32 MEMOIRS OF THE CARNEGIE MUSEUM. 



of the Monongahcla, and is abundant in its headwaters in West Virginia. In the 

 Beaver-drainage it is nearly everywhere, being absent only in the smaller creeks. 

 The same is true of French Creek. 



It is generally found in the more or less coarse gravel of our streams, often 

 in the Allegheny and Ohio among very heavj' gravel and in strong currents, but 

 it is by no means averse to fine gravel and sand (chiefly so when young). It 

 avoids mud, as most of our Naiades do. The shell attains considerable size, and 

 there is no relation between the size of the stream and that of the shell, very large 

 individuals having been often found in small creeks. 



General Distribution: Type locality, Ohio River (Rafinesque) ; according to 

 Vanatta small creeks in Kentucky. 



The range outside of Pennsylvania is very extended, and comprises practically 

 the whole of the Ohio-drainage, including the Tennessee-Cumberland system, 

 and westward, the Mississippi- and Missouri-drainages to Minnesota, Iowa, Ne- 

 baska, Kansas, and Oklahoma. Northwards it crosses into the drainage of the 

 Great Lakes at several points, and has been reported as occurring as far northeast 

 as Ottawa, Canada (Simpson, 1893, p. 592). It is found in western New York 

 in Erie, Niagara, Monroe, and Onondaga Cos. (Marshall, 1895) in tributaries of 

 the St. Lawrence sj^stem and along the route of the Erie canal. This group of 

 localities, confirmed also by our specimens from Tonowanda Creek, is important 

 (see below). It is not found in Lake Erie proper, being there replaced by A. 

 plicata. 



In Ohio it is present in both drainages (Sterki, 1907o), and has been repeatedly 

 reported from streams running to Lake Erie; from the Cayuga, Rock, and Sandusky 

 Rivers (Dall & Simpson, 1895), and also from the Maumee River (Carnegie Mu- 

 seum). This establishes one route of migration from the Oh-io to the lake. It is 

 furthermore found in southern Michigan (Walker, 1898). It also crosses over in 

 the north into the Red River of the North and the Lake Winnipeg-drainages. 



Towards the south and southwest, the boundaries of the range of the true A. 

 plicata costata are poorly known. It certainly exists in tributaries of the Cumber- 

 land in Tennessee (Wilson & Clark, 1914), and in the headwaters of the Tennessee 

 in eastern Tennessee and Virginia (Carnegie Museum). But farther in this 

 direction it is replaced by the southern form (perplicata) . In Arkansas and Okla- 

 homa forms inter grading with the latter are found. 



From the middle Ohio (region of Cincinnati) downward, and in the Mississippi 

 River, its place is largely taken by the closely allied Amblema peruviana (Lamarck)," 



-' This is the Quadrula plicata of Simpson (1914, p. 814). 



