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VERTEBRATE ANIMALS. 



are a number of small animals, characterized by the absence 

 of canine teeth, and the possession of two long curved incisor 

 teeth in both jaws, which are separated by a wide interval 

 from the molars (Fig. 155). There are seldom more than two 



FIG. 155. A, Skull of the Beaver (after Owen); B, Diagram of one of the incisor teeth 

 of a Eodent, showing the chisel-shaped point, a Enamel; d Soft tooth-substance 

 (dentine). 



incisors in the upper jaw (sometimes four), but there are never 

 more than two in the lower jaw. The molar teeth are few in 

 number (rarely more than four on each side of each jaw). The 

 feet are usually furnished with five toes each. 



The most characteristic point about the Rodents is to be 

 found in the structure of the incisor teeth, which are adapted 

 for continuous gnawing. They grow from persistent pulps, 

 and consequently continue growing as long as the animal lives. 



FIG. 156. Hamster (Cricetus vulffaris). 



They are large, long, and curved, and are covered in front with 

 a layer of hard enamel, so that the softer parts of the tooth 



