Page:The Habitat of the Eurypterida.djvu/19

 period and the evolution of distinctive characters. . . . had progressed to so sharp a differentiation that we are compelled to carry back farther in history, some of the commoner generic designations. These remains in the Frankfort [Schenectady] shale are distributed through fully 1500 feet of strata off a northeast-southwest coast line in an area of maximum deposition." Clarke and Ruedemann have described eleven species Eurypterus megalops, E. pristinus, E. ? (Dolichopterus ?) stellatus, Eusarcus triangulatus, E. ? longiceps, Dolichopterus frankfortensis, D. latifrons, Hughmilleria magna, Pterygotus nasutus, P. prolifica, Stylonurus ? limbatus.

A few fragments found as early as 1874 in the upper part of the Cincinnati group near Clarkesville, Clinton County, Ohio, were originally described by S. A. Miller (174) as Megalograptus welchi, under the mistaken supposition that they represented a graptolite, but were later determined by A. F. Foerste to be eurypterid remains. The specimens are much broken, representing two endognathites with one postabdominal segment. They occur in a blue marl three feet above a wave-marked layer of limestone, in the Liberty beds where they are associated with a typical marine fauna mainly of crinoids and some trilobites.

. Lower Siluric or Niagaran. In the Lower Siluric are several cases of the presence of eurypterid remains in marine formations. Hall's species of Eurypterus prominens from the Clinton greenish sandstone of Cayuga County, New York, was described from a single cephalon, and an unidentified species of Eurypterus is recorded from the Arisaig of Nova Scotia (39, 87). Whiteave's Eurypterus (Tylopterus) boylei from the Guelph dolomites of Ontario is a species founded upon a single somewhat crushed, but otherwise nearly complete individual. It is found in a porous, coarse-grained dolomite, and shows an unusually thickened exoskeleton, a thickening common in other members of the Guelph fauna and indicating, according to Clarke and Ruedemann, extremely saline conditions (39, 218).

Quite recently a new eurypterid horizon has been discovered by M. Y. Williams in the shales overlying the Lockport and underlying the Guelph of Ontario, Canada. Along the banks of the Eramosa River between Rockwood and Guelph the top of the Lockport formation is exposed, and is seen to consist of a series of "thin-bedded, dark