frontal emargination. The ninth segment of the fifth pair is not rudimentary as in Eurypterus but fully developed. The epimeral pieces of the postabdominal segments are more prominent than in the other groups of genera.
Eurypterus kokomoensis possesses all these characters in an initial condition. While it still has the swimming legs of an Eurypterus, they are longer and slenderer than in other Eurypteri and the ninth segment has grown out into a spur like that of the feet of Drepanopterus and Stylonurus. All the other endognathites are also longer than in typical species of Eurypterus and the fourth pair is not distinguished from the others by its lack of spines and its slenderness. The carapace already exhibits the characteristic large size, squarish outline and broad border (doublure?) and the telson is distinctly styliform. We have for this reason deemed this species worthy of subgeneric distinction and erected the subgenus Onychopterus for its reception.
From this prototype of the Stylonurus branch two genera, Dolichopterus and Drepanopterus, are clearly derived. Dolichopterus [see restoration, plates 40, 41] is principally distinguished by the development of the ninth segment of the sixth appendage which here forms a broad suboval lobe, instead of the claw of Onychopterus, exactly corresponding to the palette of Eurypterus. The same tendency to the broadening of the spines into lobes or plates is displayed on the fourth endognathite [pl. 45, fig. 2], giving this leg an aspect strikingly different from that of all other forms. All legs in the genotype are powerful organs with large coxae and the cephalothorax is of corresponding size; the gnathobase is especially large and the metastoma long, as in the whole Stylonurus branch. The genital appendage, however, resembles most that of Eurypterus. We may consider Dolichopterus as a more specialized genus derived from Onychopterus.
Like Dolichopterus the genus Drepanopterus is also derived from Onychopterus or from forms most nearly represented by that genus. The fifth pair of legs has become still more lengthened and its segments are
