“Dermatomya mactroides Dall.
Plate VIII, Fig. 8.
Poromya (Dermatomya) mactroides Dall, Bull. Mus. Comp. Zoöl., XVIII, p. 449, June, 1889.
Shell large, stout, strong., with a strong epidermis, olive gray toward the beaks, paler, inclining to greenish, toward the margins in the adult; epidermis raised into wrinkles on the posterior area, and folding in over the basal margins; young shell with a few sparse granulations near the anterior and posterior margins, adult without visible granulations, the epidermis mostly shining and the shell showing iridescent through it; the young are subrhomboidal, the adults have the beaks prominent, high, subcentral; the anterior end rounded, the posterior very slightly produced; surface sculptured only with more or less evident incremental lines; lunule and escutcheon are visible on a close scrutiny, though not marginated by a line; the former is cordate, the latter narrow and long; hinge of Poromya, strong; ligament short, half internal; interior faintly iridescent, pallial and muscular scars distinct but not emphatic; the pallial line is deeply and rather narrowly sinuated; the basal margin is perfectly plain; altitude of adult shell, 10; longitude, 18; diameter, 12 mm.
HAB.-U. S. Fish Commission Stations 2781, 2783, and 2785, on the west coast of Patagonia, on a muddy bottom in 122, 348, and 449 fathoms; bottom temperature 46º.9 to 49°.9 F. Also at Station 2793, in 741 fathoms, mud; bottom temperature 38º.4F. off the coast of Ecuador.
The superficial resemblance to a small Mactra presented by this shell needs no further comment. It is sufficiently evident.
This fine species differs from the typical form of the genus in the absence of the superficial granulations, and in the presence of a deep and strong pallial sinus, which characters indicate that it should form a special section of the group. The hinge is also remarkably coarse and strong.
In the type of Poromya the pallial sinus is obsolete; its retractor muscles are either mainly incorporated in the septum, the muscular contractions of which serve to move the siphons, or they are replaced by the septal muscles. In the present species, however, there is a large, and strong pallial sinus with its usual muscles, and the septum is consequently only very slightly furnished with muscular fibers, and does not serve to retract the siphons. The valve to the bronchial siphon is large, and the palps are enormous. The anterior edges of the anterior palps are notched or papillose toward the median line, a condition not observed in the other species. The foot is pointed and slightly geniculate. There are seven anterior and eight or nine posterior gill lamellæ; the two areas are rather narrow, and their ends closely approach one another near the middle of the foot on each side. In front of the ridge which precedes the large branchial valve, and between it and the foot, are four or five quite prominent elevations of the surface, closely resembling the branchial lamellæ, but with their length in the axial direction of the animal. There are no fissures between these, but then seem very like branchial lamellæ in process of development. Both the longitudinal branchial areas on each side are fissured, and their blood vessels reach them from behind.”
(Dall, 1889: 291-292)