34 



LINDSTRÖM, VISUAL ORGANS OF THE TRILOBITES. 



Phacops in the Lower gray Orthoceratite limestone. The eyes of the trilobites show the 

 greatest conformity with those of the recent Isopoda. 



The inöst perfect eyes amongst all the trilobitic eyes niay be those of the Phaco- 

 pidae, which are also geologically the youngest, the least developed again those of the 

 Proetidse or rather of the Bumasti. The great thickness of the cornea in these must 

 have weakened their power of vision and they had probably only a faint perception of light. 



