I9I3-] ORTMANN— THE ALLEGHENIAN DIVIDE. 323 



Chapter 2. 



Systematic Affinities of the Najades of the Interior Basin 

 and of the atlantic slope. 



In order to understand the mutual relations of the western and 

 eastern faunas, which, as we have seen, are at present rather sharply 

 distinguished, it is necessary to consider the systematic affinities of 

 the forms belonging to either. 



Up to a comparatively recent time the natural system of the 

 Najades was extremely obscure. However, the great synopsis of 

 Simpson (1900a) has paved the way for a proper understanding of 

 the relationship of our Najades, and the more recent papers of the 

 present writer (chiefly 1912a) have furnished what is believed to be 

 the natural system, expressing, as far as possible, the genetic affinities 

 within this group. 



Using this system as a guide, the following remarks are to be 

 made : 



I. The general fauna of the upper Ohio drainage (list no. 1, p. 

 291) contains no less than seventeen genera, which are not found on 

 the Atlantic side, namely: 



Fusconaia Hemilastena Amygdalonaias 



Crenodonta Ptychobranchus Plagiola 



Quadrula Obliquaria Paraptera 



Rotundaria Cyprogenia Proptera 



Plethobasus Ob ovaria Truncilla 



Pleurobema Nephronaias 



This is entirely sufficient to show the tremendous difference be- 

 tween the two faunas, and demonstrates clearly that the Allegheny 

 Mountains formed an important barrier to the eastzvard distribution 

 of the bulk of the western fauna. No further discussion of this is 

 required. 



II. The fauna of the headwaters of the Allegheny River (com- 

 bined lists 6, 7, 8, 9, p. 301) contains five species (out of fourteen) 

 which are typically western and belong to genera just mentioned: 



