154 Professor C. E. Beecher— Structure of Trilohites. 



structure cm l)e vtsiitied imletiiiitely, but has also overlooked that 

 afforded by the material illustrated by Walcott,^ Billings,- Mickle- 

 borougb,^ and Woodward.* Moreover, this single specimen of 

 Ptychopdria has led its describer to reconsider on a false premise the 

 entire question of the anatomy, ontogeny, phylogeny, and affinities of 

 the trilobite. 



It is tlie purpose of the present article to show that numerous 

 individuals of Triarthrus, as well as some material representing 

 other genera, preserve evidence of what seem to be the same 

 structures as those described by Jaekel in Ptychoparia, and also 

 present indisputable testimony as to their correct nature. It will 

 be demonstrated that they do not belong in any way to the 

 appendicular system of the trilobites, but are really the buttresses 

 and apodemes of the ventral body integument. 



The marvellous state of preservation of many of the specimens of 

 Triarthrus, whose appendages have been studied by the writer, 

 affords very satisfactory indications, not only of the presence of 

 a ventral integument, but also of some of its detailed characters. 

 Jaekel states that in his opinion the unfavourable (" ungunstigen ") 

 preservation of Triarthrus has obscured the proximal structure of 

 the legs, so that what he calls the three basal joints are equivalent 

 to the single unjointed gnathobase of the coxopodite, as described 

 by the writer. Inasmuch as Jaekel has never seen the original 

 specimens described, his statement is practically without foundation. 

 It may also be added that the types and best-preserved individuals 

 have been retained in the collections of the Yale University Museum. 

 The photographic illustrations accompanying this article, it is 

 believed, will refute his statement, and the specimens themselves 

 would serve the same purpose more completely, since from the 

 black nature of the rock and the nonactinie character of the fossils 

 the photographs feebly represent the delicate structures actually 

 preserved, which are clearly visible to the eye. 



The ventral membrane of Triarthrus, as well as of other trilobites 

 where it has been observed, is of extreme tenuity, and only under 

 the most favourable conditions has it been preserved. The mem- 

 brane itself was a thin, uncalcified, chitinous, flexible pellicle, and 

 thus was in strong contrast with the much thicker and calcified 

 dorsal test. 



In the preparation of a specimen to show the appendages from, 

 the ventral side, very little of the ventral membrane is commonly 

 exposed, owing to the crowded arrangement of the legs, but when 

 the appendages are removed it is possible to view the entire ventral 

 integument. This process has been carried out in a considerable 



1 Op. cit. 



^ E. Billings, '"Notes on some Specimens of Lower Silurian Trilobites" : Quarts 

 Journ. Geol. Soc, vol. xxvi (1870). 



^ J. Mickleborough, " Locomotory Appendages of Trilobites ": Journ. Ciuti. Soc. 

 Nat. Hist., vol. vi, No. 3 (1883). 



* Henry Woodward, "Note on the Palpus and other Appendages of Asaphus,. 

 from the Trenton Limestone, in the British Museum" : Quart. Journ. Geol. Soc.,. 

 vol. xxvi (1870). 



