316 NEW YORK STATE MUSEUM 
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 D. longicaudatus, 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 penulti- 
mate 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 
D.longicaudatus 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 Euryp- 
terus but comes through Drepanopterus from a like ancestor with 
Eurypterus. 
Drepanopterus longicaudatus nov. 
Plate 25, figure 3; plates 54-56 
Description. Body slender, of medium size, clavate in general out- 
line, 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 
