being
+flattened down, as is usually the case. The appendages under the
+pygidium are unusually well preserved. × 12.
+
+Fig. 6. Specimen 204. Photograph of the entire specimen of which a
+part is shown in text fig. 42 and pl. 3, fig. 1. × 4.5.
+
+
+PLATE IV.
+
+
+HELIOTYPE CO. BOSTON
+
+ * * * * *
+
+
+PLATE 5.
+
+Photographs of _Triarthrus becki_, made by C. E. Beecher.
+
+Fig. 1. Specimen 209. Photograph of the pygidium shown in pl. 6, fig.
+2. This specimen shows especially well the way in which the exopodites
+of the pygidium decrease in length backward, × 11.5.
+
+Fig. 2. Specimen 229. The under side of the posterior end of a
+medium-sized specimen, showing the appendages, especially the
+endopodites. On and among the limbs are scattered numerous minute
+spheres of pyrite, of the kind usually known as "trilobite eggs." They
+do not show very well in the photograph, but can be made out much more
+clearly with a hand lens, × 12.
+
+Fig. 3. Specimen 230. A specimen showing the appendages of the
+posterior part of the thorax and the pygidium. The same individual is
+also shown in text fig. 44. Note particularly the form of the segments
+of the endopodites, and the spines on them, × 13.
+
+Fig. 4. Specimen 227. The small doubly curved bodies shown in this
+figure lie under the axial portion of the cephalon and anterior part
+of the thorax. The specimen still has a very thin coating of matrix
+between it and the shell. Whether the curved bodies have anything to
+do with the trilobite is not known, × about 12.
+
+Fig. 5. Specimen 221. A small individual which shows well the
+exopodites of the posterior part of the thorax. Note the spatulate
+terminations and the spines of the shaft, × 11.
+
+Fig. 6. Specimen 202. Posterior part of the thorax and pygidium,
+showing endopodites and exopodites projecting under the dorsal test.
+Note the spiniferous plate on the median line, and the large opening
+in the anterior portion of it. × 9.75
+
+
+PLATE V.
+
+
+HELIOTYPE CO. BOSTON
+
+ * * * * *
+
+
+PLATE 6.
+
+All figures except 4 and 5, from photographs by C. E. Beecher.
+
+Fig. 1. _Triarthrus becki_. Specimen 203. A well preserved small
+individual, showing the appendages of the right side of the thorax. ×
+11.46.
+
+Fig. 2. _Triarthrus becki_. Specimen 209. A well preserved individual,
+showing the antennules and some appendages of thorax and pygidium. For
+detail of the pygidium, see pl. 5, fig. 1. × 4.
+
+Fig. 3. _Triarthrus becki_. Specimen 218. Ventral side of the pygidium
+and greater part of the thorax of an individual of medium size. Note
+especially the relation of exopodites to endopodites of the last two
+thoracic segments. A drawing of these appendages is shown on text fig.
+43. × 4,3.
+
+Figs. 4 and 5. Endopodites, probably from a species of _Calymene_.
+These specimens, with several others, are on a small slab of limestone
+from the Point Pleasant (Trenton) beds opposite Cincinnati, Ohio.
+Specimen in the U. S. National Museum. Photographs by R. S. Bassler.
+
+Fig. 6. _Acidaspis trentonensis_ Walcott. Both the specimen, No. 245,
+and the photograph are poor, but show that in this genus the
+endopodites are like those of Triarthrus. × 8.5.
+
+Fig. 7. _Cryptolithus tessellatus_ Green. Specimen 234. This specimen
+shows well the backward directed antennules and also the outer
+segments of some of the cephalic endopodites. × 11.
+
+
+PLATE VI.
+
+
+HELIOTYPE CO. BOSTON
+
+ * * * * *
+
+
+PLATE 7.
+
+Photographs of _Cryptolithus tessellatus_ Green, made by C. E.
+Beecher.
+
+Fig. 1. Specimen 233. The best preserved individual, the one from
+which Professor Beecher's drawing (text fig. 45) was made, and which
+served as the principal basis for the restoration (text fig. 20). Note
+the long, backward directed antennules, the abrupt backward turn of
+the outer portions of the endopodites, the way in which the exopodites
+extend beyond the endopodites, and the fact that alt are beneath the
+cover of the dorsal shield. The hypostoma is turned entirely around.
+× 10.9.
+
+Fig. 2. Specimen 235. Half of the thorax and pygidium, with the
+appendages revealed from the ventral side. Note the abrupt manner in
+which the outer portions of the endopodites are turned backward. See
+also pl. 8, fig. 3, and pl. 9, fig. 1 (right half). × 14.45.
+
+Fig. 3. Specimen 236. Detail from fig. 4, to show the blade-like setæ
+of the exopodites and the numerous terminal spines of the endopodites.
+× 30.
+
+Fig. 4. Specimen 236. The appendages of the thorax and pygidium, seen
+from the lower side. Specimen 236 is the right half of the same
+individual from which specimen 235 was obtained. Note the
+interarticular membranes between the segments of the endopodites and
+the blade-like setæ of the exopodites. See also pl. 9, fig. 1 (left
+side). × 19.
+
+Fig. 5. Specimen 236. The same specimen, seen from the dorsal side,
+showing, when the test is removed, the long blade-like setæ of the
+exopodites. See also pl. 9, fig. 2 (right half). × 19.
+
+
+PLATE VII.
+
+
+HELIOTYPE CO. BOSTON
+
+ * * * * *
+
+
+PLATE 8.
+
+Photographs of _Cryptolithus tessellatus_ Green, made by C. E.
+Beecher.
+
+Fig. 1. Specimen 231. A nearly complete individual, cleaned from the
+ventral side and showing obscurely the hypostoma and fragments of
+numerous appendages. Note the lines of appendifers along the sides of
+the axial lobe. × 11.
+
+Fig. 2. Specimen 232. Although this is not very well preserved, it
+shows more of the cephalic appendages than any other. Even so, only
+just enough is shown to indicate that they were similar to those on
+the thorax. × 12.
+
+Fig. 3. Specimen 235. Dorsal side of the appendages of the thorax and
+pygidium. See pl. 7, fig. 2 for the ventral view. On pl. 9, fig. 2
+(left side) is a drawing taken from the same specimen. × 11.
+
+Fig. 4. Specimen 238. Part of a thorax and pygidium, seen from the
+ventral side. The series of heavy segments shown in the upper part do
+not belong to one appendage, but are the distal ends of several
+endopodites. See also text fig. 46 for a drawing of this specimen.
+× 18.
+
+Fig. 5. Specimen 237. Pygidium and part of the thorax, with some of
+the appendages. × 11.
+
+
+PLATE VIII.
+
+
+HELIOTYPE CO. BOSTON
+
+ * * * * *
+
+
+PLATE 9.
+
+_Cryptolithus tessellatus_ Green. Upper drawing by C. E. Beecher;
+lower drawing by Miss F. E. Isham, under the direction of C. E.
+Beecher.
+
+Fig. 1. Appendages of the thorax and pygidium, seen from the ventral
+side. These are not restorations, but drawings from the halved
+individual numbered 236 (right side of drawing) and 235. For
+photographs of these specimens, see pl. 7, figs. 2, 4. × 20.
+
+Fig. 2. Appendages of the thorax and pygidium, seen from the dorsal
+side. Same specimen as in fig. 1. For photographs, see pl. 7, fig. 5,
+and pl. 8, fig. 3. × 20.
+
+
+PLATE IX.
+
+
+HELIOTYPE CO. BOSTON
+
+ * * * * *
+
+
+PLATE 10.
+
+From photographs made by C. E. Beecher.
+
+Fig. 1. _Isotelus latus_ Raymond. Ventral surface of the specimen in
+the Victoria Memorial Museum at Ottawa, Canada. Note the large,
+club-shaped coxopodites and the more slender endopodites. The first
+large coxopodite back of the hypostoma belongs to the last pair of
+cephalic appendages. The coxopodite of the appendage in front of it is
+seen turning in beneath the tip of the hypostoma. × 2.
+
+Fig. 2. _Isotelus maximus_ Locke. The ventral side of the specimen
+described by Mickleborough and now in the U. S. National Museum. The
+tips of the hypostoma may be seen at the front, and the first two
+pairs of coxopodites behind them belong to the last two pairs of
+appendages of the cephalon. Note how much stronger the coxopodites are
+than the endopodites. The appendages of the pygidium show but poorly,
+× 1.45.
+
+
+PLATE X.
+
+
+HELIOTYPE CO. BOSTON
+
+ * * * * *
+
+
+PLATE 11.
+
+_Ceraurus pleurexanthemus_ Green. A restoration of the ventral surface
+and appendages, made by Doctor Elvira Wood, under the supervision of
+the writer, from data obtained from the translucent slices prepared
+and described by Doctor Walcott. × 5.
+
+
+PLATE XI.
+
+
+HELIOTYPE CO. BOSTON
+
+
+
+
+ * * * * *
+
+
+Transcriber's Notes
+
+ Small captioned text was not converted to ALL CAPS.
+ The numer 1 and capital I both look alike in the printed version.
+ Therefore, some of the volume, plate and other roman numerals may
+ have been incorrectly converted to 1.
+ Some tables were reformatted due to space considerations.
+
+
+
+
+
+
+
+End of the Project Gutenberg EBook of The Appendages, Anatomy, and
+Relationships of Trilobites, by Percy Edward Raymond
+
+*** END OF THE PROJECT GUTENBERG EBOOK 41695 ***
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