167 



broad, projecting in a ledge on the inner side of tlie lamelliform 

 tooth of the left valve. 



Var. a. Anteriorly more prominent and hardly truncate. 



Ohs. The color of the inner surface is, in most specimens very 

 beautiful. The umbo is generally decorticated, exposing a surface 

 slightly tinged with the color of the cavity of the shell. The in- 

 ternal surface of this shell, as in several species, is minutely granu- 

 lated and undulated ; a character very sensible under the magnifier. 



Some conchologists have considered this shell a mere variety of 

 the carwi^is, nob., probably because, like that species, it is generally 

 more or less truncated before ; but there are other characters 

 which appear to me to forbid a specific union. The ahruptus is 

 always of much less breadth, the beaks much nearer to the poste- 

 rior extremity, the perpendicular length from the beaks to the 

 base much greater ; the cardinal teeth direct, much more robust, 

 that of the left valve being trifid : whereas in cariosns the cardinal 

 teeth are slightly oblique, bifid in each valve, and the plate on 

 which the teeth rest is much more slender, even when the general 

 thickness of the shells is the same. It is more closely related to 

 U. ellipticus, Barnes, by the variety a ; but although the teeth 

 are nearly similar, yet that species is never truncated, the beaks 

 are never situated so far back ; the cavity of the hinge membranes 

 is much narrower ; the anterior division of the cardinal tooth of 

 the left valve is less obvious, and the ledge on the inner side of 

 the lamelliform [tooth of the same valve is but slight ; the aspect 

 or habit also is quite difi'erent. It occurs frequently in the Wa- 

 bash. PI. 17. 



SoLECURTUS. — Shell equivalved, transversely elongated, gaping 

 at the extremities, which are obtusely and equally rounded ; hinge 

 and basal margins nearly parallel ; apex not prominent ; hinge dis- 

 tant from the extremity ; ligament external, short ; muscular im- 

 pressions two, remote, oval or angular, distinct ; impression of the 

 mantle profoundly sinuous before; teeth various, generally im- 

 perfect. 



Oha. A genus formed by Blainville to receive ten or twelve 

 species, hitherto referred to the genus Solen, and to which they 

 are indeed very closely allied. He divides this genus into three 

 parts, viz: 



A. Compressed, thin, with an anterior rib, obliquely decurrent 



