This genus is without any tubular sheath; it derives its name from the
Greek word φωλεω, to hide, alluding to the custom of its inhabitant in
forming cells in rocks, wood, &c.
In form the Pholas is generally oblong, having two large valves opposite
to each other, with a number of smaller ones attached to the back as a
substitute for a hinge. The two large valves never shut close; they are
open at one end, and sometimes at both.
The exterior of the shell is usually of a pure or dusky white, but
sometimes of a brownish cast. In some species the shell is adorned with
beautiful delicate reticulations, like the finest lace; in others the
texture is coarser, like small basket-work. They are found in the
American, Indian, and European seas, each shell in a separate habitation
formed in limestone, sandstone, wood, coral, &c.; often discovered
completely imbedded in the oak planks of ships traversing those seas; as
they advance in growth they enlarge the space within, and leave the
aperture by which they entered of its primitive size.
Shell thin, sub-transparent, finely striated, elongated oval, bivalve,
equivalve, inequilateral; the valves only touching in the middle of
their edges; the summits but little marked, and concealed by a callosity
produced by the expansion of the dorsal lobes of the mantle; near the
hinge are often developed one or more accessory calcareous pieces; an
incurved tooth interior beneath the hinge.
Pholas dactylus.
P. orientalis.
P. candida.
P. dactyloides.
P. silicula.
P. costata.
P. crispata.
P. callosa.
P. clavata.
_P. dactylus._ The prickly Pholas. Pl. 3, fig. 3.
Answers to the general description, but is beset with small calcareous
spiny nodules on the ribs, which run widening and enlarging from the
summit to the margin; colour white or very light brown.
_P. striata._ The striated Pholas. Pl. 3, fig. 5.
Oval, the dorsal callosity leaving the summit free, and extending
towards the anterior and inferior extremity in such a manner that each
valve seems to be formed of three parts, because of an oblique furrow
from the summit to the margin; a tooth running down in the inside from
the summit; one pair of accessory pieces at the posterior extremity of
the shell.
_P. candida._ The white Pholas. Pl. 3, fig. 2.
Elongated, wedge-shaped; muscular impression almost medial; a kind of
oblique tooth parting from the summit; no accessory pieces.
_P. costata._ The ribbed Pholas. Pl. 3, fig. 4.
Elongated, wedge-shaped, covered with regular elevated jagged or
scalloped ribs, elegantly disposed; three dorsal accessory pieces;
muscular impression almost medial.
_P. crispata._ The curled Pholas.
Somewhat oval, truncated behind, and as if divided into two parts by an
oblique furrow from the summit to the base; anterior part reticulated,
the other parts plain; muscular impression marginal.
_P. clavata._ The clubbed Pholas.
Short, wedge-form, little gaping, with many accessory pieces.