82 INVERTEBRATA OF MASSACHUSETTS. 



behind the beaks ; surface smooth, very slightly wrinkled by the 

 lines of growth ; epidermis light yellowish-olive ; interior glossy, 

 bluish-white ; hinge margin narrow, with a small lateral tooth on 

 the left valve, about half way along the anterior margin, and a 

 groove to receive it on the right valve ; muscular impressions 

 rather shallow ; inner margin not crenulated. Length inch, 

 height 2% inch, breadth $ inch. 



Several specimens of this small and very distinct species have 

 been furnished me for description by Dr. Prescott, of Lynn, and 

 by Dr. Storer, of Boston. They were taken from the stomachs 

 of fish caught in Massachusetts Bay. 



Its triangular form, smooth, glossy surface, and more especially the 

 small lateral tooth, at once distinguish it. In all our other species the 

 anterior margin is shortest and concave, and the lunule deeply exca- 

 vated ; but in this the posterior and anterior slopes are equally direct 

 from the very summit. 



It must be very closely assimilated to the Venus triangularis of 

 Montagu, the Cyprina triangularis of Turton, in his " British Bi- 

 valves." But I cannot make out the requisite number of teeth to 

 bring it within the genus Cyprina, or to correspond with their descrip- 

 tions. ' A hasty observer might confound it with the young of Cyprina 

 Isldndica, which is more rounded and rough. 



The existence of a lateral tooth would, literally, exclude the shell 

 from this genus. But the general characters will bring it naturally 

 into the same genus with A. castdnea and sulcdta. Moreover, a lateral 

 tooth is more or less developed in individuals of all the species ; and 

 this only shows the close alliance of the genus to VENUS, with which, 

 indeed, Blainville unites it. 



GENUS CYPRINA, LAM. 



Shell obliquely heart-shaped, beaks prominent ; hinge with three 

 unequal^ diverging, cardinal teeth, and a remote lateral one ; palle- 

 al impression simple. 



CYPRINA ISLANDICA. 



Shell ponderous, ventricose, round-ovate, inequilateral, shortest 

 and compressed anteriorly ; lunule none ; epidermis chestnut -brown, 

 thick, and coarsely wrinkled ; edge simple. 



