Page:The Eurypterida of New York Volume 1.pdf/262

 In this nepionic individual the relative width of carapace is much greater than in the type. The impression of excessive width is, however, hightened by the lengthening of the genal angles parallel to the base of the head. These horizontal genal spines, together with the equally produced third frontal angle of the triangle, serve to strongly emphasize the triangular aspect of the head. The frontal projection is the forward continuation of a strong median crest extending forward from the base. The lateral margins exhibit the sigmoidal curvature and the cheeklike convexity of the posterior portion as in the following stage.

The compound eyes are oval in form, of great size (about one third the total length of the head), strongly divergent and placed in the middle of the head instead of forward and farther inward from the margin than in the older individuals. The fact that they are surrounded by a deep depression seems to indicate that they were originally quite prominent, thereby drawing the surrounding test downward with them in becoming depressed to the level of the carapace. The ocelli have not been discerned. Finally the carapace exhibits a broad thick margin on all three sides, possibly a broad doublure on the under side.

It is easily seen that this larval form, notwithstanding its own peculiar characters, shares its most important features with the larval stages of other eurypterids. This is especially true of the large size of the eyes, the great width of the head and the presence of a median crest. The acute lobes of the angles are a character peculiar to this larval form and perhaps in line with the spinose processes of the larval forms of many crustaceans. The position of the compound eyes in the middle of the head may be of phylogenetic significance and indicate the secondary acquirement of the frontal position of these eyes in Eusarcus. It should be also noted that the convergence of the eyes in the mature form is directly reverse to that in the larval stage, obviously a consequence of the later adjustment in position of the compound eye to the converging frontal margins.

Measurements. The length of the type (carapace) is 3.2 mm; its width 4.8 mm; the width of the carapace of the original of plate 36, figure 5