340 J^ew species of Fresh Water Shells. 



with strong prominent concentric lines and undulations; beaks slight- 

 ly prominent and with undulations disposed in angular lines ', epider- 

 mis straw colored, rayed only behind the umbonial slope ; wrinkled 

 only at the two ends ; cavity most capacious behind the middle of the 

 valve ; nacre pearly white and iridescent. 



Observations. Approaches the U. abruptus of Say, and is a very 

 rare species; a specimen very much resembles the U. cariosus of 

 the Delaware and Schuylkill rivers. 



Unio arcus. pi. 1. fig. 8. 



Shell narrow-elliptical, thick and ponderous ; dorsal margin regu- 

 larly curved, or arched; beaks scarcely above the dorsal line; basal 

 margin straight, posterior side somewhat cuneate. 



Inhabits Alabama river. Length 2 inches. Cabinet of the Acad- 

 emy of Natural Sciences of Philadelphia. 



Shell narrow-elliptical, thick and ponderous ; dorsal margin form- 

 ing an arched curve, which is scarcely interrupted by the beaks, 

 ' umbonial slope abrupdy rounded posteriorly, basal margin straight ; 

 epidermis olive and wrinkled ; cardinal teeth thick, pyramidal ; dis- 

 tant from the lateral teeth ; anterior muscular impression profound ; 

 posterior rather deeply impressed ; cavity not capacious ; nacre pearly 

 white. 



Observations. This is a rare shell, and distantly related to the U. 

 phaseolus of Hildreth ; it is not however so compressed, is more 

 pointed behind, &;c. and differs altogether in the epidermal markings 

 or color. It is never rayed. 



Unio arctatus. PI. 1. fig. 9. 



Shell narrow-elliptical, elongated, much compressed ; and slightly 

 contracted over the umbo to the base ; beaks not prominent; basal 

 margin slightly arcuated, cardinal and lateral teeth distinct. 



Inhabits Black Warrior and Alabama rivers. Length 2 inches. 

 Cabinet of the Academy of Natural Sciences of Philadelphia. 



Shell elongated, much compressed, slightly contracted from the 

 beaks to the base ; posterior side much produced and sub-angulated 

 at the end ; beaks depressed ; epidermis very dark olive ; cardinal 

 teeth disposed to be single in both valves ; lateral teeth compressed 

 and a little prominent, nacre bluish white. 



Observations. This shell has somewhat the form of the U. mo- 

 nodonta, Say, but it is more nearly allied to U. purpureus of Say 



