Approximately 12 million years before the extinction of the dinosaurs, a diverse terrestrial fauna inhabited what is now northcentral Montana. The author describes and evaluates the systematic relationships of the large mammalian fauna of this area, which included rodent-like multituberculates, primitive therians, marsupials, and early eutherian mammals, and also analyzes the biogeography of mammalian faunas of the Judithian age.