Source : Webster's Revised Unabridged Dictionary (1913)

Zygobranchia \Zyg`o*bran"chi*a\, n. pl. [NL., from Gr. ? a yoke
   + ? a gill.] (Zo["o]l.)
   A division of marine gastropods in which the gills are
   developed on both sides of the body and the renal organs are
   also paired. The abalone ({Haliotis}) and the keyhole limpet
   ({Fissurella}) are examples.

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}.

Source : WordNet®

Fissurella
     n : type genus of the family Fissurellidae: keyhole limpets
         [syn: {genus Fissurella}]