THE MOLLUSCA 



1009 



144 (145) Male shell with a posterior and central radiating ridge with a 

 furrow between. Female with a greatly produced inflation 

 a little behind the center of the base. 



Subgenus Dysnomia Agassiz. 



This is one of the most remarkable groups of the genus and is represented by three species 

 from the Ohio, Cumberland, and Tennessee rivers. Type, Truncilla foliata Hild. (Fig. 1510; 

 X*). 



FIG. 1510. 



145 (142) Male shell with a wide, shallow, radiating depression in front of 

 the posterior ridge. Female with a rounded, foliaceous 

 swelling at the posterior base. . . Subgenus Pilea Simpson. 



Eight species, found mostly in the Tennessee drainage, but ranging north to southern Michigan 

 and west to Arkansas. Type, Truncilla personata Lea (Fig. 1511). 



FIG. 1511. 



146 (151) Male and female shell different. Shell ovate to elliptical, smooth; 

 hinge complete. Female shell more or less expanded in 

 the post-basal region, but the expansion does not differ in 

 texture from the rest of the shell. Marsupium occupying 

 the posterior part of the outer gill. 



Lampsilis Rafinesque . . 147 



