^913.] ORTMANN— THE ALLEGHENIAN DIVIDE. 337 



in Tinker Creek, at Roanoke, Roanoke Co., Va. (Roanoke drainage), 

 and in Mountain Run, Culpepper, Culpepper Co., Va. (Rappahan- 

 nock drainage). Although differing from C. hlandingi in not be- 

 longing to the coastal plain, but rather to the Piedmont plateau, or 

 even the mountains, the direction of its distribution apparently was 

 the same, from south to north, and thus it clearly belongs to the 

 southern element of the .Itlantic fauna. In the fact that the same 

 species is also found in the Tennessee drainage, it resembles to a 

 degree the case of Eurynia eonstricta and vanuxemensis among the 

 Najades. But this may be disregarded for the present, for it does 

 not concern the region under discussion. 



Camharus bartoni (Fabr.). All over the Atlantic side, also south 

 of Pennsylvania, and I collected it myself, for instance, at Charlottes- 

 ville, Albemarle Co., Va., and additional records are to be found in 

 my former hst of localities (1906, pp. 382-384). Here we have a 

 species of %vide and general distribution both on th'e ivestern and 

 eastern side of the mountains, going up into the very headwaters 

 within the mountains. Thus it is clear that the divide has not acted 

 as a barrier in this case, which I have explained by the exceptional 

 means of dispersal possessed by this species in consequence of its 

 ecological habits. This species is able to cross divides. 



Cambarus longidus Gir. We have seen that this is in the upper 

 Tennessee and the upper Kanawha, on the western side. On the 

 eastern side it is a common form in the upper James drainage (Jack- 

 son and North Rivers). It also has been reported from the upper- 

 most Shenandoah drainage. South River at Waynesboro, Augusta 

 Co., Va. 



This distribution clearly resembles that of Symphynota tappani- 

 ana among the Najades, and that of the genus Anculosa among the 

 Plenroceridcc, and there is no question that similar factors have con- 

 tributed to bring this about, although in each of these cases certain 

 peculiarities are observed. We shall devote more time to this far- 

 ther below. 



