Source : Webster's Revised Unabridged Dictionary (1913)

Univalve \U"ni*valve\, n. [Uni- + valve: cf. F. univalve.]
   (Zo["o]l.)
   A shell consisting of one valve only; a mollusk whose shell
   is composed of a single piece, as the snails and conchs.

   Note: Most univalves are spiral and are the shells of
         gastropods, but many belong to cephalopods and
         pteropods. A large number of univalves belonging to the
         gastropods are conical, cup-shaped, or shieldlike, as
         the limpets.

Univalve \U"ni*valve\, Univalved \U"ni*valved\, a. [Cf. F.
   univalve.] (Bot. & Zo["o]l.)
   Having one valve; as, a univalve shell or pericarp.

Source : WordNet®

univalve
     adj : used of mollusks, especially gastropods, as snails etc.
           [ant: {bivalve}]
     n : a class of mollusks typically having a one-piece coiled
         shell and flattened muscular foot with a head bearing
         stalked eyes [syn: {gastropod}]