184 MEMOIRS OF THE CARNEGIE MUSEUM. 



Remarks: This siDecies is easily recognized and distinguished from others of 

 the genus by the general shape, sculpture of the posterior slope, color of the epi- 

 dermis, and by the hinge-teeth. There is no anodontine shell, except Alasjnidonta 

 varicosa, which could be confounded with it, but it bears an external and super- 

 ficial resemblance in shape and color to a lampsiline shell, Truncilla triquetra 

 Rafinesque. The latter, however, is much smaller and has a different hinge. 



The typical form of A. marginata is a western shell. It is rather variable in 

 shape. Its chief characters are its size, strongly truncate posterior end, and, 

 in consequence of this, the more median position of the beaks, with the anterior 

 end of the shell more developed. In this it is distinguished from its eastern repre- 

 sentative, A. varicosa. But there are shells found in western Pennsylvania, chiefly 

 in the mountain-streams, in which the truncation of the posterior end is not so 

 pronounced. The existence of a peculiar race in the Susquehanna-drainage will 

 be discussed below (See under var. siisquehannce) . 



Specimens from Conneaut Creek (drainage of Lake Erie) do not differ from 

 the typical A. marginata. 



Pennsylvanian localities represented in the Carnegie Museum: 



Ohio- and Beaver-drainages : 



Little Beaver Creek, Cannelton, Beaver Co. (Miss Vera White & H. H. Smith). 



Beaver River, Wampum, Lawrence Co. (G. H. Clapp & H. H. Smith). 



Connoquenessing Creek, Ellwood City, Lawrence Co. (G. H. Clapp & H. H. Smith). 



Slipperjrock Creek, Wurtemberg and Rose Point, Lawrence Co. 



Wolf Creek, Grove City, Mercer Co. 



Mahoning River, Mahoningtown, Lawrence Co. 



Neshannock Creek, Eastbrook and Volant, Lawrence Co.; Leesburg, Mercer Co. 



Shenango River, Harbor Bridge and Pulaski, Lawrence Co.; Sharpsville, Clarksville, Shenango, and 



Jamestown, Mercer Co. 

 Pymatuning Creek, Pymatuning Township, Mercer Co. 

 Little Shenango River, Greenville, Mercer Co. 



A llegheny-drainage : 

 Allegheny River, Godfrey, Kelly, and Templeton, Armstrong Co.; Walnut Bend, Venango Co.; Tionesta, 



and Hickory, Forest Co.; Warren, Warren Co.; Larabee, McKean Co. (Dennis Dally). 

 Buffalo Creek, Harbison, Butler Co. 



Loyalhanna River, Idlepark and Ligonier, Westmoreland Co. 

 Quemahoning Creek, Stanton's Mill, Somerset Co. 

 Crooked Creek, Rosston, Armstrong Co. 

 Little IMahoning Creek, Goodville, Indiana Co. 



French Creek, Utica, Venango Co.; Cochranton, Meadville, and Cambridge Springs, Crawford Co. 

 Leboeuf Creek, Waterford, Erie Co. 

 Connewango Creek Russell, Warren Co. 



