What is "keyhole limpet"

Fissurella \Fis`su*rel"la\, n. [NL., dim. of L. fissura a fissure.] (Zo["o]l.) A genus of marine gastropod mollusks, having a conical or limpetlike shell, with an opening at the apex; -- called also keyhole limpet.

n. A marine gastropod of the genus ''Fissurella'' and allied genera.

n. marine limpet having a conical shell with an opening at the apex [syn: Fissurella apertura, Diodora apertura]