362 ORTMANN— THE ALLEGHENIAN DIVIDE. [April 18, 



Rappahannock; Lexingtonia subplana, Alasmidonta colima, Eurynia 

 constricta to the James ; and Goniobasis symmetrica to the Roanoke. 



This peculiar fact, that the southern elements in the Atlantic 

 fauna have advanced to different distances northward, is hard to 

 explain. The general tendency to migrate northward is understood 

 by what has been said under IL, i, a, but the question remains, why 

 certain forms have been unable to go as far as others. 



In part, I believe, this may be explained by the ecological prefer- 

 ences of the single species, and a comparison of a few of them will 

 show what I mean. Elliptic? complanatus is ubiquitous, and is 

 able to live under a great variety of environmental conditions. It. 

 consequently had the best chance to spread north, and actually has 

 the widest range of all. Elliptio fisherianus is a typical lowland 

 species, and it has used the easy way over the coastal plain, and has 

 succeeded in going farther north than the two allied species, E. pro- 

 ductus and lanceolatus, which, as far as I can judge, are rather up- 

 land species, which could not avail themselves so much of the op- 

 portunities offered by the lowlands ; they very likely depended more 

 on stream capture within the mountains, which naturally was a slower 

 and more difficult way of dispersal. Probably this holds good also 

 in the cases of Cambarus blandingi and C. acuminatum ; the former 

 is a lowland species and has reached farther north than the latter, 

 which seems to be an upland species. 



This, however, is only a suggestion. Our knowledge of the 

 actual distribution, and also of the ecological habits of these forms 

 is not satisfactory enough to draw positive conclusions. 



It is also possible, that the special history of these forms, chiefly 

 with regard to their geological age, plays a part in this, and it might 

 be that the oldest forms had the best chance to obtain the widest 

 range. This might be correct in the case of Elliptio complanatus, 

 while a rather recent type, Eurynia constricta, has stopped rather 

 far south. But this surely is no general explanation, as is seen in 

 the case of Lexingtonia subplana, a primitive type, which did not 

 go farther north than Eurynia constricta. 



This question should be taken up in connection with a more 

 detailed study of the origin and the distribution of the southern At- 



