The Collaborative International Dictionary of English v.0.48:

Ichthyosaurus \Ich`thy*o*sau"rus\, n.; pl. Ichthyosauri. [NL.,
   fr. Gr. 'ichqy`s, -y`os, a fish + say^ros a lizard.]
   (Paleon.)
   An extinct genus of marine reptiles; -- so named from their
   short, biconcave vertebr[ae], resembling those of fishes.
   Several species, varying in length from ten to thirty feet,
   are known from the Liassic, O["o]litic, and Cretaceous
   formations.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ichthyosaurus
    n 1: ichthyosaurs of the Jurassic