THE EURYPTERIDA OF NEW YORK 39 
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 schizochroaleyeof Phacops rana.  Itisobvious 
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 struc- 
ture 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 simi- 
larity 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. 
‘Tt may be noted in this place that a specimen in Buffalo exhibits indications 
of a faint apical depression of the papillae. 
