Definition: 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.
