The Collaborative International Dictionary of English v.0.48:
Mussel \Mus"sel\, n. [See Muscle, 3.]
1. (Zool.) 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.
2. (Zool.) Any one of numerous species of Unio, and related
fresh-water genera; -- called also river mussel. See
Naiad, and Unio.
Mussel digger (Zool.), the grayback whale. See Gray
whale, under Gray.