Metatheria
Metatheria is a grouping within the animal class Mammalia. First proposed by Thomas Henry Huxley in 1880, it is nearly synonymous with the earlier taxon Marsupialia (Illiger, 1811) though it is slightly wider. It also contains the nearest ancestors of marsupial mammals.The earliest known representatives are known from the uppermost Lower Cretaceous of North America.
The closest living relatives of the metatheres are within Eutheria (also erected by Huxley in 1880). Both are sometimes united Infraclasses within the Subclass of Theria (Parker and Haswell, 1897), which contains all living mammals except monotremes.