Who discovered the apatosaurus?

Othniel Charles Marsh discovered one species of Apatosaurus, and named it Apatosaurus ajax. When he discovered Apatosaurus Excelsus, though, he assigned it to its own genus, Brontosaurus. Later it was found that both Brontosaurus and Apatosaurus ajax belonged to the same genus, so Brontosaurus was reassigned to Apatosaurus Excelsus.