The Collaborative International Dictionary of English v.0.48:

Labyrinthodon \Lab`y*rin"tho*don\, n. [Gr. laby`rinthos
   labyrinth + 'odoy`s, 'odo`ntos, tooth.] (Paleon.)
   A genus of very large fossil amphibians, of the Triassic
   period, having bony plates on the under side of the body. It
   is the type of the order Labyrinthodonta. Called also
   Mastodonsaurus.
   [1913 Webster]