The Collaborative International Dictionary of English v.0.48:

Ichthyosauria \Ich`thy*o*sau"ri*a\, n. pl. [NL. See
   Ichthyosaurus.] (Paleon.)
   An extinct order of marine reptiles, including Ichthyosaurus
   and allied forms; -- called also Ichthyopterygia. They have
   not been found later than the Cretaceous period.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Ichthyosauria
    n 1: extinct marine reptiles: ichthyosaurs [syn:
         Ichthyosauria, order Ichthyosauria]