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_."