88 



MEMOIRS OF THE CARNEGIE MUSEUM. 



the smaller mussels, and its chief characters are the peculiar, obliquely-ovate, 

 elongated or cuneiform outline, and the light color of the epidermis, with green 

 patches, which are frequently (but not always) present. In shape, situation of 

 beaks, and obesity, it varies greatly (compare, for instance the figure of Lea's 

 patulus, Obs. I, 1834, PI. 12, fig. 20, and of Conrad's clava, Mon. 1835, PI. 5, fig. 1). 

 In old specimens the beaks are sometimes quite anterior, forming the foremost 

 point of the shell, but such extremes are rare in Pennsylvania, and generally the 

 beaks remain a httle behind the anterior margin of the shell. Also with regard to 

 the convexity of the valves no extreme cases have been observed in Pennsylvania; 

 on the contrary the tendency is more toward the flatter types (corresponding more 

 nearly to patulus). 



The peculiar color-markings are present in most specimens, but extremely 

 variable. The spots are found chiefly towards the beaks, and often an irregular 



Fig. 9. 

 • Pleurobema clava. 



series of these spots runs down just in front of the posterior slope. In old shells, 

 the epidermis tends to become uniformly brown. 



Localities in Pennsylvania, represented in the Carnegie Museum: 



Ohio-drainage: 

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

 Raccoon Creek, Raccoon Township, Beaver Co. 



Beai<er-drainage : 

 Beaver River, Wampum, Lawrence Co. (G. H. Chipp it H. IL Smith). 

 Connoquenessing Creek, Harmony, Butler Co. 

 Mahoning River, Mahoningtown and Coverts, Lawrence Co. 



