366 ORTMANN— THE ALLEGHENIAN DIVIDE. [April i8, 



Fact IL, 2, (c) 



There is a third group of forms among the Atlantic fauna, which 

 have for a common character the fact that they are conspecific or 

 extremely closely allied to western forms, and which show in their 

 distribution certain peculiar, but not quite uniform conditions. We 

 have seen (under L, 2, c, p. 339, 357) that the corresponding western 

 forms are in part characteristic for the mountain streams tributary 

 to the Monongahela and Kanawha, so that there is the appearance, 

 as if certain species had crossed the divide of the Allegheny Moun- 

 tains. It remains to be investigated, whether such a crossing of the 

 divide should be admitted, and what the means were, by which this 

 was accomplished. 



Certain cases, however, should be dismissed 18 from the beginning, 

 namely first of all those, where passive migration by transport is 

 probable or possible. The Sphaeriidcc belong here, and also Campe- 

 loma decisum. Here the whole character of the distribution is 

 such, that it does not appear to follow drainage systems at all, but 

 goes across country, suggesting exceptional means of dispersal, such 

 as transportation by birds etc. 



In other cases, active migration across divides is possible and 

 probable: this concerns chiefly, as I have pointed out in a previous 

 paper (Ortmann, 1906, p. 448), the crayfish Cambaras bartoni. 

 This species, as well as the Sphceriididcc and Campeloma decisum, 

 has a rather universal distribution east and west of the divide. 



And further, I shall disregard here Cambarus spinosus and 

 acuminatus, as belonging to the southern Appalachians, as far as it 

 concerns the distribution on both sides of the divide, and also 

 Eurynia constricta and vanuxemensis fall into the same class. 



Thus there remain the following forms to be discussed here. 



1. Strophitus edentulus. 



2. Alasmidonta marginata and marg. susquehannae. 



3. Symphynota tappaniana. 



1S Two very recent cases, Cambarus obscurus and Lampsilis ventricosa 

 (cohongoronta), in the upper Potomac must be entirely disregarded, for here 

 artificial, although accidental and unintentional, transplantation has been 

 effected by human agency (see Ortmann, 1912&). 



