30 



OBSERVATIONS. 



This shell, in having a slightly undulated disk, ap- 

 proaches the mytiloides, Raf., but is very distinct. It 

 is well known in the collections by the name we have 

 adopted, but we are informed that Dr. Hildreth has 

 not published his description of it. I have figured a 

 fine specimen from Mr. Hyde's cabinet. This na- 

 turalist observes that the species inhabits Mahoning 

 river, near Pittsburg, and that of the very many 

 specimens he has received, none has perfect beaks. 



UNIO CATILLUS. 



Plate XIII. Fig. 2. 



DESCRIPTION. 



Shell oval orbicular, slightly oblique, moderately 

 thick, convex, dark chestnut-brown, with crowded 

 wrinkles; anterior margin obtusely rounded or sub- 

 truncated; posterior margin rectilinear, nearly direct, 

 extremity rounded; umbonial slope roimded; umbones 

 inflated, narrow, prominent; ligament margin rectili- 

 near, oblique, angular posteriorly; summits slightly 

 flattened; beaks much incurved, pointed, slightly tu- 

 berculated; lunule short ovate; within rosaceous; 

 cardinal teeth thick, direct, profoundly sulcated; la- 

 teral tooth of the left valve very thick, that of the 

 right valve compressed, and rising from a very broad 

 plate or base projecting on either side; cardinal plate 

 profoundly dilated. 



Cab. A. N. S. Xo. 2103. 



