The Collaborative International Dictionary of English v.0.48:

Brontosaurus \Bron`to*sau"rus\, n. [NL., fr. Gr. bronth` thunder
   + say^ros lizard.] (Paleon.)
   A genus of large sauropod American dinosaurs of the jurassic
   era, or an individual of that genus. A length of sixty feet
   is believed to have been attained by these reptiles. The
   genus is also called Apatosaurus, and individuals of the
   genus are also called brontosaurs.
   [1913 Webster +PJC]