26 



OBSERVATIONS. 



This large species is remarkable for its resem-' 

 blance to two very different species, U. alatus, oi' 

 Say, and U. ater, of Lea; it has an outHne not unHke 

 the Ibrmer, but is less winged and much more con- 

 vex; it is less inflated and more elongated than the 

 latter, and is more angulated and truncated on the 

 posterior margin. The cardinal teeth are directed 

 obliquely forwards in U. ater; in this species they 

 are, on the contrary, directed obliquely backw^ards. 

 It is not an uncommon species in the Black Warrior 

 river, in North Alabama, where I procured them 

 in six or eight feet water, in the centre of the stream; 

 the purity of the water enabled me to detect them 

 with facility, although a very small portion of the 

 -^hell projected above the surface of the mud in which 

 they were always imbedded. I obtained them by 

 means of a long pointed stick, with the extremity 

 inserted between the gaping valves, which the ani' 

 mal immediately closed with such force, that I could 

 readily draw them from their tenacious beds. 



UN 10 TAENIA T US. 



Plate \Y.—Fig. 2. 



DESCRIPTION. 



Shell regularly convex and elliptical, moderately 

 fjn'ck, epidermis -finely wrinkled, olivaceous, with 

 regular green interrupted rays; beaks slightly promi- 

 nent; umbonial slope rounded; hgament slope with a 

 slightly impressed line; base arcuated; cardinal and 

 lateral teeth prominent; within bluish white. 



