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 tteth, 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.