Page:The Eurypterida of New York Volume 1.pdf/242

 broad, well rounded lobes on either side of the broad median sinus. Postlateral angles obliquely truncated and rounded. The second sternite has also the posterior angles of the median cleft well rounded off, while in the succeeding plates the posterior margin is continuous.

As the specimen reproduced in plate 35, figure 5 shows, the sternites were not only very convex in outline, but also highly arched, forming a high ventral vault.

The postabdomen forms a slender tail that sharply contrasts with that of the extremely broad preabdomen. The curved telson spine combines with the long tubular postabdomen to produce the singularly scorpioid aspect of the tail.

The first postabdominal segment which closes the interval between the broad preabdominal and the following narrow segments resembles the former in the dorsal and ventral aspects, as in other eurypterids. It is a short ring, about three times as wide as long, contracting posteriorly by one third of its width. The other segments are practically tubular. Their relative length increases so strongly backward that while the second segment is not quite half as long as wide, the last is six times as long as wide. The numerous marginal wrinkles and the form of the median portion of some less compressed segments indicate that the section of the last segments was nearly or quite circular.

The telson is a curved, stout spine, half as long as the postabdomen or equal in length to the last two segments. Its first two thirds are gently, and the last third strongly curved downward. Its basal portion is swollen globularly on the ventral side and hollowed out on the dorsal, thus forming an articulation specially adapted to a strong up and down movement in the vertical plane. The compressed condition of the telsons in our material has not permitted a clear view of the section but the keellike projections which appear on the lateral view indicate that the spine was probably four-sided. The edges of the keels are serrate.

Appendages. The chelicerae have been seen in position in two specimens [ pl. 32] and one well preserved chelicera has been found detached