any one of many species of marine bivalve shells of the genus mytilus, and related genera, of the family mytidae. the common mussel (mytilus edulis; see illust. under byssus), and the larger, or horse, mussel (modiola modiolus), inhabiting the shores both of europe and america, are edible. the former is extensively used as food in europe.-any one of numerous species of unio, and related fresh-water genera; -- called also river mussel. see naiad, and unio.