The Collaborative International Dictionary of English v.0.48:

Fissurella \Fis`su*rel"la\, n. [NL., dim. of L. fissura a
   fissure.] (Zool.)
   A genus of marine gastropod mollusks, having a conical or
   limpetlike shell, with an opening at the apex; -- called also
   keyhole limpet.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Fissurella
    n 1: type genus of the family Fissurellidae: keyhole limpets
         [syn: Fissurella, genus Fissurella]