MAMMALS 245 



altogether, no canines in either jaw, i upper and i lower premolar on 

 each side and 3 upper and 3 lower molars on each side, or 10 teeth in 

 the upper jaw on both sides and 10 teeth in the lower jaw on both sides, 

 or 20 teeth altogether. 



The friction pads of the feet are called the plantar tubercles (Fig. 

 163) and are employed in certain species of rodents for purposes of 

 identifications. 



In a country as large as the United States, in which the conditions 

 of climate, rain-fall, moisture and altitude are so very varied, and 

 where forests, plains and deserts, large and small rivers, lakes and 

 marshes form an endless succession of localities which harbor mamma- 

 lian life, any species which inhabits any considerable area is sure to be 

 divided into a number of geographical races or subspecies. Often the 

 best means of identifying a mammal is to tell where and under what 

 conditions it lives, and the descriptions in this book usually give such 

 details. 



History. — The Mammalia as a definite scientific group of animals 

 has existed in the literature since the time of Aristotle. Linnaeus made 

 it one of the six classes into which he divided the Animal Kingdom, 

 and Lamarck one of the classes of Vertebrata. 



About 10,000 species and subspecies of mammals are known, 

 grouped in about 15 orders, of which 9 are represented in the United 

 States. 



Key to the Orders of Mammals in the United States 



ai A marsupial pouch present ; opossums i. Marsupialia (p. 245). 



ao Xo parsupial pouch present. 



bi Hind limbs wanting; manatees 3. Sircnia (p. 247). 



b2 Hind limbs present. 



Ci Body covered with a bony shell; armadillos 2. Edentata (p. 247). 



C2 Body not so covered. 



di Flying mammals; bats 5. Cliiroptera (p. 257). 



do Non-flying mammals. 



Ci Feet with claws and not with hoofs. 

 fi Canine teeth present. 



gi Limbs used mainly for walking and running. 



hi Canines small; moles and shrews. . .4. Insectivora (p. 247). 



h2 Canines prominent 6. Carnivora (p. 264). 



g2 Limbs used mainly for swimming; seals . 7. Pinnipedia (p. 288). 



fi Canines absent 8. Rodentia (p. 289). 



62 Feet with hoofs 9. Ungulata (p. 352). 



Order i. Marsupialia. — Aplacental mammals in which the young 

 are born in a very immature condition and are maintained by the 



