Shell small, white, slightly inequivalve, excessively inequilateral, byssiferous ; with a thin brownish epidermis radiatingly streaked with thicker lines which fray out into hairs, the whole being very fugacious; hinge-line straight, the area long and narrow, the beaks situated just within the anterior third of the line ; ligamentary area extremely narrow, beginning under the beak, extending close under the posterior areal carina obliquely to the posterior third of the hinge-line, where it first reaches the margin, flat, black, with several white longitudinal strife ; beaks well marked, not very prominent, with a faint depression extending from them to the basal margin, marking the position of the byssal gape ; outline like that of Philippi’s Arca aspera (Philippi, op. cit., II. Tab. XV. fig. 1 c), but much more inequilateral, the anterior end being smaller, the posterior larger and more extended, and the line of the base much more oblique, as would naturally follow from the above circumstances ; sculpture of about a dozen (excluding those on the beaks) concentric sharp-edged subimbricate ridges, being old margins of the shell as it were forming varices, fainter and more dose set on the beaks; with strong rounded radiating riblets (near the base about four to a millimeter) continuous only from one varix to another ; posterior part of the cardinal border somewhat auriculated or compressed, anterior extreme rounded or moderately inflated, the anterior margin forming a little less than a right angle with the hinge-line, the posterior a little more than a right angle, the former slightly rounded to the base, where it is a little concavely excavated for the byssal gape, the latter first a little concave from the auriculation, then very broadly rounded out and passing without any angle into the base ; right valve a little the smaller ; interior white, both muscular scars double (by the splitting of the ends of the adductors) ; margin smooth, sharp ; byssus slender, tufted at the distal end, light brown ; hinge with six vertical teeth extending from the position of the beaks anteriorly, the first four small, vertical, gradu¬ally enlarging forward, the fifth largest, and with the sixth somewhat oblique; hinge between the beak and the ligamentary furrow edentulous, beyond that there are three horizontal teeth, the middle one largest extending posteriorly nearly parallel to the hinge-line and to each other. Lon. 8.5 ; alt. 5.0; max. diam. 3.25 ; lon. of hinge-line, 6.25 mm.

ROUGH BATHYARK Macrodon asperula Dall, 1881: 120; Arca profundicola Verrill and E. A. Smith, in Verrill, 1885: 439 (March), A. (Barbatia) pteroessa E. A. Smith, 1885: 262 (Nov.); A. (Scapharca) culebrensis E. A. Smith, 1885: 268; A. (Cucullaria) endemica Dall, 190&c: 399.
Shell trapezoidal, moderately inflated; ventral margin with a byssal gape. Sculpture of widely spaced, irregular commarginal ribs, and low radial ribs. Hinge plate narrow; teeth vertical anteriorly, horizontal posteriorly, about 4 on each side. Periostracum thick, scabrous posteriorly; setae broad. Length to 12 mm. The species is cosmopolitan in deep water, widely distributed in both the Atlantic and Pacific oceans. In the Pacific, it has been taken off Oregon (44.9°N) [LACM], south to Ecuador (8°S, 104°W) [USNM] and in the central North Pacific (36°N, 178°E) [BMNH], in 3,700 - 4,900 m.