FAMILY II. PHOLADARIA. Two genera. 1. Pholas. The Stone Piercer. Nine species. 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.