200 ANIMAL LIFE PAST AND PRESENT. 



surface of a tooth which has been somewhat worn, it will 

 be seen that this enamel forms only a comparatively 

 thin outer layer, and that the inner part of the tooth 

 consists of a yellowish substance, which is much softer, 

 and is termed the ivory or dentine. Further, when a 

 human tooth is removed from its socket, it may be 

 observed that the root, or embedded portion, is overlain 

 by a very rough-looking bony substance, known as 

 cement, which directly overlies the dentine. In the 

 human teeth this cement is, indeed, confined to the 

 root, but in some other Mammals it also extends on to 

 the crown, where it overlies the enamel. We thus see 

 that the crown of a tooth may consist of three distinct 

 elements, of which the dentine forms the central part, 

 this being overlain by the enamel, and this again by 

 the cement. 



In many Fishes teeth are distributed irregularly, or 

 at other times in a well-defined pattern, over the whole 

 of the surface of the mouth; and we also find that 

 certain Eeptiles have the roof of the mouth more or 

 less covered with teeth. In the majority of the higher 

 Keptiles such as Lizards and Crocodiles the teeth 

 are, however, confined to the margins of the upper and 

 lower jaws, where they form a single row, with the 

 base of each tooth either embedded by an undivided 

 root in a groove or socket of the jaw-bone, or absolutely 

 welded to the bone itself. A common type of Reptilian 

 tooth has the crown forming a more or less compressed 

 cone (Fig. 61), with a pair of sharp-cutting edges, which 

 may or may not be furnished with saw-like notches. 



