364 ORTiMANX— THE ALLEGHEXIAX DIVIDE. [April i8, 



latter three forms are found in Pennsylvania way up into the moun- 

 tain region in the Susquehanna, they are missing west of Blue 

 Ridge in the Potomac/' James, and Roanoke. This fact, that the 

 southward range of some of these forms falls largely within the 

 coastal plain, where there were special advantages for migration, is 

 corroborative evidence for their northern origin : they were first 

 and originally present in the northern section of the Atlantic slope, 

 where they had, in consecjuence of the longer time elapsed, a better 

 chance to spread upstream. 



I have treated of the origin of the distribution of a member of 

 this northern fauna, Cambanis limosus, in a former publication 

 ( Ortmann, 1906, p. 428 ff.). I have pointed out, that this species 

 is well marked, but possesses allied forms in the interior basin, and 

 I have not the slightest doubt that the Najadcs enumerated above 

 fall under the same head, and that the origin of their distribution 

 is to be explained in a similar way. Also these Najades are well 

 defined species, but possess allied representatives in the interior 

 basin (see above p. 325). 



According to the theory advanced for Cambanis limosus, these 

 Najades came around the northern end of the Appalachians, in 

 Preglacial times, by way of the Erigan River, which flew in the gen- 

 eral direction of the present St. Lawrence. This river received the 

 ancestral forms of these species from the interior basin (more es- 

 pecially from the lower Ohio and Tennessee drainage) in some way, 

 which is at present not fully understood. But there is no serious 

 obstacle to the assumption of this possibility on account of the prob- 

 able numerous changes of the drainage in these parts. Having 

 once reached the Atlantic coastal plain at the mouth of the Erigan 

 River (region of St. Lawrence Gulf and New Foundland), there 

 was no barrier to their farther dispersal southward, chiefly since the 

 coastal plain, as we know, extended at certain times further sea- 

 ward. This dispersal was first along the coast, but several of these 

 forms migrated thence upstream in the various rivers of the Atlan- 

 tic side. 



"C limosus is found here and there in the upper Potomac, but it prob- 

 ably reached these parts only recently by the aid of the Chesapeake-Ohio 

 Canal. 



