The closer approximation of the lateral eyes to the margins in the young than in the adult eurypterids is a feature shared to some extent by young Limuli and still more distinctly shown by the embryo of the scorpion [Laurie, 1890, pl. 17, fig. 45]. In Limulus the lateral eyes and ocelli are placed on distinct ridges, termed by Packard [1870, p. 168] the ophthalmic ridges, which separate the cardio-ophthalmic region from the lateral regions of the carapace. These ridges are most prominent in the larva and become obscured in the adult. They are faintly seen in the later embryonic stages and are there distinctly farther away from the median line of the carapace. The ocelli appear first on the under side of the head [Packard, p. 168] just in advance of the chelicerae and in later embryonic life wander to the upper side.
Strabops of the Cambric has the lateral eyes in about the position where they are seen in nearly all our larval eurypterids; E. megalops (Frankfort) and the Clinton E. prominens as well as the primitive Hughmilleria also have submarginal eyes, and we see in this approximation of the eyes to the margin a palingenetic feature which probably points to ancestors with submarginal eyes. In the Frankfort Lower Siluric fauna taken as a whole, the eyes are nearer the margin, a noteworthy fact in this connection. The genera Pterygotus, Slimonia and Eusarcus make a nearer approach to the primitive condition in this respect than Stylonurus and those species of Eurypterus with approximate eyes high up on the carapace. A comparison in point is afforded by the trilobites, in which, according to Beecher's excellent investigations, the eyes appear on the under side in both the ontogenetic and phylogenetic development.
The prominence and great size of the ocellar mound in the larvae of the eurypterids are fully in consonance in function with the early appearance and relative great size of the ocelli or "larval eyes" in the embryo and larva of Limulus and the great size of the central eye in the scorpion. There is no direct evidence to indicate that the prominence of the ocellar mound is of any other than larval or coenogenetic character.
