Shell rhomboidal to subquadrate, very elongate; dorsal margin straight. Anteroventral margin greatly produced. Sculpture absent. Periostracum thick, adherent. Posterior slope not sharply defined, with long, simple setae, broad on both sides at base. Siphons white. Length to 220 mm. As in other species of Modiolus, shape is variable, and M. flabellatus cannot be reliably separated from M. rectus; we have synonymized them. This species is close to the Japanese type species of the subgenus, M. (Modiolusia) elongatus (see Okutani et al., 1989: 48), and their relationship merits additional investigation. Tow Hill, Queen Charlotte Islands, British Columbia (54.1°N) [LACM], throughout the Golfo de California, south to Paita, Perú (5.1°S) (Olsson, 1961), in the intertidal zone to 15 m, solitary in sand, anchored by a large byssal mat in the substratum. Recorded as early as the Miocene in northwest America. Literature: U. S. Grant and Gale (1931: 249-251), Haderlie and Abbott (1980: 358-359), Hertlein and Grant (1972: 166-167), Keen (1971: 72), E. J. Moore (1983: 76), Olsson (1961: 127-128). Poutiers (1995: 155), Sool-Ryen (1955: 63-64).