ACEPHALA. 25 
tended to four times the length of the shell; the bran- 
chiz are coarsely pectinated, —the outer leaflet be- 
ing one third shorter than the inner one. The foot 
is compressed, resembling somewhat that of Venus. 
But at a point a little behind the middle of its sole 
or lower edge, I have detected a minute slit, from 
which I have once seen a byssus issuing. 
The cirrhi around apertures of the siphons in P. 
pholadiformis, are complicated, but very variable, and 
sometimes entirely wanting, or represented only by 
papillae. Much importance has been attached to 
the form of the cirhi among the characters of the 
animals of Bivalves, but in most cases but little de- 
pendence can be placed upon them. 
Saxicava arctica, Desh. F. et H. Mya arctica, L. 
S. distorta, Say. Gould. — Litt. to Corall. Whole 
Coast. 
S. rueosa, Lam. Mytilus rugosus, O. Fabr. Sazxicava 
distorta, Say. Gould. — Litt. to Corall. Whole 
Coast. 
PHOLADIDA. 
Puouas costata, Lin. Gould, 27.— Lamin. Alive 
in the mud of New Bedford Harbor, (see Gould, 
Bost. Proce. ii. 81.) 
P. rruncata, Say. Gould, Bost. Proe. ii. 81.— Lamin. 
New Bedford Harbor, (Greene.) Connecticut, (Jay.) 
P. crispata, Lin. Gould, 27.—Lamin. Eastport, 
(W. 8S.) to Stonington, (Lins.) and intermediate 
shores. Alive at Phillips Beach at low water, (‘True.) 
4 
