univalve

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Having a shell consisting of a single valve or piece. Used of a mollusk.
• adjective Composed of a single valve or piece. Used of a shell.
• noun A univalve mollusk; a gastropod.

from The Century Dictionary.

• Having one valve only, as a mollusk; not bivalve or multivalve; univalved or univalvular. See II.
• Having the carapace single, or not hinged in the middle line: specifying the cladocerous or daphniaceous crustaceans. [Now rare.] In botany, consisting of one valve or piece.
• noun In conchology, a univalve mollusk or its shell; a shell consisting of a single piece; formerly, a member of one of three Linnean divisions of Testacea, as distinguished from bivalves and multivalves.
• noun body-whorl or aperture are collectively the spire, ending at the tip, point, or apex. The opposite end of the shell is the base, which often presents a depression, the umbilicus; the circumference, a completely lipped aperture, is the peristome. The spiral line between the successive whorls or volutions is the suture. See words italicized above with various cuts there, or there cited.

from the GNU version of the Collaborative International Dictionary of English.

• adjective (Bot. & Zoöl.) Having one valve.
• noun (Zoöl.) A shell consisting of one valve only; a mollusk whose shell is composed of a single piece, as the snails and conchs.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective having one valve, typically used to refer to mollusks, notably slugs and snails.
• adjective consisting of a single valve or piece, used to refer to a shell.
• noun A univalve mollusk or its shell.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective used of mollusks, especially gastropods, as snails etc.
• noun a class of mollusks typically having a one-piece coiled shell and flattened muscular foot with a head bearing stalked eyes