178 AMERICAN MARINE CONCHOLOGY. 
Genus ARCA, Linnzus. 
Syst. Nat., edit. x. 1758. 
The animal has a long-pointed foot, deeply grooved, and heeled, 
mantle furnished with ocelli; palpi 0; gills long, narrow, less 
striated externally, continuous with the lips; hearts two, each 
with an auricle. 
The Arcas with close valves have the left valve a little larger 
than the right, and somewhat overlapping at the margin. 
There are about two hundred species ; distribution universal, 
ranging from low-water to 230 fathoms. 
1. A. Nom, Linneus. Fig. 464. 
Syst. Nat., edit. xii. 1140. 1767. 
A. zebra, Swainson, Zool. Ilust., Ne. 26, t. 118. 
Shell elongately oblong, anterior side very short, posterior side 
emarginate, with a blunt keel extending from the umbone to the 
margin; ventral margin more or less gaping; white, with waved 
brownish streaks; radiately ribbed; ligament area flatly concave. 
North Carolina to West Indies. (Medit.) 
2. A. PONDEROSA, Say. Fig. 467. 
Journ. Philad. Acad., ii. 267. 1822. 
Shell very thick and ponderous, somewhat oblique, with 25 to 
28 ribs, each marked with an impressed line. Beaks distant, op- 
posite the middle of the hinge; lower margin nearly straight or 
even somewhat contracted in the middle. 
Length 2.5, height 2 inches. 
Southern Coast. 
Fossil valves of this species sometimes occur on the beach at 
Cape May and Atlantic City, N. J. 
3. A. TRANSVERSA, Say. Fig. 465. 
Journ. Phil. Acad., ii. 269. 1822. 
Shell transversely oblong, rhomboidal, with from 82 to 35 ribs, 
umbones separated by a long narrow area; extremities of the hinge 
margin angulated; epidermis chestnut-brown. 
Length 30, height 8.5 mill. 
New England, New York, southwards. 
4. A. LIENOSA, Say. Fig. 469. 
Am. Conch., iv. t. 36, f. 1. 1882. 
This shell is described as fossil, and worn (probably fossil) 
valves are found abundantly at Beaufort, N.C. It is admitted 
