Page:The Habitat of the Eurypterida.djvu/240

232 and with one exception between the latter and the Pittsford, is easily understood and entirely to be expected. The baffling break in the phylogenetic history of Stylonurus is also explained. First found in the Pittsford, it came from Appalachia; on that continent its evolution continued through the remainder of Siluric time, its remains not being found because the continental, chiefly river flood-plain, deposits from Appalachia during the Upper Siluric and the Lower Devonic are unknown on the North American continent. The perplexities which were so detrimental to the "lagoon" theory are completely removed by the river theory. But if the latter be accepted, a new objection arises—only one, to be sure, yet at first it seems to demolish the theory altogether. How does it come to pass that E. pittsfordensis so closely resembles E. lacustris as to seem almost certainly the direct ancestor? In specimens approximately the same size the two species are found to be almost identical in the proportions of the cephalon (i.e., length:width = 2:3), and in the position and shape of the eyes. On the other hand, the posterior portion of the cephalon flares out in E. pittsfordensis or at least broadens out in a hyperbolic curve, while E. lacustris is marked by the nearly parallel sides of the cephalon. E. lacustris is not so broad a species as E. pittsfordensis, but otherwise does not differ especially in form. The telson in the latter species is unusually long, being nearly equal in length to the rest of the postabdomen.

An immature, but complete individual in the Buffalo Society of Natural Sciences Museum measures as follows:

In another specimen which is incomplete, the telson measures 11.5 cm., while in E. lacustris, in an individual of about the same size, it measures only 6.5 cm. In spite of these differences, however, the species are very much alike, though not so closely related as E. lacustris, E. remipes, and E. fischeri, which can be understood from the fact that the three latter belong to the same horizon, while the former precedes them by a long period. I am quite prepared to agree with Clarke and Ruedemann that E. pittsfordensis is the