340 New 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 so U. cariosus of 
the Delenie 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. Length2inches. 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 abruptly 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 oluted: to the U. 
phaseolus of Hildreth; it is not however so compressed, is more 
pointed behind, &c. and differs altogether in the — markings 
or pre: ‘Tt is never rayed. 
~ Unto AROTATUS. ke ie 
Shell narrow-elliptical, elongated, much scape 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 
