The Collaborative International Dictionary of English v.0.48:

Univalve \U"ni*valve\, Univalved \U"ni*valved\, a. [Cf. F.
   univalve.] (Bot. & Zool.)
   Having one valve; as, a univalve shell or pericarp.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Univalve \U"ni*valve\, n. [Uni- + valve: cf. F. univalve.]
   (Zool.)
   A shell consisting of one valve only; a mollusk whose shell
   is composed of a single piece, as the snails and conchs.
   [1913 Webster]

   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.
         [1913 Webster] Univalve

WordNet (r) 3.0 (2006):

univalve
    adj 1: used of mollusks, especially gastropods, as snails etc.
           [ant: bivalve, bivalved]
    n 1: a class of mollusks typically having a one-piece coiled
         shell and flattened muscular foot with a head bearing
         stalked eyes [syn: gastropod, univalve]