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].

Source: Webster's Revised Unabridged Dictionary (1913)
 

Marsupialia \Mar*su`pi*a"li*a\, n. pl. [NL., fr. L. marsupium a pouch, bag, purse, Gr. ?, dim. of ?, ?.]

(Zo["o]l.) A subclass of Mammalia, including nearly all the mammals of Australia and the adjacent islands, together with the opossums of America. They differ from ordinary mammals in having the corpus callosum very small, in being implacental, and in having their young born while very immature. The female generally carries the young for some time after birth in an external pouch, or marsupium. Called also Marsupiata.

Source: Webster's Revised Unabridged Dictionary (1913)