186 MEMOIRS OF THE CARNEGIE MUSEUM. 



Distribution and Ecology in Pennsylvania (See fig. 18) : This species is rather 

 frequent in the Ohio-drainage in western Pennsylvania, but decidedly avoids the 

 larger rivers, and is more or less absent (probably extinct) in the Monongahela- 

 drainage. Rhoads mentions it from the Ohio at Coraopolis, below Pittsburgh, 

 but I never found it in the Ohio proper, nor in the Allegheny below Armstrong Co. 

 In the Ohio between Pennsylvania and Cincinnati it is also missing. In the 

 Monongahela-drainage it has been found only in Cheat River, but it turns up again 

 in the upper Monongahela S3"stem (Cheat and West Fork Rivers) in West Virginia. 



In the Beaver-drainage, this species is abundant, except in the extreme head- 

 waters. From Armstrong County upwards it is found regularly in almost all 

 streams tributary to the Allegheny, and goes far up into the upper Loyalhanna in 

 Westmoreland Co., the Quemahoning in Somerset Co., the Little Mahoning in 

 Indiana Co., and the uppermost Allegheny in McKean Co. 



A. marginata is most decidedly a species of the riffles, being found there in 

 finer or coarse, but firmly packed gravel, in swift currents. It is one of the species, 

 which in the Conemaugh-drainage enter the valleys between Chestnut and Laurel 

 Ridges and the Allegheny Front. 



It occurs also in Conneaut Creek of the Lake Erie-drainage. Here it is 

 rather abundant, and does not differ at aU from the common form of western 

 Pennsylvania. It should be noted that it never has been found on the Pennsyl- 

 vanian shores of Lake Erie, although Sterki (1907a) reports a small, slight form, 

 from the lake in Ohio, and Walker (1913, p. 22) reports A. varicosa from Lake 

 Erie. I have never seen this lake-form. 



Call (1900) remarks on the strongly developed foot of this species, which is, 

 indeed, quite remarkable. When fuUy extended, the foot is firmly attached to 

 the gravel in the river bed, and it requires quite an effort to dislodge the specimens. 



General distribution: Type locality, Scioto River, ChiUicothe, Ross Co., Ohio, 

 Say. (See Fox, 1901.) 



Simpson (1900) gives this species as from the "Upper Mississippi-drainage; 

 Ohio, Cumberland, and Tennessee river systems; Michigan; Upper St. Lawrence- 

 drainage." This is about right, but it should be noted that the species advances 

 eastward into certain mountain-streams of the western slope of the Alleghenies, 

 not only in the headwaters of the Tennessee in Tennessee and Virginia, but also 

 in West Virginia and Pennsylvania, in the upper Kanawha system, and in the 

 upper Conemaugh. In the Allegheny it also goes far up (to Olean, Cattaraugus 

 Co., New York) (Marshall, 1895) and to McKean Co., Pennsylvania. It is known 

 from several other localities in western New York, partly in the St. Lawrence, 



