size of the compound eyes is a prominent feature of all very young stages here investigated and seems to be a general ontogenetic character. In one species, Pterygotus globiceps, the eyes are not notably larger than in the ephebic individual, but in this case they are very large at maturity and obviously a retained larval character. In Eurypterus maria, Pterygotus macrophthalmus and Hughmilleria shawangunk the difference in size is especially notable. In Stylonurus myops the visual area exhibits little difference in relative size, but the ocular nodes are so extended that they occupy nearly the entire dorsal side of the carapace within the flat border.
3 The compound eyes are nearer to the margin; either the anterior margin (Eurypterus), or the lateral (Stylonurus myops). In Pterygotus and Hughmilleria they are already marginal in the nepionic stage and remain so to the ephebic stage.
4 The ocellar mound or tumescence exhibits a distinct tendency to both larger size and greater prominence in the earlier growth stages than in the adult. This ontogenetic feature has been especially noted in Eurypterus remipes, E. maria, Pterygotus macrophthalmus and Hughmilleria shawangunk. There is also evidence that the ocelli themselves are relatively larger.
5 The swimming legs are, as a rule, larger. This has been noted in Eurypterus remipes, E. maria and Pterygotus buffaloensis. The great majority of all larval individuals have lost the appendages. The specimen reproduced on plate 21, figure 5, shows the relative great size and width of the swimming legs in E. maria.
Of much interest in this connection is a larval specimen of Stylonurus myops exhibiting the metastoma in fine preservation [pl. 51, fig. 6]. The latter is of immense size, almost equaling the carapace in length. As the metastoma is correlated in size to the coxae of the swimming legs it would follow that the latter must have been enormously developed in this species and the larvae highly agile. The probable activity of the larvae is further suggested by the enormously bulging eyes.
