Monodelphis domestica (Burnett 1830, Pine et al. 2013) is the extant gray short-tailed opossum. Monodelphis is traditionally considered a member of Didelphidae, a group of exclusively New World opossums thought to be basal among the living marsupials.
However, Monodelphis does not have a pouch and skeletal traits nest it at the base of the Eutheria, the placental therians. Monodelphis is approximately 50 times smaller in body size than Didelphis, The dental formula is: 184.108.40.206 and 220.127.116.11 as in other didelphids. Note (below) that even smaller species of Monodelphis, like M. kunis, are extant and known. These have three molars per side, as in many basal placentals.
Images from Digimorph.org used with permission.