68 EVOLUTION OF LIFE. / 
MAMMALIA. 
SUB-CLASS. ORDER, EXAMPLE, 
Ornithodelphia. 1 Monotremata. Ornithorhynchus. 
Didelphia. 4 Marsupialia. Opossum. 
( Carnivora. : 
Cetacea. Whale. 
Prosimize Lemur 
Simiee. Man, Monkey. 
Rodentia. Beaver, Rat. 
HMM Hyracoidea. Hyrax. 
oU 2 | Proboscidea. Elephant. 
Cheiroptera Bat. 
Insectivora, Hedgehog. 
Edentata. Sloth. 
Ungulata. Horse, Pig. 
| Sirenia. Sea-Cow, Duzong. 
MAMMALIA. 
The class Mammalia is so called from the females suck- 
ling 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 char- 
acters, 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 Mamma- 
lia can be divided into three sub-classes, each of which 
offers well-marked peculiarities, which serve to distinguish 
