!818.] A NEW GENUS GF SHELLS. 45$ 



Description of a new genus of Fresh Water Bivalve 

 Shells. By Thomas Say, 



Genus ALASMIDONTA. 



Shell transverse, equivalve; inequilateral, hinge with 

 a primary tooth in each valve; cicatrices three. 

 Animal resembling that of Unio? 



SPECIES. 



A. Marginata. Shell transversely oblong-suboval, 

 white^ covered with an olive brown epidermis, obsoletely 

 radiate with green, numerous concentric wrinkles; wnbo 

 with about three concentric undulations; ligament slope 

 abruptly depressed, with numerous, obtuse, oblique rug^, 

 decussating the concentric ones, which are obsolete in 

 that part; within bluish-white, margin white; cavity of the 

 umbo, not distinctly impressed by the external unduia- 

 tions; tooth compressed, oblique, nearly parallel with the 

 posterior slope, and terminating abruptly behind. 



Length, exclusive of the umbo, one inch and a quar- 

 ter, breadth two inches and a half. 



Cabinet of the Academy. 



The inner margin is of a chalky whitness, in this re- 

 spect resembling Anadonta marginata. It was communi- 

 cated to me by Mr. Isaac Lea, who found it in the Scioto 

 river. 



Uhio undulata of the first and second American edi- 

 tions of Nicholson's Encyclopedia, is a species of this 

 genus, but it is very distinct from the marginata. 



This genus will be properly placed between Unio 

 and Anodofitap and in conjunction with Dipsas of Leach, 



