82 



THE CHAIN OF LIFE. 



a somewhat limited group at present, and one which reaches 

 back in geological time only to the Upper Silurian, when the 

 Trilobites had perhaps already passed their culmination. 



Constructed as above described, the Trilobite could swim, as 

 is supposed, usually on its back or side. It could crawl on 

 the bottom. Using its snout as a shovel, it could burrow like a 

 modern King-crab (Fig. ySa) ; and when pressed by danger some 

 species could roll themselves into balls and defy their enemies. 



Fig. 77. — Silurian Trilobites. 

 a, Isotebis. b, Homalonotiis. c, Calymenc. 



This type of animal, entering on the stage in full force in 

 the Older Cambrian, continues under many forms through the 

 whole Palaeozoic age, dying out finally in the Carboniferous. 

 Figs. 77 and 78 show a few of the forms of the Silurian, 

 Devonian, and Carboniferous. 



Contemporaneously with the dawn of the Trilobite group, 



appear some small shrimp-like forms (Fig. 28),^ and others 



with bivalve shells (Fig. 79), which are closely allied to modern 



forms,'^ and, like the Lingulce, persist through the succeeding 



^ Ilyirenocaris. ^ Phyllopods and Ostracods, 



