38 NEW YORK STATE MUSEUM 
surface showing whitish circles in a dark reticulate mass and resting 
on this surface patches of a smooth carbonaceous test which in this 
case shows but the faintest shadow of the underlying lenticular struc- 
ture. 
On plate 72, figure 1, we illustrate an internal mold of P. buf- 
faloensis from the collections of the National Museum and the exter- 
nal mold of the same specimen from the Museum of the Buffalo Society 
of Natural Sciences is shown on plate 72, figure 2. In this external 
mold the test of the lateral eye is radially wrinkled and lacks all traces of 
facets, while in the internal cast the eyes are finely faceted but without 
radiating wrinkles. In this case, there must have taken place before the 
entombment of the specimen a partial separation and a wrinkling of an 
outer smooth cornea. 
We may hence conclude with entire safety that this Pterygotus at 
least possessed a smooth, relatively and uniformly thick cornea and below 
this a system of lenses. | 
In accordance with this conclusion the lenses were either separate 
from the overlying cornea or they were only papillary prolongations of its 
underside. If the former they have a structure like that of the holochroal 
eye of the trilobites; if the latter then the structure is in entire accordance 
with that of the eye of Limulus [see text figures 8 and g]. | 
The choice of the alternatives seems to be indicated by states IV 
and V; for both can only be explained by assuming that the lenses have 
been lifted out asa whole or system, leaving the sclera forming the sockets. 
In case III the epidermal layer into which the lenses projected is still pre- 
served; in case IV this is lost and only the impression left. In the 
latter case the cornea and the attached lenses must have been lost before 
burial by the sediment as otherwise the papillate cast should not have 
been produced. It is obvious that if the lenses had been distinct and 
separate from the cornea, we would find, after the loss of the cornea, the 
majority of the lenses still embedded in the epidermal layer, while in fact 
in these two cases none at all are thus preserved. 
