TL;DR: In this article, the temporal range of Hyracodon is extended into the early Miocene by adding a new species of rhinoceros, Diceratherium gregorii, to the fauna.
Abstract: Only one species of rhinoceros, Diceratherium gregorii, has been described from Lower Miocene deposits in South Dakota. Additional specimens with unworn dentitions adds to the knowledge of this species. D. armatum is added to the fauna and is used as a basis for correlating the deposits of Gering type lithology with the John Day formation in Oregon. The temporal range of Hyracodon is extended into the early Miocene.