The work offers a geological introduction followed by systematic, anatomical descriptions of fossil mammals collected on the voyage, with emphasis on South American extinct forms. It details crania, jaws, teeth, vertebrae, limb bones, and tesselated armour for taxa such as Toxodon, Macrauchenia, Glossotherium, Mylodon, Scelidotherium, Megalonyx, and Megatherium, illustrated by numerous plates. Comparative morphology and measurements underpin taxonomic identifications and interpretations of structural relationships, and a brief discussion addresses the geological contemporaneity of the described extinct mammals.