Thylamys is a genus of opossum in the family Didelphidae. The premaxillae are rounded rather than pointed. The females lack a pouch. The females' nipples are arranged in two symmetrical rows on the abdomen All species but T. macrurus store fat in their tails., although this is not necessarily true for all species in the genus. The genus contains the following species: Fossils belonging to the genus date back to the Miocene, with the oldest specimens being found in Argentina and Colombia. Genetic studies indicate that the genus may have originated around 14 million years ago.