The Collaborative International Dictionary of English v.0.48:

Plesiosaurus \Ple`si*o*sau"rus\, n.; pl. Plesiosauri. [NL.,
   fr. Gr ? near + ? a lizard.] (Paleon.)
   A genus of large extinct marine reptiles, having a very long
   neck, a small head, and paddles for swimming. It lived in the
   Mesozoic age.
   [1913 Webster]