A new genus and species of notharctine primate, Hesperolemur actius, is described from Uintan (middle Eocene) aged rocks of San Diego County, California. Hesperolemur differs from all previously described adapiforms in having the anterior third of the ectotympanic anulus fused to the internal lateral wall of the auditory bulla. In this feature Hesperolemur superficially resembles extant cheirogaleids. Hesperolemur also differs from previously known adapiforms in lacking bony canals that transmit the internal carotid artery through the tympanic cavity. Hesperolemur, like the later occurring North American cercamoniine Mahgarita stevensi, appears to have lacked a stapedial artery. Evidence from newly discovered skulls of Notharctus and Smilodectes, along with Hesperolemur, Mahgarita, and Adapis, indicates that the tympanic arterial circulatory pattern of these adapiforms is characterized by stapedial arteries that are smaller than promontory arteries, a feature shared with extant tarsiers and anthropoids and one of the characteristics often used to support the existence of a haplorhine-strepsirhine dichotomy among extant primates. The existence of such a dichotomy among Eocene primates is not supported by any compelling evidence. Hesperolemur is the latest occurring notharctine primate known from North America and is the only notharctine represented among a relatively diverse primate fauna from southern California. The coastal lowlands of southern California presumably served as a refuge area for primates during the middle and later Eocene as climates deteriorated in the continental interior. Hesperolemur probably was an immigrant taxon that entered California from either the northern (Wyoming/Utah) or southern (New Mexico) western interior during the middle Eocene © 1995 Wiley-Liss, Inc.