92 MAMMALIAN DENTITION 



a very modified dentition the most remarkable feature of which 

 is the increase in number of molars associated with degen- 

 eration of the antemolar teeth. These two peculiar diets are 

 invariably associated with a modification of tooth form, but to 

 avoid tedium consideration of this phenomenon will be post- 

 poned until placental Mammals exhibiting similar modifica- 

 tions come to be discussed (see pp. 161, 224, 271). 



It will now be clear that the reason for the dominant position 

 in phylogeny of the so-called trituberculate type of dentition 

 is its infinite range of adaptation to meet all possible dietary 

 conditions. The absence of this adaptability in the tooth pat- 

 terns which characterize Multituberculates, Protodonts and 

 Triconodonts is one of the factors which led to their ultimate 

 extinction. 



One peculiar modification of the tritubercular tuberculo- 

 sectorial dentition, namely that of Notoryctes the Marsupial 

 Mole, will be discussed in the next chapter with the very simi- 

 lar dentition of the Zalambdodont Insectivores. 



Although full discussion of the deciduous teeth cannot be 

 entered upon at this stage certain prominent facts concerning 

 the marsupial milk dentition should be recalled. In this Order 

 the milk dentition has become greatly reduced probably in 

 consequence of the peculiar conditions of birth and of early 

 life. In no Marsupial is there ever more than one tooth pos- 

 sessing a temporary predecessor. When replacement occurs it 

 invariably involves the last premolar. The deciduous tooth is 

 molariform and is therefore unlike its successor. The succes- 

 sional tooth moreover develops in front and not behind its pred- 

 ecessor as in the case of most replaced teeth in Placentals. 

 The condition is therefore difficult to interpret. Possibly the 

 deciduous tooth is really one of the permanent molar series 

 projected as it were over the top of the posterior premolar. 

 But whatever be the explanation the deciduous marsupial den- 

 tition illustrates very clearly the principle of reversed evolu- 

 tion. In the insectivorous-carnivorous series the deciduous 

 tooth becomes progressively reduced but in the purely herbiv- 



