Musculus impressus (Dall, 1907)

(Fig. 3 E, F)

Modiolaria impressa Dall, 1907: 172 .

Musculus impressus (Dall, 1907) — Scarlato 1981: 228, fig. 125.

Material examined: 91 specimens from the ocean and sea slopes of all the Kuril Islands and also from the Fourth Kuril, Krusenstern, Severgin, Rikord, and Snou Straits (1–500 m) .

Distribution: Kuril Islands, Bering Sea (1–500 m) (Dall 1907; Scarlato 1981).

Remarks: Previously, this species was recorded only from waters off the northern Kuril Islands (two samples, three specimens) at depths of 100–130 m (Scarlato 1981). Our data greatly expand both the geographical and the vertical ranges of this species.

Musculus impressus (Dall, 1907) was synonymized with Musculus discors (Linnaeus, 1767) (Coan, Valentich-Scott & Bernard 2000) . However, after studying the description of Mytilus discors (Linnaeus, 1767) and the photographs of a syntype of Modiolaria impressa Dall, 1907 (USNM 110467) from the USNM web site, we consider Musculus impressus (Dall, 1907) to be a valid species.

Musculus impressus is clearly distinguished from M. discors by a much more elongate shell (the shell height to shell length ratio is almost 1:2) (Scarlato 1981) with its posterior part highly raised relative to the middle part and a small number (6–8) of sparse radial ribs on the anterior part of the shell.