Unio

from WordNet (r) 3.0 (2006)

Unio
    n 1: type genus of the family Unionidae [syn: {Unio}, {genus
         Unio}]

from The Collaborative International Dictionary of English v.0.48

Unio \U"ni*o\, n. [NL., fr. L. unio unity, union, a single large
   pearl. See {Union}.] (Zool.)
   Any one of numerous species of fresh-water mussels belonging
   to {Unio} and many allied genera.
   [1913 Webster]