256 MAMMALIAN DENTITION ' 



cramped for space in the jaw but it is a very large and special- 

 ized tooth modified for sectorial purposes and as such may be 

 expected to cause some disturbance in the dentition as it erupts 

 in such animals as the Kangaroos. The question of relation- 

 ship in appearance and position of the last permanent premolar 

 to its predecessor and the special difficulties regarding tooth 

 replacement in the Marsupials must therefore be left sub judice. 

 One last feature of the temporary dentition must be men- 

 tioned. The milk teeth always lie to the outer or labial side of 

 their successors even in the case of the molars, although the 

 developing permanent premolars project between the divergent 

 roots of their predecessors. If the student examine the care- 

 fully cleaned jaws of any young Mammal (see for example the 

 infant Chimpanzee, Fig. 88), he will note foramina in the bone 

 upon the lingual or palatal aspect of the milk teeth, one imme- 

 diately behind or internal to each tooth, those related to the 

 incisors and canines being large, those near the milk molars 

 small and overhung by the corresponding tooth. These are the 

 openings in the bone through which passed the dental rudi- 

 ments of the permanent teeth and at the bottom of each hole 

 is the crypt containing the corresponding successional tooth. 

 Since these foramina are of necessity absolutely invariable 

 their presence indicates with certainty that the erupted tooth 

 adjacent to each belongs to the deciduous series. The absence 

 of the foramen gives equal assurance that the corresponding 

 tooth is of a member of the permanent set. 



