A detailed report of an early Tertiary paleontological expedition that documents the geology and stratigraphy of a Patagonian formation, the expedition’s fieldwork and localities, and a complete systematic treatment of the vertebrate fossils recovered. It examines fauna composition, feeding adaptations, and habitat, compares the new material with earlier fragmentary finds, and provides careful descriptions and taxonomic revisions across many groups of mammals and a few birds, supplying measurements, illustrations, and arguments about relationships and the origins of the constituent taxa.