96 PROF. DR. L. BOLK 



milk teeth and partially of teeth of the second dentition. The 

 best known case is that of the Erinaceus. According to the 

 very exact and ample researches of Leche, the functional set of 

 teeth of this Insectivora is composed of elements of both den- 

 titions. The above point of view therefore does not introduce 

 a wholly new principle in odontology. 



Before developing the different arguments upon which mj^ 

 hypothesis is based, I wish to summarize its esssentials by means 

 of some dental formulas. In these the symbols of the milk 

 dentition are printed in small letters, the permanent teeth in 

 capitals. 

 Dental formula of the Cebidae: 



ii. io. c. nil. nio. uis. 



Ii. Io. C. Pi. Po. P3. Ml. M2. M3. 



Dental formula of the Hapalidae: 



ii. i2. c. nil. mo. nis. 



Ii. Io. C. Pi. Po. P3. Ml. Mo. [iAl3] 



Dental fonnula of the Catarrhinae: 



ii. io. c. nil. nio. Mi. 



Ii. I2. C. Pi. P.. (P3.) M^. M3. [M,] 



In the last formula, relative also to man, the elements, whose 

 development is suppressed, are placed in brackets, as is also 

 done in the second formula. In my hypothesis two principles 

 are involved, which will be discussed separately, viz., the belong- 

 ing of our first permanent molar to the socalled milk dentition, 

 and, secondly, the disappearance of two elements in our perma- 

 nent set of teeth, the third premolar and the molar originally 

 hindmost. A further consequence of this hypothesis is that the 

 three molars of the catarrhine Primates are not homologous with 

 the three molars of the Platyrrhinae, our second molar corres- 

 ponding with the first of the latter. I question the nature of our 

 first permanent molar. Aly opinion is that this tooth is homolo- 

 gous with the third milk molar of the more primitive Primates. 

 Evidently the process by which this tooth became the first 

 molar of our permanent dentition is composed of two factors, to 



