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diff --git a/41695-8.txt b/41695-8.txt deleted file mode 100644 index b16a25a..0000000 --- a/41695-8.txt +++ /dev/null @@ -1,11258 +0,0 @@ -The Project Gutenberg EBook of The Appendages, Anatomy, and Relationships -of Trilobites, by Percy Edward Raymond - -This eBook is for the use of anyone anywhere at no cost and with -almost no restrictions whatsoever. You may copy it, give it away or -re-use it under the terms of the Project Gutenberg License included -with this eBook or online at www.gutenberg.org - - -Title: The Appendages, Anatomy, and Relationships of Trilobites - -Author: Percy Edward Raymond - -Release Date: December 24, 2012 [EBook #41695] - -Language: English - -Character set encoding: ISO-8859-1 - -*** START OF THIS PROJECT GUTENBERG EBOOK TRILOBITES *** - - - - -Produced by Thomas Cosmas. Produced from files made -available on The Internet Archive. - - - - - - - -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. 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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 THIS PROJECT GUTENBERG EBOOK TRILOBITES *** - -***** This file should be named 41695-8.txt or 41695-8.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/4/1/6/9/41695/ - -Produced by Thomas Cosmas. 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