The Collaborative International Dictionary of English v.0.48:

Astacus \As"ta*cus\, n. [L. astacus a crab, Gr. ?.] (Zool.)
   A genus of crustaceans, containing the crawfish of
   fresh-water lobster of Europe, and allied species of western
   North America. See Crawfish.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Astacus
    n 1: type genus of the family Astacidae; Old World crayfish
         [syn: Astacus, genus Astacus]