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 Pterygotus 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 seen in P. bilobus); similarly in Slimonia, where the lateral eyes lie at the anterior angles of the rectangular carapace, half of the eye is on the underside [see Woodward'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 marginal in H. socialis, the genotype, it is still submarginal in H. shawangunk and while it is smooth on the outside, it shows delicate facettae on the inside.
Homology of lateral eye in Pterygotus and Limulus. No observers have recorded the presence of corneal facets in the eyes of Eurypterus and Holm states that even in excellent microscopic preparations he has been unable to notice anything but an apparently smooth, uniformly 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 inconclusive showing either projecting round lenses in a square meshed interstitial test or sclera [cf. Huxley & Salter Monogr. I, pl. 3, fig. 1b] or lenticular depressions with a hexagonal scleral test [Woodward, pt. 2, p. 56]. More frequently it is stated that the facets are not discernible, a fact attributed 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-
