THE EURYPTERIDA OF NEW YORK 37 
cision, and to demonstrate the entire homology in structure of the eyes of 
Pterygotus and Limulus. The visual surfaces of the specimens in question 
exhibit five distinct states of preservation, here illustrated by diagrams. 
In states I and II the visual surface is perfectly smooth without a 
trace of reticulation or lenticular depressions or prominences even under 
condensed light and under water. Two specimens show this state in the 
exterior view, two more as intaglios of the exterior of the head. Among 
the former is one of the best preserved carapaces [pl. 69, fig. 7] which 
distinctly shows the fine granulation of the 
surface and hence might be expected to re- 
tain also the finest details of the visual sur- | 
face. The other specimens with smooth eyes 
are so well preserved that the corneal facets I 
should be visible. Where the smooth sur- 
face is seen in an exterior view, the presence 
of the carbonaceous film indicates that the J 
exterior of the test is actually under obser- 
vation while in case II where the smooth 
visual surface is seen in a cast, the absence 
of the test proves likewise that we do not 
have before us an interior view of that sur- 
face. V 
The second group comprises preserva-  fFieure 7 Compound eye of Pterygotus; 
tion states JI and IV. Incase III the visual diagrams of preservation states 
surface in an exterior view exhibits lenticular depressions in a network 
of squarish meshes. This is well shown in the large head [pl. 73, fig. 1]. 
In case IV the visual surface is seen in a cast and provided with a system 
of low papillae corresponding to the depressions seen in case III. Here 
the test is reduced to a mere carbonaceous film. ~~ | 
The relation between these two states of preservation, the absolutely 
smooth and the papillate, is indicated in the condition V exhibited by 
the fine head shield in the Buffalo collection. Here is a smooth visual 
