CHAPTER IV 



THE RELATION BETWEEN LIFE HABITS AND 

 DENTITION 



THE MARSUPIALS 



Distinctive features of mammalian subclasses — Origin of Mar- 

 supials — Characters of primitive and of marsupial skulls — 

 Plexodont Theory — Description of marsupial dentition — 

 Aberrant forms — Reasonableness of the persistence of the 

 tritubercular tuberculo-sectorial dentition — The marsupial 

 milk dentition — Reversion, divergence and parallelism in 

 evolution- — Species formation. 



In the last chapter the origin of modern Mammalia has been 

 considered and the Jurassic Trituberculates have been indi- 

 cated as exhibiting most clearly the features which probably 

 characterized the ancestor of the Mammals of today. Instead of 

 following the theme in Paleontology it is now proposed to dis- 

 cuss existing Mammals and to show by these whenever pos- 

 sible the adaptation of teeth in response to animal habits and 

 environment. 



We have used the terms marsupial and placental on several 

 occasions and it now becomes imperative that these terms 

 should be clearly defined. Existing Mammals are divided into 

 egg-laying (oviparous) and non-egg-laying (viviparous) sub- 

 classes. The modern representatives of the former, the Duck- 

 bill and the Spiny Anteater, we shall discuss in due course 

 and shall find that they are survivals of a very early primitive 

 type of Mammal. Such primitive Mammals, among which must 

 be referred the Protodonta already described, are known as 

 Prototheria. The higher subclass comprising the viviparous 



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