102 PROF, DR. L. BOLK 



cence of our jDlatyrrhiiie precursor — would surely be an error. 

 Among the cases of a supernumerary premolar we must look 

 for those in which the additional tooth clearly represents the 

 homologue of the third premolar in New World monkeys. The 

 best criterion in this matter is the topographical relation be- 

 tween the supernumerar}" premolar and the first permanent 

 molar. This relation, of course, must be the same as that be- 

 tween a milk tooth and its successor. As we know, the replacing 

 tooth is situated in the jaw on the inner side and a little behind 

 the tooth to be replaced. And only those cases in which an 

 equal relation between the first permanent molar and the addi- 

 tional premolar exists can be considered as conclusive proof. 

 A very fine specimen of such a variation is given in figure 5, 

 representing the lower jaw of a Macacus cynomolgus. The 

 denture is complete, the teeth are still very sound and all of 

 normal form. In the left half of the jaw a supernumerary 

 tooth developed, having nearly the same form as the second 

 premolar, and standing in the jaw on the inner side and a little 

 back of the first permanent molar. In all respects, morphologi- 

 cal and topographical, this case represents the typical conditions 

 existing between a milk molar and its successor, and furnishes 

 strong proof that in earlier times the first molar of the catarrhine 

 Primates was a deciduous tooth, replaced by an element of the 

 second dentition. There are in the odontological collection of 

 the Anatomical Museum of the University of Amsterdam several 

 cases of human dentition with an additional premolar, but in 

 none of these is the supernumerary tooth standing on the inner 

 side of the first molar, and therefore they do not illustrate my 

 hypothesis. Yet it should be kept in view that in some, per- 

 haps in the majority of these cases, the position of the super- 

 numerary tooth is a secondary one, the tooth being pushed a 

 little forward by the mechanical influences of the growing jaw. 



Still another phenomenon furnishes a strong proof of my 

 theory. It is well known that occasionally our first molar is 

 really replaced by a third premolar. In literature these cases are 

 sometimes described as examples of the so-called third dentition. 

 Now I absolutely deny the possibility of a thii-d dentition, and I 



