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 structure.
On plate 72, figure 1, we illustrate an internal mold of P. buffaloensis from the collections of the National Museum and the external 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 9].
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 as a whole or system, leaving the sclera forming the sockets. In case III the epidermal layer into which the lenses projected is still preserved; 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.
