272 THE DENTAL SYSTEM OF THE PRIMATES [SECT. A 



in question is indicated as the probable site of the missing tooth 

 which should provide the transition stage (at present lacking) 

 between the definitely incisor and canine types of tooth-crown. 



Passing from the incisor teeth to those of the premolar series, 

 the results of protracted investigations are not very much more 

 definite. In this instance, two teeth of the original four (cf. 

 formula (a) or (b)) have to be accounted for. Agreement appears 

 to be fairly general in regard to the first of those which have 

 disappeared. For there is a consensus of opinion that it is the 

 premolar immediately behind the canine tooth, viz. piUj. Thus 

 three premolars are left, as in the Lemurs, Tarsii, and Platyrrhinae. 

 Then another premolar tooth disappears, the resulting dentition 

 being characteristic of the Katarrhinae. The answer to the en- 

 quiry, which of the three Platyrrhine premolars is absent from the 

 Katarrhine (including the human) dentition, is not easy to give, 

 because opinions are divided between pnij and pm 3 of the Platyr- 

 rhine series. 



(i) On behalf of the former, it is claimed that inasmuch 

 as a premolar immediately behind the canine tooth has already 

 been lost (reducing the number from the four originally present 

 to three), the process of reduction will affect the rest in sequence, 

 viz. pnij of the Platyrrhine dentition. Professor Adloff indeed 

 claims that of the four original premolars, three may be seen in 

 Rodents to undergo degeneration from before backwards in suc- 

 cession 1 . The problem might seem to be resolved therefore in 

 favour of regarding the reduction as affecting the premolars in 

 regular succession from before backwards. 



(ii) On the other hand, those who incline to regard the 

 third premolar of the Platyrrhinae as the excluded element, have 

 to base their opinion largely on the instances of anomalous super- 

 numerary additions to the Katarrhine series. These supernumerary 

 teeth are sometimes small (v. supra, p. 258), and it is at least 

 doubtful whether they can be justly claimed as " reversions." 



But a better argument (in favour of the disappearance of the 

 hindmost premolar) has been advanced by Professor Bolk -2 , whose 

 view is so ingenious and attractive as to demand special notice. 



1 Cf. Adloff, Z.Jiir M. und A. Bel xi. Heft 2, p. 379. 



- Koninkligke Akademic van Wetenschappen, Amsterdam, April 27, 1906. 



