Why was aptosaurus used to be called brontosaurus?

Othniel Charles Marsh discovered a species of Apatosaurus in 1877, and he named it Apatosaurus ajax. Two years later, he discovered another species of Apatosaurus. Marsh didn't realize that they both belonged to the same genus, though, so he named the new species Brontosaurus Excelsus. When it was determined that Brontosaurus Excelsus was actually a species of Apatosaurus, its name was changed to Apatosaurus Excelsus.