THE MOLLUSCA 



999 



114 (115) Outer gills only serving as marsupia. Shell rounded; beaks sculp- 

 tured with numerous fine irregular corrugations; hinge com- 

 plete; nacre violet Rotundaria Rafinesque. 



The type, R. tuherculata Raf. (Fig. i486), ranges from southern Michigan through the Ohio, 

 Tennessee, and Mississippi systems, south to Texas. Another species ranges from Kentucky 

 and Tennessee to Iowa. 



Fig. 1485. 



lis (117) 



Outer gills only serving as marsupia. Shell alike in both sexes; 

 triangular to rhomboid, usually with a prominent umbonal 

 region; beaks at or near the anterior end; beak cavities shal- 

 low; hinge complete; surface smooth, brown to yellow, 

 usually not very dark, frequently rayed. 



Pleurobema Rafinesque . 116 



This is a large group, of which 

 more than seventy species are 

 known. With the exception of a 

 few species found in the Ohio and 

 Mississippi drainage, it is confined 

 to the streams of the Southeastern 

 States from Tennessee and Georgia 

 to the Mississippi. The shells of 

 this genus are easily distinguished 

 from the Quadrulae, which they 

 often resemble by the uniformly 

 shallow beak cavities. Type, P. 

 dava Lam. (Fig. 1487). 



Fig. 1482. 



