Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/96

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ORDER, Ornithodelphia, Monotremata. Ornithorhynchus. Didelphia. Marsupialia. Opossum. ' Carnivora. l^og. Cetacea. Whale. Prosimise. Lemur. ' Simise. Man, Monkey. Rodentia. Beaver, Rat. Hyracoidea. Hyrax. ° ^ P Proboscidea. Elephant. Cheiroptera. Bat. Insectivora. Hedgehog. Edentata. Sloth. Ungulata, Horse, Pig. Sirenia. Sea-Cow, Dugong.

The class Mammalia is so called from the females suckling their young, and includes the domestic animals as well as many other less well-known forms. The mammals differ from the reptiles and birds in many important characters, as in the manner in which the skull and backbone are joined together (two condyles instead of one), in the simple structure of the lower jaw, it being composed of only one piece on each side in the Mammalia, whereas in the reptiles and birds it is made up of several. The skin of the mammals is covered more or less with hairs, never with feathers, as in birds. They bring their young into the world living, and nourish them for a longer or shorter time with milk. These peculiarities, as well as others, separate the birds and reptiles from the mammals. The Mammalia can be divided into three sub-classes, each of which offers well-marked peculiarities, which serve to distinguish