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]

WordNet (r) 3.0 (2006):

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