92 



with a dilated but slightly impressed contraction in the middle ; 

 epidermis transversely wrinkled, light-brown, the raised oblique 

 portion of the shell yellowish-white; cortex with membraneous 

 scales and filaments, and covering all the anterior portion of the 

 shell to the beak. 



Modiola Americana, Leach. Zool. Misc., vol. 2, pi. 12, fig. 1. 

 Var. 



Inhabits the Southern coast. Length from the tip of the angle 

 to the middle of the base, three-fifths of an inch nearly. Breadth 

 one and one-fifth of an inch. Thickness half an inch. Cabinet of 

 the Academy and Philadelphia Museum. 



This variety difi'ers from that figured by Dr. Leach, in being 

 always destitute of oblique colored radii. 



Modiola CASTANEA. — Shell transversely oblong suboval; hinge 

 margin elevated in a right line from the beak to the alated angle, 

 from which it declines in a line slightly arquated ; anterior margin 

 rounded at the tip ; posterior margin rather large ; base with a 

 dilated but slightly impressed contraction before the middle ; epi- 

 dermis concentrically wrinkled, castaneous ; cortex not continued 

 behind the middle of the shell ; within bluish. 



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. Cabi- 

 net of the Academy and Philadelphia Museum. 



This species, like the preceding, is furnished with a membrana ■ 

 ceous expansion over a portion of the epidermis, giving rise to a 

 number of filamentous processes, by means of which the shell is 

 attached to various marine bodies. 



Arca ponderosa. — Shell somewhat oblique, very thick and 

 ponderous, with from twenty-five to twenty-eight ribs, each marked 

 by an impressed line ; interstitial 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 ; ante- 

 rior margin cordate, flattened, distinguished from the disk by an 

 abrupt angular ridge ; posterior edge rounded, very short ; inferior 

 edge nearly rectilinear, or contracted in the middle. 



Length two inches and one-eighth. Greatest breadth two inches 

 and a half nearly. Inhabits the Southern coast. My cabinet. 



