SPECIES.
1. ?arginata. Shell transversely ob. long sub-oval, white, covered with an olive brown epidermis, obsoletely radiate with green, numerous concentrie wrink. les ; umbo with about three concentric undulations ; ligament slope abruptly de• pressed, with numerous, obtuse, oblique rum decussating the concentric ones which are obsolete in that part ; within bluish-white, margin white ; cavity of the umbo not distinctly impressed by the ex ternal undulations ; tooth compressed ; oblique, nearly parallel with the posteri or slope, and terminating abruptly be hind.
Length, exclusive of the umbo, one inch and one-fourth ; breadth, two inches and a half.
Journ. Acad. Net. Sci ences.
The inner margin is of a chalky white ness, in this respect resembling Anodonta Marginate. It was found by Mr. Isaac Lea, in the river 2. U. Undulate. Shell sub oval, greenish or olivaceotts, with ob tuse concentric wrinkles and radiate with green, a little uneven before ; beaks pro minent, acute, approximate, decorticat ed, and with four or five large, obtuse, dis tant, undulations, disappearing towards the basal margin : within bluish-white, cavity deep ; teeth one in each valve, thick and strong ; that of the left valve crenated, of the right valve somewhat bifid, and gradually sloping to the hinge margin.
Length, three-fifths of an inch: breadth, nine-tenths of an inch.
Plate 3. fig. 3.
Found in the Delaware and Schuylkill rivers, but is rather rare, and resembles Anodonta Undulate.
This genus, in conjunction with Dipsas of Leach, will complete the chain of con. nection between the two genera Unic and Anodonta. It corresponds with these genera in the number of its cicatrices, but is separable from Anodonta, by it primary tooth ; from Unio by being des. titute of the lathelliform teeth ; and from Dipsas, also by the last mentioned charac• ter, as well as by the presence of a pri. mary tootb, which is wanting in that genus This new genus we proposed in the former editions of this work, when de. scribing the Undulate, under the name o' ; but as the same term ha: been applied to a genus of univalves, I have substituted that of Alasmodonta.