The Collaborative International Dictionary of English v.0.48:

Mosasaurus \Mos`a*sau"rus\, n. [NL., fr. L. Mosa the River Meuse
   (on which Meastricht is situated) + Gr. ? a lizard.]
   (Paleon.)
   A genus of extinct marine reptiles allied to the lizards, but
   having the body much elongated, and the limbs in the form of
   paddles. The first known species, nearly fifty feet in
   length, was discovered in Cretaceous beds near Maestricht, in
   the Netherlands. [Written also Mososaurus.]
   [1913 Webster]