CHAPTER X 



Order : RODEN^IA. RODENTS, OR GNAWING 

 MAMMALS : HARES AND RABBITS 



Reference has already been made to the Rodent, or gnawing 

 group of mammals, as having started probably in very ancient 

 times from some generalised group of Eutherian mammal akin 

 to the Insectivores. The marked feature in the Rodents is the 

 complete absence of canine teeth in the upper and lower jaws, 

 and the reduction of the incisors, which in the lower jaw are 

 never more than two, one on each side, and in the upper jaw are 

 equally reduced to two, except in the case of the sub-order of 

 hares and rabbits, where the upper incisors may be three on 

 each side in the case of young animals, and two on each side 

 in the adult. The premolars are always below the full number, 

 and very often either completely wanting or reduced to one each 

 side. The feet almost invariably possess the original five toes, 

 but differ from those of the Primates (man, apes, and lemurs) in 

 never having the thumb or first toe opposable. The extremities 

 of the toes and fingers are armed with claws, sometimes long and 

 sharp, occasionally in the few large forms becoming blunt, angular, 

 and almost hoof-like, but never developing a flat nail as in the 

 Primates. Nearly all the Rodents have collar bones, though 

 sometimes in a very rudimentary form. Most of them have a 

 caecum. The mouth is characterised by a very peculiar feature. 

 Behind the large incisors, or gnawing teeth, of the upper and 

 lower jaw the palate is covered with hair, so that the mouth is 

 really divided into two portions— the entrance in front between 

 the great incisor teeth, followed by a narrow passage more or 



209 14 



