412 



HISTORICAL GEOLOGY 



ornamentation : some were a fraction of an inch long (Agnostus, Fig. 

 382 Z>), while others (Paradoxides, Fig. 382 B) attained a length of 

 from one to two feet ; some had a smooth surface and few body seg- 

 ments (two in Agnostus), while others were ornamented with spines 

 and had a large number of body segments (Paradoxides had from 17 



Fig. 382. — Cambrian crustaceans. Trilobites : A y Olenellus thompsoni; B, 

 Paradoxides harlani; C, Dicellocephalus minnesotensis; D, Agnostus inter sir ictus. 

 Other crustaceans: E, Hymenocaris perfecta; F y Naraoia compacta. (After Walcott.) 



to 20) ; some had large eyes, while others were eyeless. These features 

 show that the trilobite race must have extended far back into Pre- 

 Cambrian times, since such a great diversity of form and structure 

 could only be developed as a result of long evolution. Although 

 trilobites varied greatly, it should be distinctly understood that in 



