The 27 species in the genus Marmosa are relatively small Neotropical members of the family Didelphidae. This genus is one of three that are known as mouse opossums. The others are Thylamys (the "fat-tailed mouse opossums") and Tlacuatzin, the grayish mouse opossum. Members of the genus Marmosops used to be called "slender mouse opossums", but are now just called "slender opossums". The thirteen members of the Marmosa subgenus Micoureus, known as woolly mouse opossums, were formerly considered to be a separate genus, but were moved into Marmosa in 2009. Based on a comparison of sequences of one mitochondrial and three nuclear genes, three new subgenera, Eomarmosa, Exulomarmosa and Stegomarmosa, were recognized by Voss et al. in 2014. Eomarmosa and Exulomarmosa, as well as Marmosa and Micoureus, are thought to be sister taxa, while Stegomarmosa is viewed as sister to Marmosa plus Micoureus. Exulomarmosa is a mostly trans-Andean (west of the Andes) clade. Subgenus Eomarmosa Voss, Gutierrez, Solari, Rossi & Jansa 2014 Subgenus Exulomarmosa Voss, Gutierrez, Solari, Rossi & Jansa 2014 Subgenus Marmosa J. E. Gray 1821 Subgenus Micoureus Lesson 1842 Subgenus Stegomarmosa Pine, 1972