Grier: Morphological Features of Mussel-shells. 157 



noquenessing and Slippery-rock Creek. The Shenango River, a 

 tributary, arises in northwestern Pennsylvania, and flows eighty-seven 

 and one-half miles through the glaciated area, where are many swamps 

 and small lakes, and the country is generally broad and flat, to its 

 junction with the Beaver. The principal tributaries are Pymatuning 

 Creek, Little Shenango River, and Neshannock Creek. The Mahon- 

 ing River arises in Ohio, flowing twelve miles in Pennsylvania to its 

 junction with the Shenango to form the Beaver. Its course is through 

 broad valleys and rolling hills in the glaciated region. The majority 

 of these streams are of comparatively recent (glacial) origin. 



Characteristics of the Naiad Fauna. 



The fauna of the Ohio River (37) and tributaries is that of the 

 interior basin and is largely post-glacial in origin, having migrated 

 up stream in post-glacial times. The most conclusive evidence points 

 to its original source as having been in the drainage of the Tennessee 

 River. The fauna may be traced from the Licking River up through 

 the whole Upper Ohio Drainage into the headwaters of the Allegheny 

 and the Monongahela. As a whole it may be considered a somewhat 

 depauperate Tennessee fauna (37) becoming (although richer and more 

 exuberant than that of Lake Erie), more greatly so in the rivers above 

 Pittsburgh, in the Allegheny and its tributaries to a greater extent than 

 in the Monongahela and its tributaries. In the latter and its tribu- 

 taries the rich Ohio fauna, only slightly depauperated, goes up to a 

 certain point at the lower end of canyons where begin extremely rough 

 portions of the rivers. The species of shells found in the Mononga- 

 hela, but not the Allegheny, are pre-eminently "big river forms" 

 while those of the xAllegheny are those of a small river. The Beaver 

 River is a glacial drift stream. Wetherby remarks (58) "It is a sig- 

 nificant fact that those North American rivers which contain the 

 richest Unione Fauna drain Mcsozoic and Tertiary regions, while 

 those that drain Paleozoic and Azoic regions have a comparatively 

 meagre Unione Fauna." 



Summary of the Physical Conditions in Lake Erie and the Upper Ohio 

 Drainage which affect the Naiades. 



Lake Erie resembles "big streams" in having the sandy and gravelly 

 bottom, preferred by Mollusca. The shells are not subject to the 



