Source : Webster's Revised Unabridged Dictionary (1913)

Placentalia \Plac`en*ta"li*a\, n. pl. [NL.] (Zo["o]l.)
   A division of Mammalia including those that have a placenta,
   or all the orders above the marsupials.

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].