Page:The Eurypterida of New York Volume 1.pdf/322

 neither expanded, as in Eurypterus, etc., nor excessively elongated, as in Stylonurus." The American rocks have furnished a single representative of this genus, here described as, which stands still somewhat apart from its three British allies. In general habit it is a Stylonurus, as evinced by the slender body, broad, somewhat angular carapace with broad rim, the very long and slender legs and the immense telson. At the same time it possesses differential features, some of which clearly denote intermediate stages between the highly specialized limbs of Stylonurus and those of its unknown ancestors. The characters indicative of the incomplete specialization of the legs consist in the smaller size of the last pair; that in Stylonurus reaches to the middle of the telson, but here, notwithstanding the relatively short body, only to the penultimate segment of the postabdomen; and the further fact that the preceding pair of legs is only about half as long (more exactly three fifths) as the last.

Laurie [1893, p. 519] suggests that "the form of the two last pairs of legs [of Stylonurus], which are long and pointed at the end, and are among the most characteristic structures of the genus, is possibly derived from Eurypterus through some form like Drepanopterus, though it is also possible that Stylonurus is descended from an ancestral type in which the last pair of legs were less modified than in Eurypterus." In view of the early appearance of Drepanopterus, and the fact that the young of  show no indication of eurypteroid features in their limbs, we are convinced that the last of the alternative hypotheses of Laurie is nearer the truth and that Stylonurus is not derived from Eurypterus but comes through Drepanopterus from a like ancestor with Eurypterus.

Description. Body slender, of medium size, clavate in general outline, the carapace being broadest and the body tapering to the long telson.

The carapace is subquadrate in a young specimen, but an ephebic individual has the sides so well rounded that it appears subcircular, with the