a^lde very short : anterior side with a groove, obsolete on 

 the umbo, but becoming deeper to the anterior edge, vs^hich 

 is consequently emarginate a little belov^^ its middle ; a lit- 

 tle gaping ; the inferior angle somewhat more prominent, 

 rounded and denticulated : anterior muscular impression 

 very deep : palleal impression emarginate opposite to the 

 denticulated angle : above this angle also, the concavity is 

 abruptly more concave, so as to form a definite dilated 

 groove. 



Var. a. Within deep reddish purple. 



OBSERVATIONS. 



The remarkable character of the anterior basal ande or 

 lobe of this species, renders an extended and more particu- 

 lar description unnecessary. In the figure of the annexed 

 plate we have brought this character particularly into view, 

 by a delineation of the front of the shell, and two or three 

 series of the teeth are visible in some specimens, like vari- 

 ces in the univalves, on the surface of the shell, fringing 

 the deeper undulations or wrinkles. [We may remark 

 tliat in our figure these teeth have too much of a serrated 

 appearance ; they are in reality more tooth-shaped, though 

 slender.] It has considerable resemblance to the flngella- 

 tus, Nob., in general form, brevity of the posterior side, 

 summits, cavity of the hinge membranes and radiated sur- 

 face, but the denticulated anterior edge, the more acute 

 anterior groove and other subordinate characters, amply 

 distinguish it. It is a small species, even smaller than the 

 jlagcllatus. 



A remarkable and gigantic variety occurs in Cumber- 

 land river ; a specimen is rather more than two inches 



PL ATI: V. 



