I9I3-] ORTMANN— THE ALLEGHENIAN DIVIDE. 301 



a unit, as will become evident by comparison with the next group 

 (upper Monongahela drainage). This is the most easterly advanced 

 part of the Allegheny drainage. For this reason it will be advan- 

 tageous to give the full list of all species which advance here farthest 

 toward the Alleghenian divide. 



Combined Lists: 6, ?b, 8, p. 



i. Fusconaia undata rubiginosa (Lea) 



2. Pleurobema obliquum coccineum (Conr.) 



3. Pleurobema clava (Lam.) 



4. Elliptio dilatatus (Raf.) 



5. Symphynota compressa Lea 



6. Symphynota costata (Raf.) 



7. Anodonta grandis Say 



8. Alasmidonta marginata (Say) 



9. Strophitus edentulus (Say) 



10. Ptychobranchus phaseolus (Hildr.) 



11. Ob ovaria cir -cuius lens (Lea) 



12. Lamp silts luteola (Lam.) 



13. Lampsilis ovata ventricosa (Barn.) 



14. Lampsilis multiradiata (Lea) 



This is a comparatively rich fauna. Although not all of these 

 14 species are found in every one of these streams, the average 

 number is about 7 or 8. Some of the species (Symphynota costata, 

 Strophitus edentulus) are found in all of these creeks, and five spe- 

 cies are in most of them (Pleurobema obliquum coccineum, Ellip- 

 tio dilatatus, Alasmidonta marginata, Ptychobranchus phaseolus, 

 Lampsilis ovata ventricosa). 



Looking over the Allegheny River fauna, we see that the Ohio 

 fauna, well and richly represented in the Ohio below Pittsburgh by 

 ^y forms, depauperates in the Allegheny. Although a few species 

 are added toward the headwaters, the general tendency is that one 

 species after the other disappears in the upstream direction. But 

 one feature of this should be emphasized : the decrease in the number 

 of forms is gradual, no sudden deterioration of the fauna being 



