noun.
The leading genus of bivalves of the family Unionidæ: formerly used with great latitude for many species, some of which are now placed in other families as well as in other genera.
noun.
[lowercase] A species of this genus; any river-mussel.
the GNU version of the Collaborative International Dictionary of English
noun.
Any one of numerous species of fresh-water mussels belonging to Unio and many allied genera.