The animal, as described by M. Chenu, is elongate, oval, the lobes of the mantle simple or fringed, divided at the edge into two leaves, the interior being very short, bearing a fringe of small, cylindrical, and movable fillets; the exterior leaf is united to the shell very near the edge. The opening by which water and food are introduced supplies the branchiæ at the same time. The stomach consists of a white membrane, thin, like opaline, and presenting itself in longitudinal folds; the liver is granulous, composed of greenish grains more or less deep, contained in the meshes of a whitish tissue forming a thickish bed, which surrounds the stomach, the intestines taking the direction of the median and dorsal line, and beneath the heart are received and terminate in a small appendage, floating in the cavity of the mantle near the hinge. The foot is, perhaps, the remarkable organ of the mussel: it is small, semi-lunar when not in motion, but capable of great elongation, resembling thus a sort of conical tongue, having a longitudinal furrow on its side. It is put in motion by several pairs of muscles, all of which penetrate and are interlaced with the tissue; behind it is the silky byssus. The mouth is large, and furnished with two pairs of soft palpi, which are pointed and fixed by their summit. Abdominal masses emanate, and on each side a pair of nearly equal branchiæ. The additional muscles, one anterior and small, the other posterior, large, and rounded. At the base of the foot is a gland which furnishes a viscous secretion; this viscous liquid is organized and moulded in the groove of the foot, and forms a thread, and originates the byssus; it is a bundle of hairs, mane, or thread, which holds on to its shell.