THE MOLLUSCA 



IO13 



154 USS) Shell retrorse to short oval; beaks high and central. 



Subgenus Obovaria s.s. 



A small group of species mosll\- found in the Southern States from Alabama to Arkansas, 

 The type, (), rtiiisa Lam, (l-'ig, 15 [7), occurs in the Ohio, Tennessee^ and Cumberland systems, 

 and another species ranges nortli to southern Micliigan. 



Fig, 1517- 



155 (154) Shell elliptical; beaks anterior. 



Subgenus Pseudoon Simpson. 



Two species only. The type, Obovaria ellipsis Lea {Fig. 1518; X 3), ranges from the upper 

 Mississippi and lower Great Lakes south to Tennessee and Arkansas. The other is found from 

 Arkansas to Louisiana and east to Alabama. 



Fig. 1518. 



156 (tsq) Male and female shells different, the latter being more or less 

 inflated in the post-basal region. Shell triangular ovate, 

 with a distinct, often sharp posterior ridge; hinge complete. 

 Marsupium kidney-shaped, occupying the posterior portion 

 of the outer gills, but not extending quite to the hinder end. 



Plagiola Agassiz . . 157 



