The Marsupials include the Macro pus or kangaroo; the opossums, Phalangista vulpina and P. Cookii; the opossum-mouse, Dromicia nana; Perameles or bandicoot; Hypsiprymnus or kangaroo rat; Phascolomys or wombat; while of Monotremata there are the Echidna or porcupine ant-eater and the duck-billed platypus.
One of the leading features in mammalian distribution is the fact that the Monotremata, or egg-laying mammals, are exclusively confined to Australia and Papua, with the adjacent islands.
Others however (inclusive of Tritylodon and Microlestes, if they be really mammals), seem nearer to the Monotremata; and the question has yet to be decided whether placentals and marsupials on the one hand, and monotremes on the other are not independently derived from reptilian ancestors.
The anatomical differences by which the platypus, and its only allies the echidnas, are separated from all other mammals, so as to form a distinct sub-class, are described in the article Monotremata, where also will be found the main distinctive characters of the two existing representatives of the group. It is there stated that the early stages of the development of the young are not yet fully known.