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MAMMALIA (-mā′lyä), in zoölogy, mammals, the highest class of the vertebrate sub-kingdom. The individuals are characterized by the possession of mammæ (teats), enabling them to suckle their young. The class is sometimes popularly but erroneously called quadrupeds (four-footed animals). They have red, warm blood, in this respect agreeing with birds, but differing from reptiles, amphibians and fishes. The mouth is concealed by lips and armed with bony and enameled teeth; each ramus of the mandible is composed of a simple piece of bone. The covering is of hair. Normally, there are four limbs, which in some aquatic members of the class are modified into fins. The toes are generally five. Most of the bones are solid or have cavities filled with marrow, the air cells which aid in imparting lightness to the bones of birds being, as a rule, absent. The bones of the cranium and of the face are immovably fixed to each other. The cranium is larger than in other vertebrates, the lower jaw consists of only two pieces. The vertebral column may be divided into five regions, the cervical, the dorsal, the lumbar, the sacral, and the caudal vertebræ. The heart has two auricles and two ventricles. The respiration is by lungs.