Professor C. E. Beecher— Structure of Trilohites. 155' 



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 



Fig. 1. — A specimen of Triartltrus Becki, Green; Tiewed from the ventral side. 

 The appendages have been removed and the ventral membrane exposed. In the 

 glabellar region are seen the hypostoma, and just below it the semicircular 

 convex metastoma with side lappets. Below, in the axial region, the buttresses 

 and thickenings of the sternal arches are clearly marked, as described in the text. 

 Enlarged about nine diameters. 



the condition during life, to allow space for the biramous legs and 

 for their infolding during enrolment. It should be noted, however, 

 that the dorsal and ventral integuments in the fossils are generally 

 very close together thi-oughout, leaving but a small cavity for the 



1 Jaekel has suggested the name mesotergite to supplant the terrns axis or 

 tergum, and pleurotergite in place of pleuron or epimerum, 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 

 mesosternite for the membrane beneath each mesotergite, and pleurosternite for the 

 membrane beneath each pleurotergite. The interarticular membranes are not 

 included. 



