A new genus of sciurid, Hesperopetes, contains three species: H. thoringtoni (type species), H. jamesi, and H. blacki. H. thoringtoni is from the early Chadronian (late Eocene) White River Formation of central Wyoming and represents the earliest recognized species of sciurid. The other species are from the latest Whitneyan or earliest Arikareean (late Oligocene) Blue Ash local fauna of South Dakota. Hesperopetes is closest in dental morphology to Oligopetes Heissig, which is known only from the early Oligocene of Europe. Hesperopetes may also represent the earliest occurrence of a lineage leading to pteromyine sciurids (“flying” squirrels). It is impossible at this time to construct a phylogeny of North American “flying” squirrels due to their scarcity in the fossil record and their undetermined relationships with Old World taxa.