Source : Webster's Revised Unabridged Dictionary (1913)

Naiad \Na"iad\, n. [L. naias, -adis, na["i]s, -idis, a water
   nymph, Gr ?, ?, fr. ? to flow: cf. F. na["i]ade. Cf. {Naid}.]
   1. (Myth.) A water nymph; one of the lower female divinities,
      fabled to preside over some body of fresh water, as a
      lake, river, brook, or fountain.

   2. (Zo["o]l.) Any species of a tribe ({Naiades}) of
      freshwater bivalves, including {Unio}, {Anodonta}, and
      numerous allied genera; a river mussel.

   3. (Zo["o]l) One of a group of butterflies. See {Nymph}.

   4. (Bot.) Any plant of the order {Naiadace[ae]}, such as
      eelgrass, pondweed, etc.

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

Source : WordNet®

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