DEFINITION OF THE GENUS EUMYSOPS AMEGHINO, 1888 (RODENTIA, ECHIMYIDAE) AND SYSTEMATICS OF THE SPECIES FROM THE EARLY PLIOCENE OF CENTRAL ARGENTINA A definition of the genus Eumysops Ameghino and a systematic review of its oldest species, E. laeviplicatus Ameghino and E. formosus Ameghino, from the early Pliocene of the Monte Hermoso and “Irene” Formations (southwest of Buenos Aires province, central Argentina), are provided. Eumysops has craniomandibular and postcranial features that clearly define it within the Echimyidae: posterior skull short and bowed, rostrum narrow, orbital region very wide, paroccipital apophysis ventrally protruding, condyle and coronoid processes low, distalized ventrocaudal iliac spine, and metatarsals I, II and V markedly reduced. According to the concept of Eumysops here proposed, the genus would include four species: E. laeviplicatus, E. formosus, E. chapalmalensis Rovereto y E. gracilis Rovereto species. The oldest species E. laeviplicatus, from the Montehermosan—lower Chapdamalalan, co-occurs with E. formosus at least in the lower Chapadmalalan. These species are absent in the early—late Pliocene of the Chapadmalal area (southeast of Buenos Aires province) where other representatives of the genus are recorded. This stratigraphic pattern, concurrent with that of other two caviomorph genera, supports the hypothesis that there is a higher faunal similarity between the two recognized levels of the Monte Hermoso Formation than between the upper level of this formation and the Chapadmalal Formation.