156 Professor C. E. Beecher — Structure of TriloUtes. 



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^ and Mickleborough,^ from 

 the Trenton limestone and Cincinnati shales respectively, where 

 they have apparently suffered less compression. 



Walcott showed that the membrane in Calymene and Ceraiirus 

 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 (the interarticular membrane), 

 and were aptly compared to the arches in the ventral integument of 

 many of the decapods. Similar features are present in Triarthrus, 

 as illustrated in PI. XI, Fig. 1, and Figs. 1-3 in text, where 

 it is seen that the interarticular membrane (Fig. 3) in a normally 



Fig. 2. — The same specimen enlarged only a little more than three diameters. The 

 illumination is from the side opposite to that in the preceding figure. 



extended individual is somewhat less than half the length of the 

 arches. The chitinous integument of the arches, or mesosternites 

 as they may conveniently be called, is thickened along the borders, 

 and appears to be slightly incurved on the posterior edge. The 

 arches are further strengthened by a series of median and oblique 

 longitudinal ridges, or buttresses, which are generally progressively 

 more developed in passing anteriorly from the pygidium along the 

 thorax to the neck-segment of the cephalon. 



The ventral arch of each segment has the following arrangement 

 of these ridges : — There is first a median ridge generally extending 

 from the posterior border entirely across the plate, but sometimes 

 becoming obsolescent near the anterior border. Then, on each side, 

 there is an oblique ridge making an angle of about sixty degrees 

 with the posterior edge and extending inward, but not meeting 

 the median ridge, thus enclosing a subtriangular space with the 

 anterior apex truncated. Outside of these ridges, but still within 

 the axial region, there is often a second pair of somewhat more 

 oblique ridges, enclosing rhombic areas. 



1 Op. cit. ^ Op. cit. 



