A shell consisting of one valve only; a mollusk whose shell is composed of a single piece, as the snails and conchs. [
1913 Webster]
" 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]