The Collaborative International Dictionary of English v.0.48:

Placentalia \Plac`en*ta"li*a\, n. pl. [NL.] (Zool.)
   A division of Mammalia including those that have a placenta,
   or all the orders above the marsupials.
   [1913 Webster]

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]