1913-] ORTMANN— THE ALLEGHENIAN DIVIDE. 351 



barrier, we further have noticed the fact that it forms distinctly a 

 unit from the upper Allegheny River at least to Licking River in 

 Kentucky. It is hardly necessary to discuss this, since the present 

 conditions sufficiently explain this uniformity; all these rivers, run- 

 ning westward, are united into one master stream, the Ohio. Also 

 the system of the Tennessee, which has much in common with the 

 Ohio, finally unites with this river. 



However, when we come to study the origin of this fauna and to 

 consider the fact that the Ohio drainage in its present form is a 

 modern feature of our hydrography, we have to ask the question, 

 W'hat the old conditions were? 



There is hardly any doubt that the uniform Najad-isiuna. of the 

 upper Ohio basin is, in its origin, connected with the origin of the 

 Ohio River, that is to say, that it is not older than the Glacial time, 

 probably largely Postglacial. The fact brought out above, that from 

 the upper Allegheny downstream this fauna becomes richer, and 

 that the number of species increases steadily farther down (from 

 47 in Pennsylvania to about 6o or more in the vicinity of Cincinnati), 

 makes it certain that the center of dispersal of this fauna was in the 

 region of the lower Ohio, probably also including the Tennessee 

 system, and that this fauna migrated upstream in Glacial and Post- 

 glacial times as soon as the present Ohio was formed, depauperating 

 gradually in the direction toward the headwaters. 



Fact I., 2, (a). 



The fauna of the upper Tennessee is very strongly marked. 

 Nevertheless it shows distinct affinities to the Ohio fauna. We have 

 studied only a very small part of it, and it is well known that farther 

 down in the Tennessee and also in the Cumberland River drainage, 

 this fauna becomes still richer. 



Without a closer and more exhausting study of this fauna it is 

 impossible to express any definite ideas as to the origin of it. Thus 

 we have to dismiss this topic here and it is sufficient to say that prob- 

 ably this fauna represents the common ancient stock, and the great 

 center of radiation, not only of the interior basin fauna, but also of 

 that of the Atlantic slope and the Gulf region. That the Ohic 



PROC. AMER. PHIL. SOC. , LH . 2IO E, PRINTED JULY II, I913. 



