This genus is easily known by its resemblance to the hull of a ship; the hinge is peculiar, being composed of numerous sharp teeth alternately inserted between each other. The Arca of Linnæus was divided by Lamarck into the four genera that compose this family, each possessing a strong distinctive character. All the shells of this family are found in the sea at a little distance from the shore; they are covered with a dark greenish lamellar or velvet-like epidermis, frequently ending in a deep fringe at the margin.
Shell a little varied in form, but most generally elongated, and more or less oblique at the posterior extremity; sometimes very inequilateral; summits more or less distant, and a little recurved forward; hinge anomalous, straight, or a little curved; long, and formed by a line of short vertical teeth decreasing from the extremities to the centre; ligament exterior, broad; sometimes the margin is crenated.
Arca tortuosa.
A. semitorta.
A. Noæ.
A. tetragona.
A. umbonata.
A. sinuata.
A. avellana.
A. cardissa.