DIAGNOSIS
Shell large (to 36.3 mm in height), thick, shell height and length almost equal, subequilateral. Umbonal angle 95°. Auricles small, unequal in length and shape. Right and left valve discs with well-developed, regular, commarginal undulations and well-developed, overlying radial ribs. Anterior auricle of right valve demarcated from disc, sculptured with fine, lamellate, closely spaced, commarginal riblets overlying three coarse radial ribs; posterior auricle continuous with shell disc, sculptured with thin, closely spaced, commarginal, lamellate ribs overlying rounded, radial riblets. Anterior auricle of left valve distinct from disc, with reticulate sculpture from widely spaced, lamellate, commarginal ribs and closer spaced, round, radial ribs; posterior auricle continuous with disc, sculptured with thin, closely spaced, lamellate, commarginal ribs and weak, round, radial ribs with noduloses at intersections. Byssal fasciole broad. Byssal notch deep, sharp. Inactive and active ctenolium well-developed. Prodissoconch large (length 245 µm), circular-shaped.

DESCRIPTION
Shell large (to 36.3 mm in height), opaque, thick, hard, weakly inflated, shell height and length almost equal (H/L — 1.043), subequilateral (beaks slightly posterior to midline, A/L = 0.511), inequivalve (left valve more convex than right valve). Shell disc subcircular. Dorsal shell margin straight. Anterior shell margin rounded. Posterior shell margin slightly convex, smoothly transitioning to rounded ventral shell margin. Auricles small (AL/L = 0.388), anterior auricle slightly larger and longer than posterior (AAL/AL = 0.511). Umbonal angle 950. Right and left valve discs with well-developed, regular, rounded, wide, commarginal undulations (distance between tops of undulations in disc centre 3.0-3.3 mm), becoming higher and broader towards ventral shell margin, and widely spaced, thin, well-developed, rounded, overlying radial ribs (interstices broader than ribs, 0.3-0.4 mm in disc centre), with numerous inserted radial riblets at ventral shell margin. Towards anterior and posterior shell margins undulations transforming into narrow ridges with noduloses at intersections with radial ribs. Anterior auricle of right valve sharply demarcated from shell disc by a deep furrow with a long and strong ctenolium and a sharp suture (between furrow and shell disc). Surface of auricle with fine, lamellate, closely spaced, commarginal riblets overlying three coarse radial ribs on dorsal half of auricle and forming serration of auricle dorsal margin. Byssal fasciole broad. Byssal notch deep, sharp. Active ctenolium short, strong. Posterior auricle continuous with shell disc; sculptured with thin, closely spaced, commarginal, lamellate ribs overlying rounded, radial riblets and forming small noduloses at intersections with radial riblets. Anterior auricle of left valve distinct from disc; anterior auricle margin passing vertically down. Surface of auricle with reticulate sculpture from widely spaced, lamellate, commarginal ribs and closer spaced, round, radial ribs sometimes with small noduloses at intersections. Posterior auricle continuous with disc, sculptured with thin, closely spaced, lamellate, commarginal ribs and weak, round, radial ribs with noduloses at intersections. Prodissoconch large (length 245 µm), smooth, circular-shaped, convex, distinct, sharply separated from shell disc.

Hyalopecten abyssalis is most close in the shape, proportions and sculpture of the shell to Hyalopecten arntzi, Hyalopecten frigidus, Hyalopecten hadalis and Hyalopecten pudicus. However, it differs from them in having the shell almost rounded, markedly less elongate dorsoventrally, posterior auricles continuous with the shell disc, and a sharp byssal notch. Furthermore, H. abyssalis differs from H. arntzi H. hadalis and H. pudicus in much smaller relative length of the auricles; and from H. frigidus with relatively small auricles, in having a more well-developed radial sculpture on both valves. This species is distinguished from Hyalopecten vityazi, which has a similar shape of the shell and small auricles, by the presence of commarginal undulations and the lack of reticulate sculpture on the left valve, by its posterior auricles continuous with the shell disc, a sharp and deeper byssal notch, and a smaller and circularshaped prodissoconch.

This species was recorded at the abyssal plain adjacent to the Kuril-Kamchatka and Aleutian trenches (North-western Pacific) at 4550-5020 m depth.