176 NEW-YORK FAUNA — MOLLUSCA. 
FAMILY ARCAD. 
Anima resembling those of the preceding family, partly adherent, and with a foot always 
large. Marine. Susu generally thick, regular, equivalve, inequilateral, with the hinge 
furnished on each valve with teeth in a regular series, often lamellar, straight or oblique. 
Muscular impressions two on each valve, almost always united by a. palleal impression, 
very narrow, and parallel with the margin of the shell. 
GENUS ARCA. Linneus. 
‘ 
Animal with the labial appendages very small and slender. Foot pedunculated, compressed, 
and divided throughout its length. Shell rather solid: beaks distant, separated by the 
area of the ligament. Hinge-margin straight, linear, without ribs at the extremities : teeth 
numerous, crowded, alternately inserted into each other ; ligament entirely internal 
ARCA PEXATA. 
PLATE XII. FIG. 211. 
(STATE COLLECTION.) 
Arca pexata. Say, Jour. Acad, Nat. Sciences, Vol. 2, p. 268. 
Alt -id. Goutp, Invertebrata of Mass. p. 95, fig. 60. 
Description. Shell thick and heavy, transversely ovate, inequilateral. Surface with thirty 
two to thirty-six radiating ribs, which are nearer to each other than their own diameters, and 
strongly impressed along the margin within. Beaks ventricose and prominent, obliquely di- 
rected; space between them very narrow. Valves closing accurately all round, obtusely an- 
gular on the anterior edge near the hinge margin. Epidermis consisting of long and fibrous 
threads, which are thickly distributed over the whole surface. 
Color of the epidermis dark brownish or black ; polished white within. 
Length. 20. Transverse diameter, 2°5. 
This is a common species along our coast. Its northern limits appear not to extend beyond 
Cape Cod. Tam not aware how far it ranges to the south. According to Mr. Say, this 
species, when violently opened, gives issue to a bloody sanies, whence it has derived its 
name of Bloody clam. This remarkably well characterized and very common species has 
now been described and known by American naturalists for more than twenty years, and yet 
it does not appear in the latest and best lists of species given by foreign writers. 
