Page:The Eurypterida of New York Volume 1.pdf/82

 awaiting their prey or digging for it as Limulus does. A good instance of such a species is.

We consider the genus Dolichopterus well adapted to a swimming habit. This is indicated by the notably forward position of the compound eyes and the remarkable lengthening and broadening of the swimming legs. The lengthening has been produced not only by the great lengthening of the segments, but also by the development of the ninth segment (only a minute claw in Eurypterus), into a palettelike plate. We have also shown [see generic description p. 264] that the spines on the swimming legs are transformed into expanded leaflike appendages, which serve to broaden the limb. If these appendages were arranged in whorls, as in Limulus, the claim could be made that they served in digging as in the latter genus, but it is difficult to conceive that these appendages, arranged in a series on the posterior side of the limb, could have well served that purpose, since they would fail to push the mud outward as does the leg of Limulus, but would move it backward and inward. Nor is the great length of the swimming legs favorable to the digging function, for digging organs are always short and stout; and as for serving only as anchors in the mud it would not seem necessary to lengthen the limbs to such extent.

The last group, represented by Drepanopterus and Stylonurus, shows again a different adaptation. Here the legs exhibit a distinct tendency to become greatly lengthened without being broadened. It is obvious that these forms were not mud dwellers, and if we consider Drepanopterus as the ancestor of Stylonurus, they were originally crawlers.

Drepanopterus [see restoration pl. 54] possesses five pairs of walking legs which increase regularly in length backward and exhibit no differentiation, except that the frontal legs have longer spines and the last pair is spineless. The very broad shovellike carapace with its elevated eyes and the long styliform telson are, however, quite suggestive of a mud-grubbing mode of feeding.

In Stylonurus the tendency of Drepanopterus to lengthen the last pairs of legs has reached its extreme. While all its species exhibit these