The Collaborative International Dictionary of English v.0.48:

Marsupialia \Mar*su`pi*a"li*a\
   (m[aum]r*s[=u]`p[i^]*[=a]"l[i^]*[.a]), prop. n. pl. [NL., fr.
   L. marsupium a pouch, bag, purse, Gr. marsy`pion, dim. of
   ma`rsypos, ma`rsipos.] (Zool.)
   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.
   [1913 Webster] marsupialian

The Collaborative International Dictionary of English v.0.48:

Mammalia \Mam*ma"li*a\, n. pl. [NL., from L. mammalis. See
   Mammal.] (Zool.)
   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.
   [1913 Webster]

   Note: Mammalia are divided into three subclasses;
         [1913 Webster] I. Placentalia. This subclass embraces
         all the higher orders, including man. In these the
         fetus is attached to the uterus by a placenta.
         [1913 Webster] 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.
         [1913 Webster] 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 mammae.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Marsupialia
    n 1: coextensive with the subclass Metatheria [syn:
         Marsupialia, order Marsupialia]