NORTHERN RIBBED MUSSEL Mytilus demissus Dillwyn, 1817: 314, ex Solander MS; Modiola plicatula Lamarck, 1819: 113; M. semicostata Conrad, 1837:244. Shell elliptical, elongate; sculpture of strong radial ribs; interior flushed blue or purple. Length to 100 mm. This Atlantic species was introduced before 1890 in a number of localities on the Californian coast, and sporadic populations now occur from San Francisco Bay (37.7°N) [CAS] to Newport Bay (33.6°N) [SBMNH], California; native in northwestern Atlantic from the Gulf of St Lawrence. Canada, to northern Florida; in the intertidal zone of bays and esturaries. Sarver et al. (1992) have shown that the closely similar G. granosissima (G. B. Sowerby III, 1914) is a separable species that occurs from southern Florida to South America