as i believe someone pointed out, there is no such dinosaur. the brontosaurus was an apatasaurus who's head somehow got changed with that of a camarasaurus.
An incidental point of fact, but: this is actually a mashing of the truth. There was a reconstruction of an Apatosaur with the wrong head, however that is not what the difference between a Brontosaurus and an Apatosaurus. The actual difference is:
nothing. They're different names for the same dinosaur - the international committee that decides these things chose Apatosaurus as the official name.
(See Stephen J. Gould's
Bully for Brontosaurus for further details)