I'liOBLEMS OF HUMAX DENTITION 103 



think that the above-mentioned i)ecuHar phenomenon can be 

 exphiined very simply by my hypothesis. I have in my collec- 

 tion two cases of a replacing third premolar, but regarding only 

 one have I complete data. In this case the first upper molar 

 on the left side of a man of thirty-five was extracted because 

 of pain. Nearly ten months after this treatment the gap in the 

 dental arch was filled by a new tooth, possessing nearly the same 

 form as the second premolar. Tn this case a replacing tooth was 

 really substituted for the hrst molar. After the loss of his first 

 molar and the eruption of the third premolar this indi\'idual l)ore 

 on one side of the upper jaw a dentition closely resembling 

 that of the Hapalidae: three premolars and only two molars. 

 It is evident that the pain in the first molar was caused by the 

 third premolar which was unable to push out its ])redecessor in 

 the physiological ]:)ainless way. 



Summarizing, I belie\'e that the embiyology, morphology and 

 anomaly of the denture of catarrhine Primates furnish sufficient 

 grounds to pro\'e that my supposition regarding the nature of 

 our first permanent molar is a correct one. Originally it was a 

 deciduous tooth, like the first and second milk molar, and like 

 these it belongs to the first dentition. Still another pnx^f will 

 follow : 



It is clear from my hy])othesis that the three molars of the 

 platyrrhine monkeys are not homologous with the three molars 

 of man. Our second permanent molar is identical with the 

 first of the Platyri'hinae, and our third with the second of this 

 group of Primates. Consecjuently an element efjuivalent to the 

 third molar of the American monkeys is wanting in our denture. 

 This logical consequence of my hypothesis, throws new light 

 upon a well-known and often discussed anomaly in human den- 

 ture, viz.: the so-called fourth molar. 



A fourth molar is not a rare occurrence in the present foi'ni of 

 human denture. But a typical, well-developed fourth molar is 

 very seldom met with. Harrison has found such a one, "full- 

 sized and typical" in an Irish skull. Two years ago, I had 

 more than 30,000 human skulls at ]\\y disposal. Among these, 

 collected from inhabitants of Amstei'dam deceased during the 



