Actually, the following three species were all described by Marsh in 1877: Apatosaurus ajax, Atlantosaurus ajax and Brontosaurus ajax. All three have since been rendered synonymous, though Bakker had attempted to resurrect B. ajax in 1986, and Steel attempted the same with Atlantosaurus ajax in 1970. However, since there isn’t sufficient reason to separate the latter two from Apatosaurus, both attempts failed.
It should be noted, however, that things could well have been different: when Riggs re-examined Apatosaurus, and its type species A. ajax, the A. ajax specimens were too young and fragmentary to allow for a reliable species designation. He could well have used this information to argue against Apatosaurus as a valid genus and used Brontosaurus as the valid taxon, instead.
Incidently, when Marsh first described Brontosaurus excelsus and Apatosaurus ajax, he recognized them as closely related. However, he fell victim to a very low sample size: the differences he noted (number of fused sacral vertebrae) turned out to be due to age (the A. ajax specimens were juveniles), not differences between genera - something that’s difficult to tell when you’ve only got two or three specimens total between the two supposed genera. Also, the fact that Marsh did not officially describe the genus Brontosaurus until 1879 (with B. excelsus as the type, rather than B. ajax) established a definite priority for Apatosaurus (though, as noted above, Riggs could have argued that Apatosaurus was not valid).
And, as yet another side note, Marsh described Brontosaurus montanus in 1877 as well. This fellow, based on fragmentary material, was later referred to the genus Atlantosaurus.
