NORTHERN HORSEMUSSEL Mytilus modiolus Linnaeus, 1758: 706; Modiola papuana Lamarck, 1801:113; M. gibhsii Leach, 1815:34; M. vulgaris J. Fleming, 1828: 412; M. grandis R. A. Philippi, 1844c: 51; Mytilus modiolus ovata Jeffreys, 1864: 112; Volsella difficilis Kuroda and Habe, 1950: 30, non Modiola difficilis Deshayes, 1863; Modiolus kurilensis F. R. Bernard, 1983b: 19, nom. nov pro Volsella difficilis Kuroda and Habe, non Deshayes. There are additional synonyms in the eastern Atlantic. Shell rhomboidal, moderately inflated. Periostracum dehiscent, with sparse, short, non-serrate setae, broad on both sides at base. Anteroventral margin projecting slightly in front of umbones. Shell beneath periostracum violet. Length to 180 mm. Rarely epifaunal, this species usually forms subtidal nestling colonies in broken rock, or is shallowly infaunal in mud or gravel, with only the posterior end of the shell at the surface. These specimens may not have a hirsute periostracum. F. R. Bernard (1983b) recognized M. kurilensis as separable species occurring in both Asia and the eastern Pacific. However, specimens vary greatly in each of the characters he cited to distinguish the two. In the western Atlantic, M. modiolus squamosus Beauperthuy, 1967, is now regarded as seperable southern subspecies. Circumboreal; St. Paul Island, Pribilof Islands, Bering Sea (57.8°N) [LACM], throughout the Aleutian Islands, south to Monterey, California (36.6°N) [CAS]; south to Honshu Island, Japan; from Greenland to New Jersey; along the European coast to the Mediterranean; in the intertidal zone to 200 m, often in aggregations binding rocks and gravel. Reported as early as the Pliocene of western North America. Literature: Avdeeva-Markovskaia (1983), R. A. Brown (1984), R. A. Brown and Seed (1977), R. A. Brown et al. (1976). Coleman and Trueman (1971), Comley (1978), Davenport and Kjørsvik (1983), De Schweinitz and Lutz (1976), Gardiner (1927), Gogolev (1983a, b). U. S. Grant and Gale (1931: 249), Jokumsen and Fyhn (1982), Kas'lanov et al. (1980: 118), Lutz and Hidu (1979), M'Intosh (1894), E. S. Morse (1919: 154), Okutani et al. (1989: 46),