The Collaborative International Dictionary of English v.0.48:

Ceratodus \Ce*rat"o*dus\, n. [NL., fr. Gr. ke`ras, ke`ratos horn
   + ? tooth.] (Zool.)
   A genus of ganoid fishes, of the order Dipnoi, first known as
   Mesozoic fossil fishes; but recently two living species have
   been discovered in Australian rivers. They have lungs so well
   developed that they can leave the water and breathe in air.
   In Australia they are called salmon and baramunda. See
   Dipnoi, and Archipterygium.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ceratodus
    n 1: extinct lungfish