

OF THE UNITED STATES. 269 



of the hinge margin angulated ; posterior edge round- 

 ed : inferior edge rounded, that of the light valve ex- 

 tended a little heyond the regular cuvve in the mid- 

 dle ; anterior margin cordate, flattened; anterior 

 edge nearly rectilinear. 



Length two inches. 



Breadth rather more than two inches. 



Inhabits the estuaries of the United States. 



Cabinet of the Academy and Philadelphia Mu- 

 seum. 



This species, which is very abundant on our coast, 

 strongly resembles A. rhomhea, but, agreeably to the 

 figure in the Encyc. Meth., it differs in the width of 

 the space on the hinge margin, in the width of the 

 spaces between the ribs, and in its more rectilinear 

 anterior edge. 



4. A. ^transversa. Shell transversely oblong, 

 rhomboidal, with from thirty-two to thirty-five ribs, 

 placed at nearly the length of their own diameters 

 distant from each other ; apices separated by a long 

 narrow space, and situate at the termination of the 

 posterior third of the length of the hinge margin ; ex- 

 tremities of the hinge margin angulated ; anterior 

 edge, superior moiety rectilinear; posterior edge 

 rounded ; inferior edge nearly rectilinear, or very 

 obtusely rounded ; on the hinge space one or two an- 

 gulated lines are drawn from the apex, diverging to 

 the hinge edge. 



Length less than seven-twentieths of an inch. 



Breadth one inch and one-fifth. 



