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In 1977 Marsh mentioned Eohyus as the oldest known artiodactyl but failed to describe or illustrate a type specimen, and thus Eohyus was for many years anomen nudum. The validity of its two species, E. distans and E. robustus, both named by Marsh in 1894, has been the subject of numerous and often conflicting opinions. Reexamination of the type specimens reveals that E. distans is a subjective synonym of Phenacodus primaevus Cope, 1873, and E. robustus is a subjective synonym of Periptychus carinidens Cope, 1881. The type of E. distans is from early Eocene (Wasatchian) age strata of the San Jose Formation, San Juan Basin, New Mexico; that of E. robustus is from middle Paleocene (Torrejonian) age strata of the Nacimiento Formation, also in the San Juan Basin.