The Collaborative International Dictionary of English v.0.48:

Mosasauria \Mos`a*sau"ri*a\, n. pl. [NL. See Mosasaurus.]
   (Paleon.)
   An order of large, extinct, marine reptiles, found in the
   Cretaceous rocks, especially in America. They were
   serpentlike in form and in having loosely articulated and
   dilatable jaws, with large recurved teeth, but they had
   paddlelike feet. Some of them were over fifty feet long. They
   are, essentially, fossil sea serpents with paddles. Called
   also Pythonomarpha, and Mosasauria.
   [1913 Webster]