1913.] ORTMANN— THE ALLEGHENIAN DIVIDE. 289 



Thus the present paper treats in the first line of the Najades. 

 But there are other groups, which are important, yet they vvih be 

 interesting only in so far as they confirm the results obtained from 

 the Najades. These are certain freshwater Gastropods, and the 

 crayfish-genus Combariis. However, in the Gastropods we are 

 handicapped by an insufficient knowledge of their mutual natural 

 affinities ; and in the Crayfishes the number of forms, which are to 

 be considered, is rather small, so that it would be difficult to obtain 

 general results from them alone. 



In the present paper, the writer is going to pay attention only to 

 that part of the Alleghenian divide which lies between the New 

 York-Pennsylvania state line and the northern line of Tennessee 

 (see Plate XII.). In the north we have a rather natural boundary: 

 from about the New York state line northward the Glacial area 

 begins, ofifering geological and physiographical features which are 

 of rather recent age, and have created special conditions, which 

 should be investigated separately. In the south, in the region of the 

 headwaters of the Tennessee drainage, the conditions form the con- 

 tinuation of those farther north, but become here so complex ,, that 

 they deserve special study, to which additional, and much more ex- 

 tended investigations are necessary, involving the " Tennessee- 

 Coosa problem."^ I have considered the upper Tennessee only so 



'This is the problem in which Johnson (1905) is especially interested. 

 The old idea is (see chiefly Hayes, 1899) that the headwaters of the Tennessee 

 once continued in the direction of the Coosa (Appalachian River), and that 

 the present course of the Tennessee is due to a deflection in consequence of 

 stream capture. Johnson believes (and also White, 1904) that the present 

 course of the Tennessee is original, and I consider his physiographical evi- 

 dence as perfectly sound and satisfactory. But since the Najades (and other 

 freshwater groups) have been used to demonstrate the correctness of the 

 assumption of the existence of the Appalachian River (see: Simpson, 1900b, 

 p. 133, Adams, 1901, p. 846, and Ortmann, 1905, p. 130), we must take cogni- 

 zance of this line of evidence, and dispose of it in some way. Johnson did 

 this by dismissing it as not convincing, as not apt to demonstrate stream 

 capture. However, as I have said, he is wrong in this, and I believe that 

 the distril)Ution of the Najades docs indicate stream capture in this region, 

 but in the opposite direction : the original fauna belonged to the old Tennessee 

 (similar to the present in its course), and certain southern tributaries of it 

 have been captured by the Coosa-Alabama system. This idea is already im- 

 plied in Simpson's (1900b, p. 135) sentence: "it is probable that nearly all the 



