I9I3-] ORTMANX— THE ALLEGHEXIAX DIVIDE. 365 



The southward migration was unequal, but the causes of this 

 are not very clear, but might be compared with the similar phenom- 

 enon in the case of the southern elements. 



When the Glacial period set in, the ice coming from the north 

 separated the eastern range of these forms from that on the west- 

 ern side. Habitudinal segregation was thus effected, and this in- 

 duced differentiation into species. The final consequence is, that the 

 Atlantic forms developed into well marked species, which have a 

 rather young age (Glacial), and still are closely allied to correspond- 

 ing forms in the interior basin. In Postglacial times, after the ice 

 had disappeared, a reaction, a northward migration set in, and these 

 species reoccupied a good deal of the territory lost in Glacial times. 

 In this advance they were accompanied by certain southern types, 

 which also invaded the glaciated area (Elliptio complanatus, Alas- 

 midonta undulata). 



Thus the origin and the history of this part of the Atlantic fauna 

 appears rather clear. The most interesting fact is, that the case of 

 Cambaras lim o sus has a number of parallel cases among the Na- 

 jades. This element in the Atlantic Naj ad-fauna, however, has 

 been recognized already by Simpson (1896&, p. 337), who also 

 explains its origin by migration around the northern end of the 

 Appalachians. 



Considering the two elements together, the northern and the 

 southern, and the fact that the species belonging to them migrated 

 to various extents south or north, we obtain a satisfactory explana- 

 tion of the fact, mentioned above (p. 315, 318), that the Susquehanna, 

 and also the Potomac, fall short, in the number of species, of the 

 rivers both to the north (Delaware) and south (James)» Certain 

 forms of the northern fauna have not gone south beyond the Del- 

 aware, and certain southern forms have not gone north beyond the 

 James, and this leaves a balance against the intermediate systems of 

 the Susquehanna and Potomac. In the Susquehanna, this short- 

 coming has been in part supplemented by an indigenous form (Alas- 

 midonta marginata siisqnehannce) , and in the Potomac by a southern 

 form (Elliptio prodiictus) . This peculiar condition is a point which 

 very strongly speaks for our assumption of two distributional cen- 

 ters in the Atlantic fauna, a northern and a southern. 



