THE EURYPTERIDA OF NEW YORK 133 
frontal emargination. The ninth segment of the fifth pair is not rudimentary 
as in Eurypterus but fully developed. The epimeral pieces of the post- 
abdominal 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 Euryp- 
terus, 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 sub- 
genus Onychopterus for its reception. 
From this prototype of the Stylonurus branch two genera, Dolichop- 
terus and Drepanopterus, are clearly derived. Dolichopterus [see restora- 
tion, plates 40, 41] 1s principally distinguished by the development of the 
ninth segment of the sixth appendage which here formsa 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 
