i88 



OUR COMMON BRITISH FOSSILS. 



again as the body. Like the Agnosttis and several 

 others, the Trinucleus had no eyes. In this respect 

 we find the various genera of Trilobites differing very 

 much from each other. Some have a very large 

 number, as Asaphus tyrannus ; and thence we find 



them decreasing until they 

 are absent altogether. All 

 the genera of the order 

 Trinucleidce, however, are 

 not eyeless ; and this illus- 

 trates the uncertainty with 

 which the power of vision 

 seems to have heen distri- 

 buted among these ancient 

 crustaceans. Doubtless,this 

 variation was the result 

 of special conditions of 

 existence, eyes being al- 

 ways possessed when they 

 were required. Thus the 

 living male Bopyrus, or 

 shrimp-parasite (Fig. 149), 

 has rudimentary eyes, 



Fig. ^.-Paradoxides Tessini. wh jj st the f emale has none; 



but this is entirely due to the very different habits of 

 life of the two sexes. Trinucleus is abundant in the 

 Caradoc shales of Shropshire. 



From the Cambrian to the Carboniferous forma- 

 tions we find certain Trilobites peculiar to the various 



