See text Thylamys is a genus of opossums in the family Didelphidae.

The females' nipples are arranged in two symmetrical rows on the abdomen.

[4] Fossils belonging to the genus date back to the Miocene, with the oldest specimens being found in the Cerro Azul Formation of Argentina and the Honda Group of Colombia.

[5] Genetic studies indicate that the genus may have originated around 14 million years ago.

[7][8] T. karimii (Petter 1968) Reig, Kirsch & Marshall 1987 (Karimi's fat-tailed mouse opossum) T. velutinus (Wagner 1842) Miranda-Ribeiro 1936 (Dwarf fat-tailed mouse opossum) T. tatei (Handley 1957) Reig, Kirsch & Marshall 1987 (Tate's fat-tailed mouse opossum) T. elegans (Waterhouse 1839) Gray 1843b (Elegant fat-tailed mouse opossum) T. pallidior (Thomas 1902) Gardner & Creighton 1989 (White-bellied fat-tailed mouse opossum) T. citellus (Thomas 1912c) [9] T. pulchellus (Cabrera 1934) [10] (Argentine fat-tailed mouse opossum) T. pusillus (Desmarest 1804) Reig, Kirsch & Marshall 1987 (Common fat-tailed mouse opossum) T. macrurus (Olfers 1818) Gardner & Creighton 1989 (Paraguayan fat-tailed mouse opossum) T. venustus Thomas 1902 (Buff-bellied fat-tailed mouse opossum) T. cinderella (Thomas 1902) Flores, Díaz & Barquez 2000 (Cinderella fat-tailed mouse opossum) T. sponsorius (Thomas 1921) Flores, Díaz & Barquez 2000 (Argentine fat-tailed mouse opossum) Other species of Thylamys.