118 Mr. Barnes On the Genera Unio and Alasmodontu. 



muscular impression compound, hinge with two^ teeth in 

 each valve ; the Cardinal one, short, irregular, simple or 

 divided into two, substriated ; the other elongated, com- 

 pressed lateral, extending beneath the corselet. Liganient 

 exterior. 



1 &i 2, Generally, but not always when young. 



3, Others consider the divisions as separate teeth. 



Divisions. 



A. Cardinal teeth direct. 



B. Cardinal Teeth, Oblique 



Sections. 



L * Cardinal teeth, very thick. 



A. < ** Cardinal teeth, moderately thick. 

 ^ ***C'ardinal teeth, small, 

 f **** Cardinal teeth, broad, compressed. 

 \ ***** Cardinal teeth, narrow, compressed. 



* Cardinal teeth, very thick, direct. 

 Species. 

 I. UnioCrassus. Fig. 1. {^[oa'slde. 

 Shell very thick, tumid ; Cardinal teeth, lobed, angula- 

 ted ; Posterior cicatrix, deep, rough. 



Unio Crassus. Mr. Say. 

 Unio Crassidens. M. LamarcJc. 

 Mya ponderosa ? Mr. Dillwynp. bl. 

 Mr. Say's Amer. Conch, pi. 1, fig. 8. 

 Habitat. The Ohio, Mississippi, and the Lakes. 

 Diameter 2*4 Length 3-2 Breadth 4*8 inches. 



My Collection. 



Shell oval, ponderous, rounded behind, angulated before; 

 Epidermis blackish brown ; surface waved. Cardinal 

 tooth deeply sulcated ; anterior cicatrix wrinkled and 

 striated ; Naker pearly white and iridescent. 



Remarks. — The varieties of this shell are numerous, and 

 they differ considerably in form and surface. In some, the 

 beaks are large, prominent, re-curved, projecting backwards 



