268 COMPARATIVE ZOOLOGY OF CHORDATES 



forms of most groups of mammals have molars of this tri- 

 tubercular and tuberculo-sectorial type. 



When the cusps remain separate as in the pig, the tooth is 

 called bunodont. In other forms, the cusps may be joined 

 to one another by ridges running at right angles to the length 

 of the jaw, as in the tapir (lophodont condition). In others, 

 again, the cusps are splayed out to form crescents running in 

 the line of the length of the jaw, as in the camels (selenodont 

 condition). 



It is characteristic of mammalian molars to have divided 

 roots or " fangs." 



Normally, a tooth grows to a certain size (not very big), and 

 after that the pulp-cavity becomes almost closed at the base. 

 Such a tooth may have one or more " roots " or fangs, and when 

 these have formed, the tooth ceases growing. This is the 

 brachyodont type, the name being derived from the fact that 

 the teeth are comparatively short, and as a rule their possessors 

 do not make use of them for grinding hard materials. Where 

 the diet consists of resistant material which requires grinding, 

 and in other cases where the teeth are subjected to hard wear, 

 the pulp-cavity remains widely open at the base, and the teeth 

 are capable of continuous growth. These teeth are described 

 as being " rootless," or possessing persistent pulps, and from 

 the fact that they are usually long, this condition is known as 

 hypsodont. Examples of hypsodont teeth are to be found in 

 the premolars and molars of the horse, the incisors of the 

 rabbit, the incisors (tusks) of the elephant, and the canines of 

 the boar, to mention only a few. 



In the carnivores (cats and dogs) one tooth in each jaw on 

 each side becomes enlarged and modified for tearing flesh, 

 forming the so-called carnassial tooth. It is the last premolar 

 (4th) in the upper jaw and the first molar in the lower jaw. 

 Other carnivores (bears and seals) do not have the carnassial 

 tooth well developed. 



In addition to dentine and enamel, it is common for the 

 teeth of mammals to have a complete or partial covering of 

 bone which is called " cement." This may be restricted to the 

 roots of the teeth, as in man, or it may form a complete covering 



