We provide systematic description and discussion for each of 47 species of eutherian (“placental”) mammals from the type Ferris Formation of the western Hanna Basin, south-central Wyoming. All records are new for the basin, and for southern Wyoming in general. Although fossiliferous strata are both of Lancian (latest Cretaceous) and Puercan (earliest Paleocene) age, all but a few specimens (restricted to two genera) represent the Puercan. We describe five species of previously known genera as new, all of Puercan age. We also assign a previously described Puercan species to a new genus, to emphasize its distinctness. Ordinal categories are represented by Leptictida, Proteutheria, Condylarthra, and Cete. Except for fossils from lowest parts of the Puercan, the local early Paleocene fauna is biased taxonomically in favor of condylarths, animals of relatively large body size in early parts of the epoch. The taxonomic bias probably is due in part to fluvial size-sorting of specimens related to generally sandy, relatively high-energy depositional environments. The local Puercan mammalian fauna is of keen paleobiogeographic and biostratigraphic importance. Among the 59 species of Puercan mammals now known from the Hanna Basin (combining recently reported multituberculates and peradectians with eutherians considered here), at least 25 represent significant extensions of previously recognized geographic ranges (18 in northerly extent, 7 in southerly). Because many of those extended ranges involve major parts of the continent, confidence becomes greatly heightened in biostratigraphic utility of Puercan mammalian species for temporal correlations, even across vast distances of western North America. Composition of the Hanna Basin's mammalian fauna confirms utility of most elements of the formalized interval-zones of Puercan time, established in the San Juan and Williston basins. We do, however, abandon use of Puercan Interval-zone Pu0 as impractical, and use an expanded concept of Pu1 instead. Other than that adjustment, we strictly apply original faunal criteria for recognition of Puercan Interval-zones Pu1 through Pu3 to zonation of the type Ferris Formation. The unusually great thickness of Puercan parts of the formation, combined with our high density of sampled fossiliferous localities, has led to recognition of many extensions of temporal ranges beyond those known previously for individual species. The biostratigraphic importance of this section, therefore, becomes elevated for potential studies in: (1) long-distance temporal correlation of Puercan strata; and (2) many aspects of biological evolution across North America during earliest Cenozoic time. Basal parts of Interval-zone Pu2 in the Ferris Formation uniquely show persistence of some mammalian species known elsewhere only in Pu1. Faunal change within Interval-zone Pu2 greatly exceeds that observed between Pu2 and Pu3, although the evolutionary distinctiveness of Pu2 and Pu3 are confirmed in the present study. The newly discovered mammalian assemblages of Puercan age from southern Wyoming exhibit virtually no geographic endemism. Although they share species both from northern and southern fossil-bearing realms of the North American continent, composition of the Ferris assemblages clearly is more similar to southern than to northern faunas under comparison.