90 THE STORY OF THE EARTH. 



but still survives in the deep oceans, where it is 

 . by the genus Calveria dredged in 445 



fathoms of water. 



Trilobites. 



Trilobites are a group of Crustacea, entirely 

 extinct, which appear in the oldest stratified rocks, 

 and survive till the close of the carboniferous 

 period. The group is characterised by having 

 the body divided, first into a head -shield, termed 

 the cephalo-thorax, which may theoretically con- 

 sist of five segments of the immature animal, 

 blended into one plate. In this shield, in a suture 

 between the central part called the glabella and 

 the free cheeks, the eyes are placed, when the 



- have a recognisable development. Some 

 trilobites appear to be blind, being without eyes. 

 On the under side of the head is the labrum, from 

 which the long-jointed antennae extend forward in 

 a genus named Triarthrus, The middle part of 

 the animal, known as the abdomen, consists oi a 



number of separate overlapping plates, like those 



in the tail of a lobster, which are capable of mov- 

 ing freely upon each other, ami are sometimes 

 arranged SO that the animal can roll into a 

 ball, like the living terrestrial crustacean (hi; 

 known a- the WOOdloUSe. The number of these 

 joints in the abdomen, termed sopites, varies from 



two m Agnosias^ to about twenty m genera like 

 Pat 1 . which is one of the oldest, and Aula- 



coplt'ion, which is a Silurian type. Thirdly, there 



a tail-plate, known .is the pygidium, which is 



sometimes marked with external ornament, cor- 



ling t<> that of the separate p the 



tnd sometimes smooth. The < 



