Arca transversa — « Shell transversely oblong, rhomboidal, with from thirty-two to thirty-five ribs, placed at nearly the length of their own diameters distant from each other ; apices separated by a long narrow space, and situate at the termination of the posterior third of the length of the hinge margin ; extremities of the hinge margin angulated ; anterior edge, superior moiety rectilinear ; posterior edge rounded ; inferior edge nearly rectilinear, or very obtusely rounded ; on the hinge space one or two angulated lines are drawn from the apex, diverging to the hinge edge. Length less than seven-twentieths of an inch. Breadth one inch and one-fifth. Inhabits the coast of the United States. Cabinet of the Academy and Philadelphia Museum. This species which is abundant in some districts, is remarkable by its transverse form, and may be known among the foregoing species, by the apex being situate opposite to one-third of the distance from the posterior termination of the hinge margin. »