The Collaborative International Dictionary of English v.0.48:

Littorina \Lit"to*ri"na\, n. [NL. See Littoral.] (Zool.)
   A genus of small pectinibranch mollusks, having thick spiral
   shells, abundant between tides on nearly all rocky seacoasts.
   They feed on seaweeds. The common periwinkle is a well-known
   example. See Periwinkle.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Littorina
    n 1: type genus of the family Littorinidae: periwinkles [syn:
         Littorina, genus Littorina]