THE EURYPTERIDA OF NEW YORK 4I 
there may here exist a difference from that of Pterygotus, for in the other 
eurypterids no facets have been observed. It is obvious that both 
Eurypterus and Eusarcus possessed an exteriorly smooth cornea just as 
Pterygotus; as to the interior of their compound eye, however, several 
possibilities present themselves: either the lenses were separate from the 
cornea and thereby lost in fossilization, not being united by the sclera 
as in the holochroal eyes of the trilobites; or the facets were so feebly 
Figure 8 Limulus. Two ommatidia Figure 9 Diagram of the compound 
shown side by side, partly schematic. eye of Limulus, the black, heavy 
The thick unshaded body is the chitin- line representing the ectoderm 
ous covering of the eye. L. lens cone, and each depression in this layer: 
fitting into the depression of the skin. corresponding to an ommatidium. 
_ Rt. retinula. G. ganglion cell. (From (From Watase) 
Watase) 
developed as to escape observation. There are no records as far as we 
are aware, either among living forms, or among the fossil merostomes, 
to support the former hypothesis. Numerous recent crustaceans, among 
them the venerable Apus, either lack the facets entirely or have them 
so poorly developed that they are hardly noticeable,! and this fact seems 
well suited to shed light on the failure to detect the facets in Eurypterus 
and Eusarcus. 
In some arthropods the crystalline cone assumes a transparent 
semiliquid state [see Watase, 1890, p. 147], and it may perhaps be assumed 
1 See Parker, G. H., The Compound Eyes in Crustaceans. Mus. Comp. Zool. Bul 
r8gr. V. 21. 
