I9I3-] ORTMANN— THE ALLEGHENIAN DIVIDE. 295 



29. Crenodonta plicata undulata (Barn.) 



30. Rotundaria tuber culata (Raf.) 



31. Alasmidonta marginata (Say) 



32. Lampsilis multiradiata (Lea) 



33. Truncilla triquetra (Raf.) 



Comparing these two lists (nos. 2 and 3), we see that they are 

 practically identical: 23 forms are in either list, to which probably 

 five others should be added, which should be expected in this part 

 of the Monongahela. Thus there would be 28 forms common to 

 these rivers. 



Even those species, which are peculiar to only one of these rivers, 

 might exist or might have existed in the other. In a general way, 

 those species found in the Monongahela, and not in the Allegheny, 

 are preeminently big-river-forms (for instance Fusc.undata trigona, 

 Obliquaria reflexa, Obovaria circulas), while, vice versa, those of 

 the Allegheny are small-river- forms (Pleurobema obliquum coc- 

 cineum, Pleurobema clava, Truncilla perplexa rangiana). This is 

 in keeping with the general character of these rivers; the Monon- 

 gahela is, although not appreciably larger, more quiet and steady, 

 with finer bottom (gravel, sand), while the Allegheny is rather 

 rough, with coarser gravel and rocks. 



One thing is very evident : that the Ohio fauna extends into both 

 rivers above Pittsburgh, but somewhat depauperated, decreasing 

 from 37 to about 30 Najad-forms. 



III. The Upper Allegheny and its Tributaries. 



Going up the Allegheny River, we meet first a section, which is 

 utterly polluted (from northern Armstrong Co., to Oil City, Ve- 

 nango Co.). But above Oil City the river is in good condition, up 

 to Warren Co., and the New York state line. In this stretch (Ve- 

 nango, Forest, and Warren Cos.), the following Najades have been 

 collected by the writer : 



List No. 4. 



1. Crenodonta plicata undulata (Barn.) 



2. Rotundaria tub er culata (Raf.) 



