MAMMALIA 371 



that were used in driving in the daggers. The lower canines are 

 much reduced in size. 



Order 5. Rodentia (Gnawing Mammals). — The rodents are for 

 the most part rather small mammals, though a few of them have 

 reached a considerable size. It has been claimed by some authorities 

 that there are more species of rodents living to-day than of all other 

 mammals combined. Unquestionably they are the most typical mam- 

 malian group to-day, as well as the most successful. Because they are 

 so extremely proUfic, because they are omnivorous, and because many 

 of them lead a nocturnal burrowing life, they seem Ukely to be the 

 main mammalian rivals of Man in the next geological period. The 

 rodents are characterized by absence of canine teeth ; and the incisors 

 are long and strong, with persistently growing pulp and enamel con- 

 fined chiefly to the anterior edge. This arrangement of the enamel 

 makes the teeth wear down to a chisel edge, which is self-sharpening 

 with use. The brain is smooth, with few furrows, and the intelligence 

 is usually low. The testes are usually abdominal in position; the 

 placenta is discoidal and deciduate. Two sub-orders are distin- 

 guished: Duplicidentata (Hares and Pikas) and SimpUcidentata 

 (Rodents Proper). 



Sub-Order 1. Duplicidentata {Hares and ^Pifcos) .^These animals 

 are characterized by two pairs of incisor teeth in the upper jaw, the 

 inner being small and lying behind the outer. The tail is short. The 

 group is regarded by some as a distinct order. 



Family 1. Leporidce (the Hares) are distinguished by long ears, 

 long hind legs, and short though obvious tail. 



Family 2. Lagomyidoe (Pikas) are distinguished by short ears, short 

 hind legs, and no external evidences of a tail. 



Sub-Order 2. SimpUcidentata (True Rodents). — The members of 

 this sub-order are divided into three sections: represented by squirrel- 

 Uke,* rat-like, and porcupine-like rodents. 



Section 1. Sciuromorpha (Squirrel-like Rodents). — This large sec- 

 tion includes the squirrels proper, the flying squirrels, the ground 

 squirrels and chipmunks, the gophers, the prairie dogs, the marmots, 

 the beavers, and others. The flying squirrels (Fig. 192, A) are 

 parachuting animals, with a membrane stretched between the fore 

 and hind limbs. The prairie-dogs are burrowing rodents of the west- 

 em plains, that live in large colonies and share their burrows with 

 ground owls and rattlesnakes, as well as other messmates. The habits 



