DIAGNOSIS
Shell large (to 40.0 mm in height), thin, slightly higher than longer, inequilateral. Umbonal angle iooG. Auricles small, unequal in length and shape. Right valve disc with well-; developed, commarginal undulations and closely spaced, very thin, overlying radial riblets; anterior auricle demarcated) from disc, with fine, lamellate, commarginal riblets overlying 4-9 coarse radial ribs; posterior auricle almost continuous with shell disc, with reticulate sculpture. Left valve disc with reticulate sculpture from commarginal, lamellate ridges and thin, more closely spaced, intercalated, radial ribs; anterior and posterior auricles weakly separated from shell disc, with reticulate sculpture. Byssal fasciole broad. Byssal notch moderately deep, rounded. Inactive and active ctenolium well-developed. Prodissoconch very large (length 300-340 pm), D-shaped.

DESCRIPTION
Shell large (to 40.0 mm in height), opaque, thin, fragile, weakly inflated, slightly higher than longer (H/L= 1.070-1.142), inequilateral (beaks slightly posterior to midline, A/L = 0.550-0.556), inequivalve (left valve more convex than right valve). Shell disc tear-shaped. Dorsal shell margin straight. Anterior margin rounded. Posterior margin straight or slightly convex, smoothly transitioning to rounded ventral shell margin. Auricles small (AL/L = 0.367-0.458), anterior auricle slightly larger and longer than posterior
(AAL/AL = 0.529-0.587). Umbonal angle 100.
Right valve disc with well-developed, numerous, rounded, variously high and broad, rather regularly spaced, commarginal undulations and closely spaced, very thin, rounded, overlying radial riblets slightly changing direction in between undulations and separated by interstices broader than riblets. Anterior auricle slightly twisted to the right, sharply demarcated from shell disc by a deep furrow with a long and well-developed ctenolium and a sharp suture (between furrow and shell disc). Surface of auricle with fine, lamellate, commarginal riblets overlying 4-9 coarse, radial ribs on dorsal half of auricle and forming barely visible noduloses at intersections with radial ribs and serration of auricle dorsal margin. Byssal fasciole broad. Byssal notch moderately deep, rounded. Active ctenolium short. Posterior auricle weakly separated and almost continuous with shell disc; sculptured with thin, closely spaced, commarginal, lamellate ribs and very weak, dense, radial riblets.
Left valve disc with reticulate sculpture from rather regularly spaced, numerous, commarginal, lamellate, slightly corrugated ridges with wide interspaces (0.8-0.9 mm) and low, thin, rounded, more closely spaced (0.3-0.4 mm) intercalated, radial ribs. Anterior and posterior auricles weakly separated from shell disc; surface with reticulate sculpture from closely spaced, thin, lamellate, commarginal and rounded, radial ribs.
Prodissoconch very large (length 300-340 pm), smooth, D-shaped, convex, distinct, sharply separated from shell disc.

Hyalopecten vityazi differs from all species of the genus Hyalopecten by its almost regular teardrop-shaped shell but with a slighdy produced anterior part, the relatively small length of the auricles, the presence of well-developed undulations only on the right valve and prominent reticulate sculpture on the left valve, and the presence of a large prodissoconch. This is the only species of the genus Hyalopecten that has undulations and reticulate sculpture on different shell valves. Hyalopecten mireilleae, Hyalopecten neoceanicus and Hyalopecten profundicola also possess variously expressed reticulate sculpture of the shell. However, these species are readily distinguished from the new species by having reticulate sculpture on both valves, a strongly produced anterior part of the shell, and large auricles. Moreover, H. mireilleae and H. profundicola lack undulations on both valves, while H. neoceanicus, on the contrary, in addition to reticulate sculpture, has undulations on both valves. In terms of shell size, form and proportions, H. vityazi is most similar to Hyalopecten frigidus, Hyalopeten hadalis and Hyalopecten pudicus; however, it markedly differs from them by having no well-developed undulations and by having reticulate sculpture on the left valve, as well as a less distinctly marked and non-pointed posterior auricles, a less deeper byssal notch and a much larger prodissoconch.

The variability of shell form and proportions of Hyalopecten vityazi is difficult to assess, as all the shells and separate valves examined were damaged to a varying degree. Nevertheless, it can be noted that the degree of separation of the posterior auricle from the shell disc varies in this species. In most specimens from the Aleutian Trench, the posterior auricles are not separated and are almost continuous with the shell disc, and the posterior shell margin is straight, as opposed to specimens from the Kuril-Kamchatka Trench, in which the posterior auricles are more separated and the posterior shell margin is slightly convex.