30 



posterior one almost effaced. Breadth sometimes 

 six inches. 



The U. alatus of SAY, which is found in Lake 

 Erie, appears to resemble this species, and to 

 differ from it only in its wing, which is rugose 

 inside ; contour marked, flexuous ; lamellar tooth 

 simple in the right valve ; length four fifths of 

 the breadth, &c. It appears that the two follow- 

 ing species of SAY, should also be referred to 

 this genus, but he does not point out the lamel- 

 lar tooth as being flexuous. 



U. ochraceus. 



U. cariosus. 



Fourth Genus TRUNCILLA. 



Shell semi-triangular, axis nearly medial : 

 ligament oblique ; the truncation plane, oblique, 

 posterior; cardinal tooth smooth, denticulated, 

 compressed ; lamellar tooth compressed, oblique ; 

 animal similar to that of the Unio. 



This name is derived from the remarkably 

 oblique truncation, which is more distinctly 

 marked than in all the other species of this 

 family. 



