Page:The Eurypterida of New York Volume 1.pdf/51

 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 basal and second segments. This fact and the form indicate that it aided in swimming, but probably 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 character 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 Eurypterus, 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 spiniferous, while the fourth and fifth pairs were enormously extended, of subequal length and without spines. From the observations of the writers on the species from Otisville which exhibits the last four pairs of legs, these formed a continuously increasing series of long