36 NEW YORK STATE MUSEUM 
the second group, being borne near the anterior angle of the subtriangular 
carapace. 
The second group of eyes is typically represented by Pterygotus, 
and found also in the genera Slimonia and Hughmilleria. In these the 
visual area occupies the whole node, is marginal and faceted. In Ptery- 
gotus the immense eyes [pl. 73] occupy the antelateral angles. .The 
side of the somewhat globose carapace is impendent in the antelateral 
region so that in flattened individuals part of the lateral eyes is pressed over 
on the underside (particularly well seenin P. bilobus); similarly in 
Slimonia, where the lateral eyes le at the anterior angles of the 
rectangular carapace, half of the eye is on the underside [see Wood- 
ward’s restoration, 1872, pl. 20]. The connection between the Pterygotus 
and Eurypterus eyes is afforded by that of Hughmilleria as we show 
in the generic discussion of that genus. While it is marginalin H. soci- 
alis, the genotype, it is still submarginal in H. shawangunk 
and while it is smooth on the outside, it shows delicate facettae on the inside. 
IIomology of lateral eye in Pterygotus and Limulus. No observers 
have recorded the presence of corneal facets in the eyes of Euryp- 
terus and Holm states that even in excellent microscopic preparations 
he has been unable to notice anything but an apparently smooth, uni- 
formly thick cornea; the faceted eyes of Pterygotus, however, have been 
known to the earliest writers on this group of fossils. Beyond the fact 
that the lateral eyes of Pterygotus possess faceted corneae, nothing can 
be gleaned from the literature, and the figures given are equally inconclu- 
sive showing either projecting round lenses in a square meshed interstitial 
test or sclera [c?. Huxley & Salter Monogr. I, pl. 3, fig. rb] or len- 
ticular depressions with a hexagonal scleral test [Woodward, pt 2, p. 56]. 
More frequently it is stated that the facets are not discernible, a fact attri- 
buted to their extremely small size. | 
Several carapaces of Pterygotus macrophthalmus and 
P. buffaloensis, their visual surfaces excellently retained, permit 
us to elaborate the anatomy of this type of eye with a fair degree of pre- 
