An infraclass of MAMMALS, also called Metatheria, where the young are born at an early stage of development and continue to develop in a pouch (marsupium). In contrast to Eutheria (placentals), marsupials have an incomplete PLACENTA.

Monotremata is an order of mammals, including the platypus and echidnas, which are unique for laying eggs instead of giving birth to live young and for having a single body opening for both excretion and reproduction, known as a cloaca.

A genus of large OPOSSUMS in the family Didelphidae, found in the Americas. The species Didelphis virginiana is prominent in North America.

A genus of short-tailed OPOSSUMS in the family Didelphidae found in South American, chiefly Brazil. They are opossums least well-adapted to arboreal life.

New World marsupials of the family Didelphidae. Opossums are omnivorous, largely nocturnal and arboreal MAMMALS, grow to about three feet in length, including the scaly prehensile tail, and have an abdominal pouch in which the young are carried at birth.