MAMMALIA 



123 



them live upon the surface of the ground, but some burrow and 

 some climb, while others have taken to the water. The general 

 organization is of rather a low type, but there is much speciali- 

 zation in the most characteristic structures, i. e. the teeth. 

 Rodents walk entirely or partly upon the soles of the feet, and 

 the digits are usually provided with claws, though more rarely 

 they possess small hoofs. The most striking feature regarding 

 the teeth is the character of the incisors, of which in the adult 

 never more than four are developed to a useful extent. These 

 are large chisel-ended structures, growing continuously throughout 

 life, as they are worn away by the constant gnawing which is 

 so characteristic of the order. This is strikingly seen in the 

 cases of unfortunate animals which have lost one incisor, as a 

 result of which the corresponding tooth has nothing to bite 

 against, and not being therefore subjected to wear, grows to an 

 enormous length, ultimately killing its unlucky owner. A similar 

 state of things may result from malformation or accident, if 

 thereby the upper and lower teeth are prevented from coming 

 together. Canine teeth are always absent, and the cheek teeth 

 are commonly reduced in number, and more or less adapted to 

 act as grinders. There is an interesting feature, already alluded 

 to (p. 28), regarding the jaw-joint, which permits of the free 

 backward-and-forward movement necessary for gnawing. This 

 is due to the fact that the condyles of the lower jaw are elongated 

 from front to back, 

 and fit into sockets of 

 corresponding shape. 



There are four 

 great groups of exis- 

 ting Rodents, which 

 may broadly be called: 

 I. Rabbits; 2. Squir- 

 rels; 3. Mice; and 

 4. Porcupines. 



I. The group of 

 Rabbits, including 



also Hares and Pikas, is distinguished by the presence of two 

 small upper incisor teeth, situated immediately behind the two 

 large teeth of the same kind present here as in all Rodents 

 (fig. 83). It may also be added that the two bones of the lower 



Fig. 83.— Skull of Hare. Observe the small upper incisors i' behind the 

 large functional ones i; c.p. coronoid process; z.a. zygomatic arch. 



