UNIO ABRUPTUS. 



SPECIFIC CHARACTER. 



Subquadrate, truncate before. 



DESCRIPTION. 



Shell oval-quadrate, yellowish olive or brownish, slight- 

 ly radiated with blackish-brown narrow lines, which do 

 not extend to the base : umbo not veiy prominent, round- 

 ed, slightly compressed, generally radiated with obscure 

 greenish, and placed far back, almost terminal : disk con- 

 vex, anteriorly with a hardly raised, very obtuse elevation, 

 becoming obvious towards the margin and a little promi- 

 nent at the anterior basal angle : hinge margin a little de- 

 pressed, so as to form an angle with the disk, more acute 

 towards the umbo : ligament higher than the beaks : an- 

 terioi' margin truncated by an almost straight line : basal 

 margin very slightly contracted : posterior margin very 

 short, gaping considerably from the middle of the base to 

 the cavity of the hinge membranes : cavity of the hinge 

 memhrayjes very obvious, rather wide, as long or longer be- 

 hind the beaks than broad : within wdiitc, or tinged with 

 very fine fulvous-pink, or salmon colour : posterior cica- 

 trix profound ; smaller cicatrix rather large, oblique, ar- 

 quated : palleal impression hardly more prominent than 

 the anterior cicatrix : cardinal teeth thick, direct ; that of 

 tlie left valve subtrifid : tooth phi I c rather broad, project- 

 ing in a ledge on the inner side of the lanK^lliform tooth 

 of tlie left valve. 



Var. a. Anteriorly more prominent and hardly truncate. 



PLATB XVII. 



