62 



Presented to the Academy by Mr. L'Hermenier of Chai-leston, 

 am intelligent and zealous naturalist ; he assured me that this species 

 inhabits near Charleston. It somewhat resembles large specimens 

 of the P. trivolvis of the American edition of Nicholson's Encyc.^ 

 but differs in the total absence of carina, and in having a more 

 smooth and polished surface, as well as a declining and more 

 obliqvie aperture, and a more profound and much- more regularly 

 concave umbilicus. 



[J. A. N. S. vol. i. p. 459, 4G0, Dec, 1818.] 



Genus Alasmidonta. — Shell transverse, equivalve ; inequila- 

 teral, hinge with a primary tooth in each valve ; cicatrices three. 

 Animal resembling that of Unio ? 



Alasmidonta marginata. — Shell transversely oblong-suboval, 

 white, covered with an olive brown epidermis, obsoletely radiate 

 with green numerous concentric wrinkles ; umbo, with about three 

 concentric undulations ; ligament slope abruptly depressed, with 

 numerous, obtuse, oblique rugae, 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 undu- 

 lations ; tooth compressed, oblique, nearly parallel with the poste- 

 rior slope, and terminating abruptly behind. 



Length, exclusive of the umbo, one inch and a quarter, breadth 

 two inches and a half. Cabinet of the Academy* 



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

 .sembling Anadonta mar(jinata. It was communicated to me by 

 Mr. Isaac Lea, who found it in the Scioto river. 



Unio tmdulata of the first and second American editions of 

 Nicholson's Encyclopi^dia, is a species of this genus, but it is very 

 distinct from the marginata. 



This genus will be properly placed between Un!o-A\\^ Aiiodonta, 

 and in conjunction with Dispas of Leach, it will complete the 

 chain of connection between those two genera. Alasmidonta 

 coiTCsponds with those genera in the number of its cicatrices, but 

 is separable from Anodonta by its primary tooth ; from Unio by 

 being destitute of the lamelliform teeth ; and from Diapas also by 

 the last mentioned character, as well as by the presence of a pri- 

 mary tooth, which is wanting in that genus. 



