Type species: Mytilus edulis Linne 1758.
Shell wedge-shaped, very inequilateral, thin; umbos acute, terminal, anterior; valves wider and rounded behind, gaping a little for the byssus which is well developed; smooth or radially sculptured; hinge with a few small teeth under the umbos, or edentulous; ligament linear, marginal, deep-seated; margins smooth; periostracum conspicuous; interior with a thin nacreous layer; anterior adductor scar small, under the umbo, posterior very large.
Distribution: The genus is cosmopolitan, the maximum of species in Arctic and Antarctic seas.
Remarks: Gregarious, from high water-mark down to a few fathoms on rocky, stony, or muddy bottoms, usually attached by a byssus in masses to stones, wrecks or floating bodies. The grooved foot is the medium for spinning the silky material of the byssus to secure temporary anchorage.

Mytilus Linnaeus, 1758. Type species (SD Anton, 1838; ICZN Opinions 94, 1926, and 333, 1955): M. edulis Linnaeus, 1758. Recent, northeastern Atlantic. Eumytilus von Ihering, 1900. Type species (OD): Mytilus edulis Linnaeus. Crassimytilus Scarlato and Starobogatov, 1979. Type species (OD): Mytilus coruscus A. A. Gould, 1861. Recent, northwestern Pacific. Pacifimytilus Kafanov, 1984. Type species (OD): Mytilus californianus Conrad, 1837. Recent, northeastern Pacific. Shell inequilateral, subtriangular, inflated. Periostracum thick, usually black or dark brown. Umbones terminal. Sculpture absent, or of low radial ribs, divaricate in some species, sometimes with commarginal undulations. Hinge with small ventral dysodont teeth and obscure tubercles, with marginal crenulations in some. Ligament external, attached to nymph. A cosmopolitan temperate and cold-water genus, due to the very wide dispersal of members of the Mytilus edulis complex, in part as a result of shipping activity the past 500 years. Morphologically, the group is remarkably consistent, but several additional genera have been proposed, one or two of which may be useful at the subgeneric level after more careful evaluation. Additionally, Mytilus californianus Conrad, 1837, has sometimes been assigned to Crenomytilus Soot-Ryen, 1955, but absence of marginal crenulations on the pitted nymph show it to be a Mytilus, s.s. The Asian Mytilus coruscus A. A. Gould, 1861, has been collected live on fishing nets adrift off Washington [LACM], but the species appears not to have settled on the coast. This species grows to a very large size, and specimens may be very flattened. Mytilus crassitesta Lischke, 1868, is apparently a synonym. There has been some confusion in the literature between this species and the very different Crenomytilus grayanus (Dunker, 1853). The genus is known back to the Oligocene. The name Mytilus is derived from the diminutive of the Latin mya, a sea mussel; the gender is masculine. There has been much recent study of the species of the Mytilus edulis complex - the smooth-shelled Mytilus from various points of view. At least three distinct taxa are involved. In the well-studied case of M. edulis and M. galloprovincialis in Europe, although the two taxa hybridize, and the hybrids are fertile, apparently larvae of hybrids are less viable, and this, together with differences in breeding times, are sufficient to maintain species integrity. The native smooth mytilid of the North Pacific is Mytilus trossulus, which also occurs across the Arctic and in northern Europe. Mytilus galloprovincialis has been introduced into the eastern Pacific. The two form hybrids in central California, and it remains to be seen how distinct the species will remain. Populations in the Pacific north of Washington and in Asia have yet to be studied in detail, so there are many unresolved questions, and it is not easy to assign some material to a particular species.