MARSUPIALIA - (n. pl.) 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.