[1] : The Collaborative International Dictionary of English v.0.48

placentalia \plac`enta"lia\, n. pl. nl. zool.
   a division of mammalia including those that have a placenta,
   or all the orders above the marsupials.
   1913 webster

[2] : The Collaborative International Dictionary of English v.0.48

mammalia \mamma"lia\, 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
see also:
mammal placentalia marsupialia monotremata echidna ornithorhynchus 

Placentalia >> P Words
Placentalia, definition of term: Placentalia
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