Source : Webster's Revised Unabridged Dictionary (1913)

Mammalia \Mam*ma"li*a\, n. pl. [NL., from L. mammalis. See
   {Mammal}.] (Zo["o]l.)
   The highest class of Vertebrata. The young are nourished for
   a time by milk, or an analogous fluid, secreted by the
   mammary glands of the mother.

   Note: Mammalia are divided into three subclasses; -- I.
         {Placentalia}. This subclass embraces all the higher
         orders, including man. In these the fetus is attached
         to the uterus by a placenta. II. {Marsupialia}. In
         these no placenta is formed, and the young, which are
         born at an early state of development, are carried for
         a time attached to the teats, and usually protected by
         a marsupial pouch. The opossum, kangaroo, wombat, and
         koala are examples. III. {Monotremata}. In this group,
         which includes the genera {Echidna} and
         {Ornithorhynchus}, the female lays large eggs
         resembling those of a bird or lizard, and the young,
         which are hatched like those of birds, are nourished by
         a watery secretion from the imperfectly developed
         mamm[ae].

Marsupialia \Mar*su`pi*a"li*a\, n. pl. [NL., fr. L. marsupium a
   pouch, bag, purse, Gr. ?, dim. of ?, ?.] (Zo["o]l.)
   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}.

Source : WordNet®

Marsupialia
     n : coextensive with the subclass Metatheria [syn: {order
         Marsupialia}]