I9I3.] ORTMANN— THE ALLEGHENIAN DIVIDE. 377 



Mountains have formed and are forming a sharp barrier between 

 the western and eastern fauna. But this is exactly what was to be 

 expected, for the distribution of freshwater animals is primarily 

 governed by the conformation of the drainage systems and their 

 boundaries, provided there are no exceptional means of dispersal 

 which permit a transport or migration over land. 



Special Cases. 



So far we have attempted to explain those cases which submitted 

 to a classification such as has been given above (Chapter 4, pp. 

 338-341). But perusing the end of Chapter 2 (pp. 324, 325), we see 

 that not all forms have been treated and that there are among the 

 Najades at least three others which show special features. These 

 are : Margaritana margaritifera, Eurynia constricta, Eurynia nasiita. 



We may pass over Eurynia constricta with a few words. This 

 species belongs undoubtedly to the southern element in the Atlantic 

 fauna, and has been treated with it above. The peculiarity in this 

 case is that it has an extremely closely allied species in the head- 

 waters of the Holston (and elsewhere in the Tennessee drainage). 

 It might be possible that here we have evidence of a direct crossing 

 from the Holston into the Atlantic drainage. But as far as we 

 know, the two species do not come in close contact with each other 

 in the region investigated, and if there is any contact it is some- 

 where else, probably in the southern Appalachians, and this case 

 thus would belong to the Tennessee-Coosa problem. It should be 

 added that probably also two crayfishes fall into the same class, 

 Cambarus acuminatiis and C. spinosus. 



The other two cases must be treated separately, each forming a 

 class by itself. 



Margaritana margaritifera. 



In our region this species is found exclusively in the upper 

 Schuylkill drainage in Pennsylvania (Schuylkill Co.). This is the 

 only locality known outside (to the south) of the terminal Moraine. 

 Farther to the northeast, within the Glacial area, in New York and 

 New England, and all the way to New Foundland, this species is 

 rather abundant. In addition, it is found (in a somewhat dififerent 



