THE EURYPTERIDA OF NEW YORK 49 
posterior one of each pair much longer than the anterior, an arrangement 
that also aided in pushing the body forward. The fourth pair shows 
a different form; it is comparatively slender, its segments are flat and lack 
the long spines, except the penultimate segment whose two spines, together 
with the terminal spine (forming a ninth segment), make a flat extension 
of the leg in the plane of the greatest length. Holm pointed out that 
this leg had its principal articulation between the basaland second segments. 
This fact and the form indicate that it aided in swimming, but prob- 
ably had as its principal function the balancing of the animal in swimming.' 
This differentiation of the fourth pair of legs for another function than 
that of walking is most distinct in the genus Eurypterus. 
Dolichopterus [pl. 40] is most nearly like Eurypterus in the char- 
acter of the first four pairs of postoral appendages. They form a similar 
series, with the difference, however, that the fourth pair is considerably 
longer than the third. The first to third pairs are stouter than in Euryp- 
terus, the spines much longer and the spines of each pair of subequal size. 
The fourth pair is still better adapted to its swimming and balancing 
function through the greater length of the leg, the greater breadth of the 
segments, and especially the lobelike character of the spines of the eighth 
segment which clearly exhibit the tendency of the leg to enlarge its lateral 
surface. 
Stylonurus represents the extreme end of a branch that has developed 
through Drepanopterus. Its legs are hence to be regarded as derived 
from those of that genus. As restored by Woodward and by Beecher [see 
under Stylonurus] the first three pairs were conceived as short and spini- 
ferous, while the fourth and fifth pairs were enormously extended, of sub- 
equal length and without spines. From the observations of the writers 
on the species from Otisville (S. cestrotus) which exhibits the last 
four pairs of legs, these formed a continuously increasing series of long 
‘Our common crab (Callinectes hastatus) also appears to use the 
fourth pair of legs, that in front of the paddle-shaped swimming legs, as a kind of 
balancer in swimming. 
