DIAGNOSIS:
Shell small (to 9.7 mm in height), thick, slightly higher than longer, inequilateral. Umbonal angle 1050. Auricles large, unequal in size and shape. Right and left valve discs with 5 -6 wide, commarginal undulations and reticulate sculpture from about 40 widely spaced, well-developed, radial ribs and more closed, commarginal, lamellate, overlying riblets. Anterior auricle of right valve demarcated from disc; sculptured with four distinct radial ribs and coarse, closely spaced, commarginal, overlying, lamellate ribs. Anterior auricle of left valve separated from disc by a shallow sulcus; sculptured with closely spaced, lamellate, commarginal ribs and a few, weak, wide radial ribs. Posterior auricles weakly separated and continuous with disc; sculptured with widely spaced, commarginal, lamellate ribs. Byssal fasciole broad. Byssal notch moderately deep, broadly rounded. Active and inactive ctenolium well-developed. Prodissoconch large (length 260 pm), D-shaped.

DESCRIPTION
Shell small (to 9.7 mm in height), opaque, thick, hard, inflated, slightly higher than longer (H/L = 1.032), inequilateral (beaks slightly posterior to midline, A/L= 0.521), inequivalve (left valve more convex than right valve). Shell disc oblique, slightly drawn out anteriorly. Dorsal shell margin straight. Anterior shell margin rounded. Posterior shell margin slightly convex, smoothly transitioning to rounded ventral shell margin. Auricles large (AL/L = 0.596), anterior auricle larger and longer than posterior (AAL/AL = 0.571). Umbonal angle 105°. Right and left valve discs with wide, commarginal undula-tions (five in right and six in left valve) strongly smoothed near beak and almost indiscernible. Undulations in form of broad steps with a rounded edge widening (up to 1.7-1.8 mm) towards ventral shell margin. Valve discs with reticulate sculpture from about 40 widely spaced, well-developed, rounded, radial ribs (interstices broader than ribs, 0.2-0.5 mm in disc centre) and more closely spaced, commarginal, thin, lamellate, overlying riblets (with interspaces 0.2-0.3 mm) becoming denser in lower part of undulations. Anterior auricle of right valve slightly twisted to the right, sharply demarcated from shell disc by a sharp, thin suture and a long, strong ctenolium. Surface of auricle with four distinct radial ribs on dorsal half of auricle and coarse, closely spaced, commarginal, overlying, lamellate ribs forming serration of auricle dorsal margin. Byssal fasciole broad. Byssal notch moderately deep, broadly rounded. Active ctenolium short. Anterior auricle of left valve separated from disc by a shallow sulcus; anterior auricle margin extending vertically down. Surface of auricle with closely spaced, lamellate, commarginal ribs, forming indistinct serration of auricle dorsal margin, and a few indistinct, weak, wide radial ribs. Posterior auricle of right and left valves weakly separated from shell disc; dorsal and posterior auricle margins producing a rounded acute angle; sculptured with widely spaced, commarginal, thin, lamellate ribs, forming indistinct serration of auricle dorsal margin. Prodissoconch large (length 260 µm), smooth, D-shaped, convex, distinct, sharply separated from shell disc.

Hyalopecten kurilensis is most close to Hyalopecten arntzi, Hyalopecten neoceanicus and Hyalopecten profundicola in terms of the dimensions, shape and proportions of the shell, as well as the presence of cancellate sculpture. However, unlike H. arntzi, H. kurilensis has a thick, inflated and opaque shell with much fewer and much wider commarginal undulations, a much smaller number of wider and more widely spaced radial ribs, and well expressed widely spaced lamellate, commarginal ribs. Hyalopecten kurilensis is distinguished from H. neoceanicus by having a less drawn-out anterior part of the shell, the beaks less displaced posteriorly, non-pointed and smaller auricles, a rounded byssal notch, and fewer rounded and more widely spaced radial ribs. Hyalopecten kurilensis differs from H. profundicola in having a shell less drawn-out anteriorly, less posteriorly displaced beaks, shorter auricles, commarginal undulations, and fewer wider and more widely spaced radial ribs.

This species was recorded at the abyssal plain adjacent to the Kuril-Kamchatka Trench (North-western Pacific) at 4995-5045 m depth.