1 86 OUR COMMON BRITISH FOSSILS. 



the bottom ; but for horizontal vision in every direction 

 the contrivance is complete." I cannot refrain from 

 further quoting a well-known passage from the same 

 author, in which a logical inference is drawn from the 

 structure of the eyes of Trilobites. "The results 

 arising from these facts are not confined to animal 

 physiology ; they give information also regarding the 

 condition of the ancient sea and the ancient atmo- 

 sphere, and the relations of both these media to light, 

 at that remote period when the earliest marine animals 

 were furnished with instruments of vision in which the 

 minute optical adaptations were the same that impart 



Fig. 150. Simplest kind of Trilobite (Agnostu 



the perception of light to crustaceans now living at the 

 bottom of the sea. . . . With regard to the atmosphere, 

 we infer that had it differed materially from its actual 

 condition, it might have so far affected the rays of 

 light that a corresponding difference from the eyes 

 of existing crustaceans would have been found in the 

 organs on which the impressions of such rays were 

 then received. Regarding light itself also, we learn, 

 from the resemblance of these most ancient organiza- 

 tions to existing eyes, that the mutual relations of 

 light to the eye, and of the eye to the light, were the 



