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)