Note on the Legs of Trilobites. 61 



NOTES ON SOME SECTIONS OF TRILOBITES 



FROM THE TRENTON LIMESTONE.* 



By C. D. Walcott. 



In a former article,! the writer expressed the view that the perfect state of pres- 

 ervation of the delicate branchial appendages and ventral membrane of the Trilo- 

 bite, precluded the idea of the destruction of anything of a stronger texture than 

 fleshy, swimming lobes attached to the axial appendages. Within a month after 

 the above was written, a section was obtained, showing that not only was the 

 axial appendage articulated to the ventral surface, but, also, that it was a jointed 

 appendage. Six months later, the branchial appendages were found. Although 

 the structure of the Trilobite, in detail, is not yet determined, evidence of great 

 interest has been obtained. Numerous sections of Calymene senaria show axial 

 appendages with three joints. The coxa or basal joint is about as broad at 

 the upper end as long; it has a slight projection upon its upper outer side for 

 the attachment of the branchial arm. In a longitudinal section of the Trilobite, 

 it is articulated to the ventral surface by a narrow process at or near its poste- 

 rior margin. Transverse sections show the articulation to be at or near the 

 center. The second joint is broadest at the base, and tapers toward the lower 

 end, to which the third joint is attached. The third joint, in all appendages of 

 this species seen (seventy-seven in number), terminates in a round, blunt end. 

 As yet, no transverse sections showing more than one joint have been obtained. 

 In transverse sections of Ceraurus pleurexanthemus, the fifth joint terminates in 

 a short claw.t There may be six joints in the leg. In the sections five only 

 can be distinguished. Longitudinal sections of this species show a few appen- 

 dages terminating at the third joint, as in Calymene senaria. The finding of 

 good transverse sections of the latter species will probably prove that the legs 

 had five or six joints, with a terminal claw. Longitudinal sections cannot show 

 the number of joints, as the leg extends obliquely downward, and only an oblique 

 section of two or three joints is obtained. Attached to the basal joint of the 

 leg there is a slender, jointed arm of two, and probably three joints. Portions 

 of pinnulae are attached to the terminal joint. Whether they are branchial 

 tubes cannot be satisfactorily determined from the section. In other sections, 

 rows of pinnulae are shown, which are undoubtedly branchial tubes. From the 

 character of the remaining portion of the respiratory apparatus, they must have 

 been attached to the arm. It is also quite probable that a branchia was attached 

 to the basal joint of the arm. It may be that its occurrence in this position, in 

 the section, is owing to the displacement of one of the branchiae attached to the 

 thoracic cavity. These branchiae are attached above the basal joint of the leg. 

 The branchia in Calymene senaria projects out a short distance, and then bifur- 

 cates, sending two spirals nearly to the edge of the dorsal shell. In some sec- 

 tions the base appears to be a portion of the ribbon or band forming the spiral, 



* This paper (with the exception of the Note on the Legs of Trilobites, and the two figures 

 of the same, plaie 1, figs. 6,7) and the two following papers, were published and distributed, in 

 pamphlet form, September 20th, 1877, in advance ofthepresent Report. 



t Preliminary Notice of the Discovery of Natatory and Branchial Appendages of Trilobites, 

 and Additional Evidence upon the same. Twenty-eighth Annual .Report New York State Mu- 

 seum of Natural History, December, 1876. 



+ As yet seen only beneath the central segments of the thorax. As in this species the axial 

 appendage occurs upon each segment of the pygidium, and its first, second and third joints are 

 similar to those beneath the thorax, it is probable that there is little change from the modified 

 appendages beneath the head to the posterior segment of the pygidium. 



