450 SCIENCE PROGRESS. 



mammalia must have been the development of the mammary 

 glands, which by providing the young animal with food would 

 do away with the function of these teeth, which were so neces- 

 sary to the young reptile, who must feed himself as soon as 

 born. 



The phase in which the polyphyodont dentition was re- 

 duced to a single functional set (the milk dentition of the 

 higher mammalia) is probably represented by the earlier 

 mesozoic mammals, in which we have no indication of any 

 tooth change. 



As the mammalia became more advanced in their or- 

 ganisation this heterodont second dentition took a stronger 

 hold and developed earlier, then the third set, which had 

 perforce lain dormant, began to reappear. If we are to re- 

 gard the tooth succeeding the fourth (third) premolar of the 

 marsupials as a replacing tooth, which I have shown reasons 

 to doubt, it would appear to have been the first of the third 

 set to attain functional importance ; and in favour of this view 

 we may note that this tooth is one which, owing to the late 

 development of the molars, is primarily the most posterior 

 cheek tooth, and is of great importance to the young animal 

 and early becomes much worn and so needs replacing more 

 than the anterior teeth. Thus in the marsupials we find 

 the last vestiges of the first set of teeth (premilk dentition), 

 then we have a well-developed functional set, the second or 

 milk dentition, under which in the embryo we find traces of 

 a complete set of germs of the replacing or third dentition, 

 the one replacing tooth being doubtfully referred to this set. 



In the Insectivora we find the next stage in the develop- 

 ment of the third set of teeth, for in the hedgehog replacino- 

 teeth are developed to the following — 



ii, J2, C, pm3, pm4. 

 i2, pm4. 



the rest of the dentition consisting of persistent milk teeth. 



In the majority of mammals replacing teeth of the third 

 set are developed to all the incisors, the canines and the 

 three posterior premolars, only a few like Hyrax developing 

 a successor to the first premolar which in other cases is a 

 persistent milk tooth. 



