6 INTRODUCTORY. 



outer layer of the skin. The centre of each tooth is filled with 

 pulp, around which is the bone-like dentine with an outer coat 

 of hard, glossy enamel. In the incisors or cutting teeth of the 

 Rodents, while the front of the tooth is protected by a thick 

 plate of hard enamel, the back portion consists only of dentine 

 which wears away whilst the enamel front maintains a chisel- 

 like cutting edge. In the grinding teeth or molars, especially 

 noticeable in the Ungulates, the enamel is thrown into ridges 

 and tubercles, so that the action of these in the upper and 

 lower jaws upon each other is like that of "the upper and the 

 nether millstones " in grinding corn. 



Four forms of teeth are recognised in the Mammals : 

 the incisors in the front of the jaw, the pointed, round canines 

 or " eye-teeth " next to them, and at the sides the cheek 

 teeth, separated into premolars and molars. In describing the 

 teeth in any species a simple formula is adopted which shows 

 at a glance the number of each kind in one side of each jaw. 

 Taking our own normal dental equipment as an example, it 

 would be expressed in this fashion : 



i f , c \, pm |, m = 32 



the upper figures representing the number of each kind in the 

 upper jaw and the lower figures the teeth of the lower jaw, and 

 the total being reached by multiplying by two for the two sides 

 of the skull. Often in our rambles we may come across the 

 skull of some animal, and an examination of the teeth will help 

 us to the identity of its late owner. 



For the purposes of the present work it is unnecessary to 

 enter minutely into all the characters that distinguish the 

 Mammals from the other back-boned animals. One is really 

 sufficient the possession of glands (teats) in the skin of the 

 female which secrete milk for the nourishment of the new-born 

 young. There are, in addition, differences in the structure of 

 the skull and the articulation of the lower jaw. The skin is 



