MULTIVALVES —_PHOLAS. 11 
Puotas.— STONE-PIERCER. 
DESCRIPTION OF PLATE III. 
Fig. 1. P. Striata. (In the wood. ) 
Fig. 2. P. Candida. Fig. 4. P. Costata. 
Fig. 3. P. Dactylus. Fig. 5. P. Papyracea. 
Shell having two primary valves, divaricate, with several smaller, 
differently-shaped accessory valves at the hinge:—hinge recurved, 
united by a cartilage; in the inside, beneath the hinge, is an in- 
curved tooth. 
IT appears that only twelve species of this ge- 
nus have been enumerated, and some of these are 
so alike, that in many iustances, they might be con- 
sidered as mere varieties rather than different species; 
however, they all possess sufficient determinate cha- 
racters to prevent their being blended with bivalves. 
The form of the Pholas is in most species ovate 
or oblong, constituted by two large valves oppo- 
site to each other, and to which is attached, on the 
back of the shell, a number of smaller ones, that act 
as substitutes for a hinge, which in bivalves gene- 
rally determines the generic character. 
Another character of the Pholas is, that the valves 
(the two large ones) never shut close, they invariably 
are open at one end, and, in most instances, at both. 
In the interior of the shell, in each valve, nearly in 
the middle, is an incurved tooth, sometimes of consi- 
derable length, and which has been considered pecu- 
liar to the genus. 
The exterior of the Pholas is generally destitute of 
color; sometimes it partakes of a brownish cast, but 
