254 MAMMALIAN DENTITION 



tional premilk premolar occurs in the Tapir. Although this 

 seems to be a uniquely surviving feature of the ancient mam- 

 malian dentition it is of scarcely sufficient importance in the 

 present connection for treatment at length. 



Earlier in this chapter the statement has been made that 

 usually it is the more advanced species of an order which have 

 lost their milk dentition. This is not invariably the case. The 

 Rat for example, by no means the advanced of Rodents, possesses 

 so far as we know at present not even a vestige of temporary 

 teeth. On the other hand the two very specialized Marsupials, 

 Myrmecobius, the Banded Anteater and Phascolomys, the Wom- 

 bat, both possess clear evidence of a definite milk dentition 

 whereas in the less aberrant Marsupials the milk dentition is 

 still more vestigial since only a single lacteal tooth persists. 



This brings us to the consideration of Marsupial teeth in 

 general. We have noted previously some of the many curious 

 specialized and aberrant features of this order and in conse- 

 quence it is not surprising to learn that only one tooth is re- 

 placed in these animals. During the discussion of the marsupial 

 dentition this has already been noted and reference should be 

 made to Chapter IV for details. That one tooth only should be 

 replaced is no new feature of the dentition since the Jurassic 

 Marsupial, Triconodon, shows the same phenomenon. A very 

 striking feature of the case is the fact that in the modern 

 Marsupials as in Triconodon it is the same tooth, the forerun- 

 ner of the last premolar, which is replaced in all species. 

 Whereas in most other tooth changes in Mammals the suc- 

 ceeding tooth develops rather behind the tooth which it re- 

 places, in this instance the successional tooth develops some- 

 what in front of its predecessor. 



Opinion at first favored the view that the functional dentition 

 of the Marsupials in general represented the permanent set. 

 Later views inclined toward homology with the milk dentition. 

 Recently however the former conclusion in modified form is 

 becoming more generally accepted and the evidence grows 

 clearer that the Marsupials of today are specialized in their 



