A subclass of Mammalia, including nearly all the
   mammals of Australia and the adjacent islands, together with the
   opossums of America. They differ from ordinary mammals in having the
   corpus callosum very small, in being implacental, and in having their
   young born while very immature. The female generally carries the young
   for some time after birth in an external pouch, or marsupium. Called
   also Marsupiata.