Shell rugged, sinistral, obscurely three sided. Right valve deep, obscurely carinate; umbo produced incurved, forming a nearly complete volution. Left valve nearly flat, but slightly vaulted at the umbo, which is incurved some distance from the margin, but not externally elevated ; tooth wide and strong. Anterior tooth of the right valve large, corrugated on the outer side ; posterior tooth small, corrugated on the inner side. Ligament stout, almost buried beneath the incurved umbones. Anterior muscular scars largest, shell channelled between the scars. Interior white; margins smooth, the posterior edge of the left valve with a narrow band of purple. Exterior without plications, smooth, except for the lines of growth, covered with a strong light yellow brown epidermis, which is only roughened by the lines of growth.
Alt. 3,1; lat 2,1 inch. Depth of right valve, 1,3 inch. Habitat.—Middle America. " Puget Sound," Rowell, in error.
This very singular shell is covered with parasitic Vermeti, &c., of Central American species. An anomaly appears in the fact that a large part of the shell, outside of the epidermis, is covered with a white, calcareous deposit, evidently a part of the shell itself, and marked only by coarse lines of growth. The edge of the epidermis appears on the margin some distance within the actual edge. This is the only instance known to me of an epidermis being secreted by the mantle beneath the external surface of the shell. It may be accounted for as follows : The edge of the epidermis may be secreted at one period, after which the edge of the mantle might secrete a calcareous layer extending beyond the epidermal margin. The mantle, contracted in a period of rest, would again add to the layer of epidermis, after which, expanding, another outreaching calcareous layer might be deposited, which would, of course, fall upon the previous one, and so on. Tins view is strengthened by the fact that a careful examination of the epidermis with a strong lens, in places where the external calcareous layer has been removed, shows that the continuity of the epidermis is frequently interrupted by a very thin layer of shelly matter. The outer calcareous layer does not cover the whole of the shell. In any case the fact is very curious and interesting. It is nearest Chama iostoma, Conrad, from the Sandwich Islands, which is a dextral shell with radiating plicae and marginal produced laminae.