\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].