The sole case which might be taken to exhibit separate lenses is the last (V), where a smooth faceted surface is seen. As stated before, the lenses appear as light circular spots in the brown carbonaceous test. Close examination shows that they consist of semilenticular dolomite fillings of depressions that correspond with those observed in case III. They could be taken either as demonstrating that the cornea with the papillae was lifted out of the sclera before the burial of the specimen in the sediment and thereafter the depressions filled with mud, except where patches of the cornea adhered to the eye; or as representing the filling of corneal cavities which function as lenses and are homologous to the anterior corneal cavity observed by Clarke [1888, p. 258] in the schizochroal eye of Phacops rana. It is obvious that the former explanation is, in view of the preservation states III and IV, by far the more plausible, especially since the presence of a continuous smooth cornea precludes the comparison with the schizochroal trilobite eye.[1] There are besides these direct arguments for the limuloid structure of the Pterygotus eye, other facts which point, though less directly, to the same conclusion. These are the distinct continuity and at the same time strange tenuity of the large cornea of Pterygotus in contrast with the solid holochroal eye of such a trilobite as Asaphus. These characters show themselves in its wrinkling (often distinctly radiate, as in plate 72, fig. 2, more often concentric) or bursting in other specimens. That this cornea is continuous with the integument of the head is indicated by the fact that it is not divided from it by any distinct line and in macerated heads does not separate from it.
This evidence may be supplemented by two a priori reasons for similarity of eye structure in the eurypterids and Limulus: a) in all other organs these organisms have been found to agree with that ancient genus, b) the eyes of Limulus are of a remarkably primitive type such as would actually be expected in these archaic arthropods.
- ↑ It may be noted in this place that a specimen in Buffalo exhibits indications of a faint apical depression of the papillae.
