40 NEW YORK STATE MUSEUM 
In view of this inferred structural identity in the lateral eyes of Ptery- 
gotus and Limulus, the stage of development which the latter has attained 
becomes a matter of interest to us here. 
Packard [Am. Nat. 1880, 14:212] describes the eye of Limulus as 
follows: | | 
The structure of the eye is very unlike that of any other arthropod 
eye. The cornea is simply a smooth convex portion of the integument, 
which is much thinner than the adjoining part of the chitinous skin. There 
are no facets, the cornea externally being structureless, simply laminated 
like the rest of the integument. In the internal side of the cornea are a 
series of solid chitinous conical bodies, separated from one another by a 
slight interspace and in form resembling so many minié-rifle balls; the 
conical ends of these solid cones project free into the interior of the body, 
and are enveloped in a dense layer of black pigment. Within the base of 
these cones are secondary shallow cuplike bodies or shallow secondary 
cones. It is these primary cones which, seen through the smooth convex 
translucent cornea, give the appearance of a faceted surface to the 
external eye. 
All the parts thus far described except the pigment layer, are molted 
with the rest of the crust, and the large slender cones can be easily seen 
by viewing a piece of the cast-off eye; the solid cones being seen projecting 
from the inner surface of the cast-off cornea. 
The author adds: “So far as we can ascertain, no arthropod eye 1s 
so simple as that of Limulus.”’ 
Watase [Biol. Studies, Johns Hopkins Univ. 1880, 4:287] in conse- 
quence of his investigation of the compound eyes,of arthropods considers 
the ommatidium of the lateral eye of Limulus as making the nearest 
approach to the primitive condition. ‘‘It is nothing more and nothing 
less,’ he states, ‘‘ than a depression in the skin, with the thickened chiti- 
nous cuticle fitting in the open cavity and acting as a lens to condense 
the light.” We have copied two of Watase’s excellent figures to illustrate 
this structure [text figs. 8, 9]. 
In view of the close relationship clearly uniting Pterygotus with the 
other eurypterids, notably Eurypterus and Eusarcus, we have little reason 
to doubt that the lateral eyes in all were of like structure. Nevertheless, 
