As Entodesma scammoni Dall:
Shell inequilateral, inequivalve, subovate. Left valve slightly the smaller. Shell tumid, umbones inconspicuous, nearly in the middle of the shell. Interior with a brilliantly pearly, white nacre ; hinge margin narrow, nearly straight ; interrupted under the beaks ; right valve; with a small rounded projecting process, fitting into an excavation in the opposite valve, which is thickened behind it. Ligament and ossicle moderate. Pallial line simple, continuous. Anterior margin evenly rounded; posterior ditto, a little more effuse ; ventral margin with the faintest possible indication of flexuosity; shell gaping behind, but with no perceptible ventral gape. Exterior smooth except for lines of growth, which are more emphasized at intervals, forming three concentric waves from the umbo toward the margin. Shell covered with a thin yellowish brown pubescent epidermis, which is produced over the margins, and under a lens is seen to be very closely, finely, radiately striate. The pubescence is formed by little projecting points of the epidermis at the intersections of the striae with the lines of growth. Shell solid. Lon. 0,9, alt. 0,44, diam. 0,33 inch.
Habitat, Port Simpson, British Columbia, Capt. C. M. Scammon, U. S. Revenue Service.
This shell comes nearest to Eritodesma saxicola, Baird, with which Dr. Carpenter would unite it; but it appears to me perfectly distinct. That species takes every imaginable form, nestling or boring in rocks and among the roots of fuci. I have examined a very large series, extending from Sitka to Monterey. In such cases the only satisfactory way of determining specific differences is to compare normal undistorted specimens of the same age.
E. saxicola is characterized by a straw yellow or dark brown, thick, strong epidermis, which always breaks the thin shell in drying. This epidermis is marked by a peculiar smoothness of its exterior, though puckered up, wrinkled and folded to a great extent. The pinched up wrinkles are characteristic. It is distantly radiately striate, especially toward the posterior end. These stride are more properly raised folds. The substance of the shell has a characteristic porcellanous texture and appearance, but slightly nacreous, usually livid, though rarely white, but never brilliantly pearly. In normal specimens the shell is more or less truncate at each end, especially the anterior end, which is further marked by two more or less evident ridges, extending from the beaks. It always has a large ventral gape.
E. Scammoni can then be readily distinguished by its nacre, its characteristic epidermis, which does not contract and break the shell, and which is striated in a totally different way from that of saxicola. It has not the ridges or ventral gape alluded to. My specimens agree exactly with each other, in all respects, and are not distorted in the least.