Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

ichthyosaurus
     n : ichthyosaurs of the Jurassic
     [also: {ichthyosauri} (pl)]