93 



A remarkable species and readily recognised. 



Arca pexata. — Shell covered with a hairy epidennis, trans- 

 versely subovate, with from thirty-two to thirty-six ribs, placed 

 nearer to each other than the lenirth of their own diameters; um- 

 bones moderate; apices approximate, placed far backward very 

 near the posterior termination of the hinge ; posterior edge round- 

 ed, destitute of an angle; anterior edge rounded, with an angle at 

 the termination of the hinge ; inferior edge regularly rounded. 



Length one inch and seven-tenths. Breadth two inches and 

 three-tenths. Inhabits the coast of the United States. Cabinet 

 of the Academy and Philadelphia Museum. 



A common species, distinguished by the name of the bloody 

 clam. It is covered with a hairy epidermis, and when violently 

 opened, an effusion of a red sanies proceeds from the animal. In 

 the young shell, an angle is perceptible on the posterior edge, at 

 the termination of the hinge margin^ but this disappears with 

 age. 



Arca incongrua. — Shell somewhat rhomboidal, with from 

 twenty-six to twenty-eight ribs, placed nearer to each other than 

 the length of their own diameters, and crossed by elevated, ob- 

 tuse, equal and equidistant lines, which are altogether wanting on 

 ten rays of the disk of the left valve ; apices opposite to the middle 

 of the hinge, distant from each other, with a lanceolate space be- 

 tween them, of which the breadth is about one-third of its length ; 

 extremities of the hinge margin angulated ; posterior edge rounded; 

 inferior edge rounded, that of the right valye extended a little be- 

 yond the regular curve in the middle ; anterior margin cordate, 

 flattened ; anterior edge nearly rectilinear. 



Length two inches Breadth rather more than two inches. In- 

 habits the estuaries of the United States. Cabinet of the Academy 

 and Philadelphia Museum. 



This species, which is veiy abundant on our coasrt, strongly re- 

 sembles A. rhomhea, but agreeably to the figure in the En eye. 

 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 recti- 

 linear anterior edge. 



Arca 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 



