I9I3-] ORTMANN— THE ALLEGHENIAN DIVIDE. 357 



a. Youghiogheny : Strophitus edentulus. 



b. Cheat: Anculosa dilatata. 



c. Tygart : Symphynota costata, Strophitus edentulus, Anculosa 



dilatata. 

 2. Kanawha drainage — ■ 



a. Greenbrier: Elliptio dilatatus, Symphynota tappaniana, Alas- 



midonta marginata, Anculosa dilatata, Cambarus longulus. 



b. New River: The same as in Greenbrier, and in addition (at 



Hinton only) : Quadrula tuberculata and Rotundaria tuber- 

 culata. 



Two classes may be distinguished among these : those which have 

 no relations on the eastern side, and those which are represented 

 there by identical or very closely related forms. The former are: 

 Symphynota costata of the Tygart, and Quadrula tuberculata, Ro- 

 tundaria tuberculata, Elliptio dilatatus, and Alasmidonta marginata 

 of the upper Kanawha. These are species rather generally distrib- 

 uted in the upper Ohio region, and they probably belong to this 

 fauna, representing forms, which for certain special reasons, pos- 

 sibly by mere chance, were able to ascend somewhat higher in the 

 mountain streams than the bulk of the Ohio fauna. 



The other forms, Symphynota tappaniana, Strophitus edentulus, 

 and the crayfish Cambarus longulus, are represented on either side 

 of the divide by the identical species, while in the case of Anculosa 

 two extremely closely allied species, A. dilatata and carinata, are 

 found west and east of the divide. 



These latter facts are very interesting, and touch upon the ques- 

 tion, whether and how it was possible that certain forms of fresh- 

 water life were able to cross the divide. For the present, we shall 

 only indicate this problem, but we shall take it up again, when we 

 come to speak of the Atlantic forms, which are more or less nearly 

 related to western ones (see below, under fact IL, 2, c) . 



It also should be pointed out, that an additional interesting ques- 

 tion is involved here. We have seen, that the general Naj ad- fauna 

 of the Ohio, which goes up to the lower end of the canyons, is of 

 Postglacial age. This fact suggests, that also the falls line of the 

 canyons is comparatively recent, and that it marks a last rejuvena- 



