SUCCESSION OF MAMMALIAN TEETH. 451 



If we do not regard the molar teeth as representing a 

 fusion of several dentitions as advanced by Rose and Ku- 

 kenthal, to which dentition are they to be relegated ? 

 Leche, Beauregard and Owen have considered them as 

 belonging to the milk or second dentition, while I have 

 tried to show that they belong to the third or replacing set. 

 Owing to their late development and position in the jaw 

 these teeth are much specialised, and they rarely show the 

 slightest trace of any additional dentitions ; but such struc- 

 tures have been observed both preceding and succeeding 

 these teeth. Thus we find traces of three out of the four 

 dentitions present for one tooth, and the whole question 

 then turns on the point as to which set has been suppressed ; 

 if it is the first then the molars belong to the third replacing 

 set, as Lataste, Magitot and I have suggested ; if, on the 

 other hand, it is the fourth dentition which is wanting, then 

 Leche is right in supposing the molars to belong to the 

 milk or second set. 



The whole process of the first appearance of the molars 

 is so involved owing to the loss of connection between the 

 dental lamina and the gum in the region of their develop- 

 ment that it seems very doubtful if we shall find any very 

 distinct traces of additional preceding or succeeding teeth. 



Bateson has recently described (45) a large and varied 

 number of abnormalities affecting teeth. Many of these 

 are simple sports or teratological cases, of which it is very 

 difficult to offer any reasonable explanation ; but, on the 

 other hand, I see no reason to doubt, as he does, that the 

 majority of those cases where additional teeth are present 

 may be explained by the supposed increased development 

 of some pre-existing tooth germ in the fcetus, especially 

 as we now know of so many cases where these vestigial 

 germs do exist (9, 12, 15, 27, 32, 2>3* 34, 4-8), and as these 

 can in most instances be identified with definite teeth in the 

 generalised mammalian dentition, I maintain that we are 

 perfectly justified in trying to determine by comparison 

 the individual homologies of these additional teeth, and 

 further that they are often of great importance, as shown by 

 Thomas in Phascologale (10). Bateson's comparison be- 



