C. E. Beecher — Ventral Integument of Trilobites. 167 



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 them- 

 selves would serve the same purpose more completely, since 

 from the black nature of the rock and the nonactinic charac- 

 ter 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 favorable conditions has it been pre- 

 served. The membrane 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 number of specimens, and some 

 of the more evident characters are herewith described. 



The membrane under each pleuron (pleurotergite, Jaekel), or 

 the pleurosternite, as it may be termed,* was smooth and 

 extremely thin, and in the fossils it is invariably concave. This 

 was probably the condition during life, to allow space for the 

 biramous legs and for their infolding during enrollment. It 

 should be noted, however, that the dorsal and ventral integ- 

 uments in the fossils are generally very close together through- 

 out, leaving but a small cavity for the soft parts of the animal. 

 The space inside has doubtless been considerably reduced by 

 partial collapse from the decay of the soft parts of the animal 

 and also by the pressure of the sediments. The size of the 

 body cavity is unquestionably more correctly shown in the 

 specimens described by Walcott 10 and Mickleborough, 9 from 

 the Trenton limestone and Cincinnati shales, respectively, 

 where they have apparently suffered less compression. 



Walcott showed that the membrane in Calymmene and 

 Ceraurus was strengthened in each segment by a transverse 

 arch, to which the appendages were attached at the sides of the 

 axis. These arches were connected by a thinner membrane 



* Jaekel has suggested the name mesotergite to supplant the terms axis or 

 tergum, and pleurotergite in place of pleuron or epimerurn, as applied to the 

 trilobites. This seems a useful terminology since the older terms are often 

 loosely used and have somewhat different meanings in other groups. 

 Applying this system of nomenclature to the ventral integument, the writer 

 would propose the terms mesostemite for the membrane beneath each meso- 

 tergite, and pleurosternite for the membrane beneath each pleurotergite. 

 The interarticular membranes are not included. 



