I9I3-] ORTMANN— THE ALLEGHENIAN DIVIDE. 383 



by Adams (1902 and 1905). But this route was available also for 

 aquatic forms of life and lies probably mainly upon the Coastal 

 Plain, where barriers are largely removed by base-leveling. To a 

 smaller degree stream piracy in the uplands may have played a part 

 in the dispersal of the Atlantic forms. 



8. In the mountains zve know a few cases which indicate crossing 

 of the divide, but compared with the mass of the fauna, these cases 

 are very insignificant. However, they are zoögraphically of the 

 greatest interest in so far as they indicate probable cases of stream 

 capture. In order to properly understand these cases, the physiog- 

 raphy of the region involved should be studied more closely. 



9. In addition, we have on the Atlantic side a few cases of ab- 

 normal distribution for which special explanations have been ad- 

 vanced. One of them concerns a form, Margaritana margaritifera, 

 which differs in the origin of its distribution entirely from all North 

 American Najades, 31 and which is a stranger in our fauna. The 

 other case, Eurynia nasuta, possibly is due to Postglacial migration 

 from the St. Lawrence basin to the Atlantic slope, and may be in 

 part quite recent. 



10. Further investigations should be made primarily in the region 

 of the southern Atlantic slope and in the southern Appalachians, and 

 should be connected with the study of the Tennessee-Coosa problem 

 from the zoögeographical side. In this region there are extremely 

 interesting conditions, which, however, are very unsatisfactorily 

 known, and have led Johnson (1905) to the erroneous assumption 

 that the evidence taken from the Najades is unreliable with regard 

 to the reconstruction of the old drainage systems. 



In addition, other freshwater groups should be studied. In the 

 present paper the Najades have furnished the chief evidence, but it 

 has been shown that also certain Gastropods and the Crayfishes are 

 or might be valuable ; but there are surely other groups, chiefly the 

 Fishes. 



31 At present, only a land snail, Helix hortensis Muell., falls under the 

 same head. 



PROC. AMER. PHIL. SOC. LH. 2IO G, PRINTED JULY 14, I9I3. 



