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The Collaborative International Dictionary of English v.0.48 (gcide)

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]


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]

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