TRILOBITA 



325 



new somites being added in front of the telson while those at the front 

 end of the pygidium become free. 



Fig. 221. Development of a trilobite. After Barrande. A-D, Sao hirsuta, 

 Cambrian, Bohemia. A, Earliest stage (Protaspis), x 12. B, Later stage, with 

 three somites behind the head, x 12. C, With more distinct glabella furrows 

 and four somites behind the head, x 12. D, With six somites behind the 

 head, x 10. 



It is probable that the majority of the trilobites lived upon the sea- 

 bottom in shallow to moderately deep water, but others appear to have 

 been adapted to burrowing, pelagic, and deep-sea conditions. 



