Page:The Eurypterida of New York Volume 1.pdf/353

 There are nine distinct segmental division lines in the nepionic specimen, indicating the presence of nine segments, with possibly a shorter tenth one directly behind the head. There is a slight break in the outline at the end of the third segment, the fourth setting in a little further. This feature suggests that the postabdomen was already complete and consisted of six segments, while the preabdomen possessed thus far only three to four segments. This inference is supported to some extent by the fact that the postabdominal segments already exhibit the proportions of the mature forms in their gradual narrowing and lengthening. It is therefore probable that the new segments, at least the tergites, are added in successive moltings in the preabdomen, perhaps directly behind the carapace, as in the trilobites; or in other words, that the head and tail appear first and the thorax is completed gradually.

The swimming legs are also shown in the nepionic specimen. They are relatively shorter and broader than in the mature age.

The telson spine is of the same form and proportion to the body as in the ephebic stage.

The neanic stage is represented by the originals of the figures 1–11, 13. The first two of these are fair representatives of the earliest neanic (ananeanic) stage. They still retain the nepionic proportions of the carapace while the eyes have already been reduced to about one third of the length of the carapace, but are still distinguished from the ephebic eyes by their greater prominence. The large size of the ocellar mound and the distinctiveness of the ocelli themselves are also a characteristic feature of this stage [figures 1–3]. The carapace is surrounded by a thick, flat border in these specimens, possibly the doublure of the underside, exposed by the breaking away of the upper margin. The preabdomen is complete in one of the two specimens and remarkable for its uniform width which is equal to that of the base of the head. As specimen figure 13 shows, this width still remains nearly uniform in individuals a little older, while a somewhat abrupt reduction takes place to the postabdomen at the first postabdominal segment.