We describe small-bodied archaeohyracids of transitional Eocene-Oligocene age from Chubut, Argentina, including two from Santiago Roth's important but poorly known Cañadón Blanco locality and two occurring in the Tinguiririca Fauna of the Andean Main Range of central Chile. Three taxa are recognized. We refer specimens from both Patagonia and the central Andes to one of these taxa, initially named Arachaeohyrax gracilis by Roth (1903) but receiving a new generic designation here. A diminutive form from Cañadón Blanco and an intermediate-sized form from Chile are each recognized as new species. These taxa help to clarify the temporal correlation of lithostratigraphic units currently located on opposite sides of the Andean divide, and aid in the recognition of a biochronological interval, the early Oligocene (to possibly late Eocene)-aged Tinguirirican, interposed between the classical Mustersan and Deseadan "ages" of the South American land mammal succession Includes bibliographical references (p. 16-17) We describe small-bodied archaeohyracids of transitional Eocene-Oligocene age from Chubut, Argentina, including two from Santiago Roth's important but poorly known Cañadón Blanco locality and two occurring in the Tinguiririca Fauna of the Andean Main Range of central Chile. Three taxa are recognized. We refer specimens from both Patagonia and the central Andes to one of these taxa, initially named Arachaeohyrax gracilis by Roth (1903) but receiving a new generic designation here. A diminutive form from Cañadón Blanco and an intermediate-sized form from Chile are each recognized as new species. These taxa help to clarify the temporal correlation of lithostratigraphic units currently located on opposite sides of the Andean divide, and aid in the recognition of a biochronological interval, the early Oligocene (to possibly late Eocene)-aged Tinguirirican, interposed between the classical Mustersan and Deseadan "ages" of the South American land mammal succession Fieldiana series has been published as Geological Series by Field Columbian Museum (1895-1909) and Field Museum of Natural History (1909-1943), and as Fieldiana: Geology by Chicago Natural History Museum (1945-1966) and Field Museum of Natural History (1966-1978). Fieldiana Geology New Series No. 1 began June 29, 1979