■9.3.1 ORTMANN— THE ALLEGHEXIAN DIVIDE. 371 



In the localization of its eastern range, this form differs from 

 Strophitiis. lUit jnst this fact points to a connection across the 

 divide with the western range of Al. marginata. This comes up, on 

 the western side, close to the divide, and although the corresponding 

 form is not known from the West Branch Susquehanna, the dis- 

 tribution on the eastern side suggests that it must have crossed the 

 divide in this general region, presumably in consequence of stream 

 capture. This is the more probable, since the western race of Al. 

 marginata found in the headwaters of the Allegheny in Indiana, 

 Westmoreland, and Somerset Cos., in Pa., approaches the Susque- 

 hanna-form nuich more closely than the typical marginata, as found, 

 for instance, in the Beaver drainage. 



This leads us to consider this as a parallel case to that of Stroph- 

 itiis cdentuhis. Alasmidonta marginata crossed the divide by similar 

 means and in about the same region as Strophitus; but there is the 

 difference that it did not spread beyond the Susquehanna drainage, 

 This may be explained by the assumption that this crossing, in the 

 case of Alasmidonta, falls into a later time. 



Of course, this explanation is only tentative, but according to our 

 present knowledge, it is the only possible one. The fact of the 

 restriction of Al. marginata susquehanncc to the Susquehanna drain- 

 age is of the greatest weight for our argument, since we cannot 

 imagine that this form reached its present area by any other way. 



Syuiphynota tappaniana. 



Up to shortly ago, this species was known only from the Atlantic 

 slope, where it has a wide distribution from New England to Vir- 

 ginia (allied species are in North and South Carolina). On account 

 of its relation to the western vS". comprcssa, it appeared to fall into 

 the group which has been designated as the northern element in the 

 Atlantic fauna (indeed, Simpson, iSgGi", places it there). But after 

 I discovered that this species is also found in the western drainage, 

 but only in the upper Kanawha system ( Greenbrier and New 

 rivers), where it is extremely abundant, in fact the prevailing form 

 of Najad-life. the history of it must be different. 



Its general distribution in the east, and its localization in the 

 west, might suggest that we have here a case like that of Alasmi- 



