TEETH 319 



the method of taking in and biting or masticating various kinds 

 of food. The frequent presence of vestigial, functionless teeth 

 shows that a diminution in number lias taken place in the course 

 of time, 1 and such reduction is more marked in the lower than in 

 the upper jaw. An increase in number, on the other hand, such 

 as is met with in Toothed Whales, is due to a secondary differentia- 

 tion, during ontogeny, of primarily multicuspidate teeth, and the 

 homodont dentition and conical form of the teeth in these 

 Mammals is therefore a specialised and not a primitive condition. 



As already mentioned, the succession is nearly always limited 

 to two functional sets, the so-called deciduous or milk-teeth, and 

 the successional or permanent teeth, and in many cases even one of 

 these may be vestigial. 2 The milk-teeth represent a historically 

 older generation than the successional teeth : they, however, show 

 numerous adaptations and modifications, and may even be retained 

 in the adult permanently (e.g. Marsupials) or for a considerable 

 time (e.g. Chrysochloris, certain Centetida?). Traces of a still earlier 

 set occasionally occur most frequently and distinctly in the more 

 primitive orders (e.g. Marsupials, Insectivores, Rodents) : 'this may 

 be spoken of as a prelacteal dentition. As occasionally also certain 

 teeth appear which replace the corresponding "permanent" teeth, 

 indications of at least four sets can be recognised in Mammals. 3 



In each of the two functional sets, incisors, canines, and cheek- 

 teeth or grinders, can as a general rule be distinguished. The teeth 

 which replace the milk-grinders are distinguished as premolars ; 

 the molars are situated further back in the jaw and have no pre- 

 decessors. 4 The latter may therefore be considered as belonging 

 primarily to the milk-series ; or more probably they arise from 

 germs, which, owing to an abbreviation of development, really 

 represent more than one set. 



All the teeth are embedded in well-developed alveoli of the 



1 The last molar of Man, or so-called " wisdom-tooth," for instance, seems to 

 be gradually disappearing ; it appears last, is usually lost first, and often does not 

 reach the grinding surface. 



2 Thus in the Hedgehog, which presents an intermediate stage between the 

 diphyodont and monophyodont conditions, and also in the Mole and certain 

 Rodents, the milk-dentition is partially reduced. In Scalops and Condylura also 

 most of or all the milk-teeth become absorbed without cutting the gum, and a 

 similar condition is seen in the Pinnipedia, in which, however, they may not be shed 

 until shortly after birth. Thus the succession may take place in the fcetus, 

 so that the milk-teeth are functionless. This is also the case in certain Bats, 

 while in others the milk-teeth may be retained in order to hold the young fast to 

 the teats, and they thus form an interesting example of the retention of organs 

 owing to a change of function. 



' Thus the polyphyodontism of the lower Vertebrates is replaced in certain 

 Reptiles by an oligophyodont condition, and this again, by the diphyodontism 

 of Mammals, which, by higher differentiation of the individual teeth, may lead to 

 a monophyodont condition. 



4 It must, however, be remembered that in some cases the so-called premolars 

 have no predecessors, and therefore apparently belong to the milk-dentition. 

 Moreover, in Toothed Whales the persistent "milk-teeth" are said to include 

 representatives of an earlier and a later generation, and the same applies to 

 the Manatee. 



