TOOTH-GEIfESIS IN THE CANIDiE. 459 



From these considerations I am of opinion that the upper 

 permanent carnassial does really belong to the so-called Per- 

 manent dentition ; and that by the great development and ex- 

 tension backwards and upwards of the Metacone the anterior 

 molar has been pushed downwards so as to cut the gum with 

 the milk-teeth. 



These facts are of additional interest in that they may afford 

 an explanation of the somewhat similar condition found in 

 some of the Marsupials, in which the posterior premolar alone 

 replaces in position a tooth molariform in character. This 

 deciduous tooth has been regarded as the only one " comparable 

 to the milk-teeth of the Eutheria " (4) ; and the study of its 

 relationships led Flower and Thomas to regard the functional 

 set of the Marsupials as belonging to the permanent series. 



Since Kiikeuthal's discoveries in the Didelphyidae (9), this suc- 

 cession has been explained otlierwise. The dentition of the 

 Marsupials is now regarded as a persistent milk series, this tooth 

 alone having a permanent successor. I have given reasons above 

 for reverting to the former view ; and the fact that this tooth 

 alone is replaced is explained by the entire absence of any func- 

 tional milk-teeth. I regard this single deciduous tooth as in 

 reality the anterior molar pushed downwards by the overlapping 

 as it were of the premolars and molars at this point, due possibly 

 in the first instance to the gradual shortening of the jaw, and 

 assisted, as in the Carnivora, by the greater development of the 

 Metacone, which by its extension upwards and backwards would 

 tend to force the first molar through the gum. This may be 

 represented diagrammatically in this way : 



PREMOLARS (;7""^""(^''''1[;|^7r^ 



In cases where the first of the two factors (namely the 

 shortening of the jaw) is alone in operation, the result would be 

 simply to delay the appearance of the successional tooth, as is 

 seen in such forms as Potorous ; but when the additional factor 

 (namely the extension backwards of the Metacone) comes into 

 play also, the result would be that the deciduous tooth would be 

 shed at an earlier age — that is, it would be accelerated, as is the 

 case in Thylacinus and the Carnivora. 



The next poiut of interest in the microscopic examination of 



