Metatheria An infraclass of mammals containing the marsupials. The female bears an abdominal pouch (marsupium) into which the newly born young, which are in a very immature state, move to complete their development. They obtain nourishment from the mother's mammary teats. Modern marsupials are restricted to Australasia (where they include the kangaroos, koala bears, phalangers, and bandicoots) and America (the opossums). Marsupials evolved during the late Cretaceous period, 80 million years ago. In Australia, where the marsupials have been isolated for millions of years, they show the greatest diversity of form, having undergone adaptive radiation to many of the niches occupied by placental mammals elsewhere. Compare Eutheria; Prototheria.

Metatheria (class Mammalia, subclass Theria) An infraclass that comprises the marsupials and their extinct relatives, sometimes included in the single order Marsupialia and now more usually divided into several orders.