Crossword clue: a shell consisting of one valve only; a mollusk whose shell is composed of a single piece, as the snails and conchs.
1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).
Possible answers
a class of mollusks typically having a one-piece coiled shell and flattened muscular foot with a head bearing stalked ey