Metatheria is one of two mammalian clades, the other being Eutheria, with extant members that diverged in the Early Cretaceous or perhaps the Late Jurassic, and which includes all mammals more closely related to marsupials than to placentals. First proposed by Thomas Henry Huxley in 1880, it is a slightly more inclusive group than the marsupials; it contains all of the living mammals with abdominal pouches (most female marsupials) as well as their more primitive ancestors and relatives. Metatherians are one of three main classes of extant mammals: monotremes (egg laying mammals like the platypus and the echidna), metatheria (or marsupials, which includes the three American orders (Didelphimorphia, Paucituberculata and Microbiotheria) and the four Australasian orders (Notoryctemorphia, Dasy