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