The Collaborative International Dictionary of English v.0.48:
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. [1913 Webster]WordNet (r) 3.0 (2006):
Plesiosauria n 1: any of the plesiosaurs [syn: Plesiosauria, suborder Plesiosauria]