TRILOBITE HUNTING. 207 



a strong lid. In many species, the eye is compound, 

 like that of the house-fly or dragon-fly. As already 

 observed, some trilobites were eyeless, these being 

 generally the simplest in structure, such as the 

 Agnostus. They are, however, found in company 

 with more highly developed forms. 



The compound eyes of the trilobites in their 

 fossilised condition present the appearance of a 

 crescentic ridge, on which the numerous facets, 

 in some cases as many as four hundred, are 

 geometrically arranged (Fig. 56). Dr. Buckland 



FIG. 56. Compound eye of Trilobite (Asaphus caudatus) 

 and Ocelli ; of ditto magnified. 



described these ridges as being "like a circular 

 bastion, ranging nearly round three-fourths of a 

 circle, each commanding so much of the horizon 

 that when the distinct vision of one eye ceased that 

 of the other began." Their particular form and 

 position he regarded as " peculiarly adapted to the 

 uses of an animal destined to live at the bottom 

 of the water ; to look downwards was as much 

 impossible as it was unnecessary for a creature 

 living at the bottom ; but for horizontal vision in 

 every direction the contrivance is complete." 

 It follows then that the environment of the 



