depths of water from tide-marks outwards. Some of them affix themselves firmly to rocks, by means of a stout byssus.
Genus Arca.
From the square boat-like form of the valves in this genus, resembling that commonly attributed in engravings to the Ark of Noah, these shells have derived their name. The valves are generally equal, much broader than long, and more developed on one side than on the other; four-sided in outline, and usually solid in texture. Their surface is covered with radiating close-set grooves, concealed, however, by a rough, loose, horny epidermis. The hinge is nearly or quite straight; consisting of numerous minute teeth, which are parallel in the centre, but diverge at the sides.
The animal is oblong; the mantle, which, as stated above, is open, is either fringed or simple at its edges. The foot is large, oblong, bent, grooved throughout, and capable of expanding into a disk, with plain, or slightly plaited edges; it is furnished at its base with a gland from which is spun a strong compact byssus. The mouth is surrounded by lips, which are formed by the extremities of the gills.
These Mollusks inhabit all depths of water, but are generally found near the shore; perhaps, however, this is because their peculiar habits render them less accessible in other situations. They chiefly live in holes and fissures of rocks, moored by their powerful byssus. M. Rang remarks that they sometimes adhere by their disk-like foot: and Mr. Swainson states, that when young, they
