Placentalia

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. [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 mamm[ae]. [1913 Webster]