OBSERVATIONS. 



The colour of the inner surface is, in most specimens 

 very beautiful. The umbo is generally decorticated, ex- 

 posing a surface slightly tinged with the colour of the ca- 

 vitj' of the shell. The internal surface of this shell, as in 

 several species, is minutely granulated and undulated ; a 

 character very sensible under the magnifier. 



Some conchologists have considered this shell a mere va- 

 riety of the cariosus, Nob., probably because like that spe- 

 cies it is generally more or less truncated before ; but there 

 are other characters which appear to me to forbid a speci- 

 fic union. The abruptus is always of much less breadth, 

 the beaks much nearer to the posterior extremity, the per- 

 pendicular length from the beaks to the base much great- 

 er : the cardinal teeth direct, much more robust, that of 

 the left valve being trifid : whereas in cariosus the cardinal 

 teeth are decidedly 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. eJUpticus, Barnes, by the va- 

 riety a. ; but although the teeth are nearly similar, yet 

 that species is never truncated, the beaks are never situa- 

 ted 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 fre- 

 quently in the Wabash. 



REFERENCE TO THE PLATE. 



Upper figure outside of a vulvc. Lower figure insido of a valvo. Middle 

 figure back of a valve. 

 PLATE XVII. 



