Two genera of Miocene beavers (Castoridae), Hystricops and Priusaulax are referred to a new tribe Hystricopini of the subfamily of giant beavers, Castoroidinae based on cranial and dental morphologies. Previously undescribed and more complete specimens of Hystricops demonstrate that it is referable to the Castoroidinae rather than the Castorinae. Along with Priusaulax, it is believed to represent a group of basal castoroidines, distinct from later Castoroidinae by the retention of the primitive morphology of the contact of the lacrimal bone with the jugal on the cranium, as well as more primitive dental morphologies (lower crown-height and simple occlusal pattern of cheek teeth). A partial cranium and several isolated teeth from the Hemingfordian of Colorado, originally referred to Anchitheriomys?, are referred here to a new species, Priusaulax wilsoni sp. nov. Monosaulax senrudi from the Barstovian of Montana is also referred to Priusaulax. Hystricops is limited to the type species H. venustus from the Clarendonian and Barstovian North American Land Mammal Age (NALMA), and H. browni from the Hemphillian NALMA.

Key words: Mammalia, Rodentia, Castoridae, Castoroidinae, cranial anatomy, dental terminology, North American Land Mammal Ages, Colorado.

William W. Korth [wwkorth@frontiernet.net], Rochester Institute of Vertebrate Paleontology, 265 Carling Road, Rochester, New York 14610, USA.