Source : Webster's Revised Unabridged Dictionary (1913)

Plesiosauria \Ple`si*o*sau"ri*a\, n. pl. [NL. See
   {Plesiosaurus}.] (Paleon.)
   An extinct order of Mesozoic marine reptiles including the
   genera Plesiosaurus, and allied forms; -- called also
   {Sauropterygia}.

Source : WordNet®

Plesiosauria
     n : any of the plesiosaurs [syn: {suborder Plesiosauria}]