167 
broad, projecting in a ledge on the inner side of the lamelliform 
tooth of the left valve. 
Yar. 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 car7os7is, 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. elUpticuSj 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 different. 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- 
]3ressions two, remote, oval or angular, distinct ; impression of the 
mantle profoundly sinuous before; teeth various, generally im- 
perfect. 
Ohs. 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 
