REVISION OF THE GENUS TRANSPITHECUS AMEGHINO, 1901 (NOTOUNGULATA, INTERATHERIIDAE) FROM THE MIDDLE EOCENE OF PATAGONIA, ARGENTINA. The genus Transpithecus Ameghino is up to now known from middle Eocene levels of Santa Cruz and Chubut Provinces, Argentina. Only a single species is known, T. obtentus Ameghino, which is represented by very scarce remains. Traditionally, this species has been placed in the Subfamily “Notopithecinae” (Notoungulata, Interatheriidae) together with Notopithecus Ameghino, Antepithecus Ameghino, and Guilielmoscottia Ameghino. According to recent phylogenetic analyses, this group constitutes a paraphyletic assemblage conformed by basal interatheriids. Since Ameghino's description and except for the contribution made by Simpson, no recent studies on Transpithecus have been performed. In this contribution, the associated upper and lower dentition of a same individual is assigned for the first time to T. obtentus. This specimen comes from Valle Hermoso (Chubut Province, Argentina), and has led to the recognition of other unpublished materials as T. obtentus. The present study allows: (1) extending the diagnosis of T. obtentus; (2) defining an ontogenetic sequence of its dental morphology; (3) formally proposing Patriarchippus annectens Ameghino as synonymous to T. obtentus; (4) establishing similarities between Transpithecus obtentus and the specimen MNHN CAS 69, previously assigned to Paginula parca Ameghino; and (5) discarding the presence of Transpithecus in the Kibenikhoria faunal zone (Itaboraian South American Land Mammal Age), restricting its biochron to the Casamayoran South American Land Mammal Age (Barrancan subage; middle Eocene).