OF THE UNITED STATES. 267 



Inhabits the southern coast. 



Length, from the tip of the angle to the middle of 

 the base, three-fifths of an inch. 



Breadth one and one-tenth of an inch. 



Thickness nearly half an inch. 



Cabinet of the Academy and Philadelphia Mu- 

 seum. 



This species, like the preceding, is furnished with 

 a membranaceous expansion over a portion of the 

 epidermis, giving rise to a number of filamentous pro- 

 cesses, by means of which the shell is attached to 

 various marine bodies. 



ARC A, Lin. Lam. 



1. A. *ponderosa. Shell somewhat oblique, very 

 thick and ponderous, with from twenty-five to twenty- 

 eight ribs, each marked by an impressed line ; inter- 

 stitial spaces equal to the width of the ribs ; umbones 

 very prominent ; apices remote from each other, and 

 opposite to the middle of the hinge, spaces between 

 them with longitudinal lines as prominent as their 

 corresponding teeth ; anterior margin cordate, flat- 

 tened, distinguished from the disk by an abrupt an- 

 gular ridge ; posterior edge rounded, very short ; 

 inferior edge nearly rectilinear, or contracted in the 

 middle. 



Leugth two inches and one-eighth. 



Greatest breadth two inches and a half nearly. 



Inhabits the southern coast. 



My cabinet. 



A remarkable species and readily recognised. 



