*mammalia*

‖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 ]

☞ 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 ]