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



(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 

 Plate IV, figure 1, and Plate V, figures 2-4, where it is seen that 

 the interarticular membrane (Plate Y, fig. 4) in a normally 

 extended individual is somewhat less than half the length of 

 the arches. The chitinous integument of the arches, or meso- 

 sternites, 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 but- 

 tresses, 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 arrange- 

 ment 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. 



The ridges are clearly produced by a thickening of the 

 ventral integument, and can be seen when viewed from the 

 dorsal side of a specimen in which the dorsal test and filling of 

 the body cavity have been removed. They are thus partly or 

 wholly of the nature of apodemes, or plates of chitin, which 

 joass inward from the mesosternites and divide as well as sup- 

 port* internal organs, and they are not, therefore, in any sense 

 the proximal joints of legs. Besides serving in this manner 

 they were doubtless efficient in giving the necessary firmness to 

 the ventral arches for the attachment of muscles. 



Were these observations confined wholly to the specimens of 

 Triarthrus, there might still be some chance of error, although 

 it is believed that the evidence presented by this genus alone is 

 quite sufficient. Additional data, however, will now be given, 

 regarding other genera and families of trilobites, described 

 independently by other authors, and with no intention of 

 representing the detailed characters of the ventral arches. In 

 the search for trilobite appendages by various investigators, 

 the ventral membrane has naturally been of secondary consid- 

 eration, and in the case of Jaekel's work was of no considera- 

 tion whatever. 



