Page:Second Geological Survey of Pennsylvania Report of Progress PPP.djvu/18

 near the summit of this tubercle as in the ocelli of Eurypterus, etc. The fact that facets cannot be discerned is no criterion of their absence in the animal, for numerous trilobites and Eurypterus have as yet revealed no evidence of the compound character of their eyes, and on the other hand many species have large and distinct facets. It has also been noticed by Professor McCoy that this spot on the carapace is darker in the fossil than the general surface of the carapace, indicating the presence of pigment in the animal at this point. In the specimens of Elymocaris siliqua, Tropidocaris bicarinata and T. interrupta of the present paper this character is well marked.