THE TEETH OF MAMMALS. 287 



following exceptions may be given. In bears the second 

 premolar is often absent, the first being very constant ; the 

 same thing is true of many bats ; in Dasyurus the third, or 

 hindmost (it being a Marsupial) is absent, the first two being 

 present. ( J ) 



A difficulty at times occurs in deciding whether a tooth 

 is to be regarded as a premolar, or as a milk tooth, as there 

 are many so-called permanent teeth which are lost early in 

 the lifetime of the animal. 



Professor Flower gives an instance of this in the hippo- 

 potamus : the first premolar appears with the milk teeth ; 

 it probably has no predecessor, and is shed in middle life. 

 But in allied forms the corresponding tooth remains in place 

 throughout the creature's life. 



The wart-hog is a conspicuous example of the early loss 

 of teeth which clearly belong to the permanent series (see 

 page 328), all the teeth (premolar and molar) in front of the 

 last great molar being cast off, and the dentition ultimately 

 reduced to 



.21 1 



i c m 



3 1 1 



That general correspondence, which is found to exist 

 between the dentitions of various animals, extends also to 

 the patterns of individual teeth, so that we are able to trace 

 out the various stages by which complexity of pattern has 

 been arrived at. 



In what might be termed a typical tooth we should have 

 a single central pulp cavity surrounded by a body of hard 

 dentine ; over the crown this is coated by enamel, whilst the 

 whole, crown and root, would be invested by a layer of 

 cement. 



The layer of coronal cement may be so thin as to be 



( ! ) This is ascertained by the examination of allied forms, in which the 

 hird premolar is found to be so small as to be rudimentary. 



