362 



CRUSTACEA ENTOMOSTRACA. 



Trilobites abound in Cambrian and Silurian strata in many 

 parts of the world, but appear not to have survived to the 

 secondary period. Though the preservation of the head and 

 trunk is often as perfect as can be desired no traces of appendages 

 were for a long time found, but within the last few years the 

 Utica shales (Lower Silurian) of the State of New York have 

 yielded abundant examples of a trilobite, Triarthrus Becki, 

 the organic parts of which are replaced by iron pyrites. These 



have been made the subject 

 of a series of reports by 

 Beecher. Many features of 

 the appendages of these speci- 

 mens are preserved in minute 

 detail, and their discovery has 

 put our knowledge of the group 

 on a new level. Unfortunately, 

 however, notwithstanding the 

 care which has been devoted 

 to their elucidatien, we are 

 still left in tantalizing uncer- 

 tainty on several points. 



The body of Trilobites is 

 oval, and dorso-ventrally flat- 

 tened. It consists of a head 

 and of a segmented trunk the 

 anterior somites of which were 

 movable on one another, while 

 the posterior are united, as in 

 many Isopoda, to form the 

 pygidium ; and, like woodlice, 

 the animals possessed the power 



of rolling themselves into a ball, in which position they are 

 often preserved. 



In both head and trunk a central part is divided by longitu- 

 dinal grooves from pleura! regions, causing the tripartite division 

 of the body in allusion to which the group was named. 



On the central part of the head, known as the glabella, trans- 

 verse grooves usually indicate a division into five segments, of 

 which the posterior or occipital segment is most distinct (Fig. 244). 

 The anterior and lateral regions of the head end in a sharp 



FIG. 243. Diagram of Dnlmanites. Ge 

 gena ; 6/glabella ; eye ; PI. pleuron ; 

 Py pygidium ; Rh rhachis ; Sf facial or 

 ocular suture (the leader extends a little 

 beyond the suture). From Claus, after 

 Pictet. 



