LIFE HABITS AND DENTITION 



69 



Only one successional 

 tooth at most (last 

 premolar). 



Different teeth modified 

 for sectorial purposes. 



Mandible: Condyle sessile. 



Sigmoid notch shallow. 

 Angle inflected. 



Skull: Brain case very narrow. 



Zygomata p r o j ecting 



greatly. 

 Sutures remain open 



late. 

 Nasals broader behind. 

 Malar reaches glenoid 



fossa. 

 Palate fenestrated. 



Milk dentition usually well repre- 

 sented. 



P4 



modified as sectorial or carnas- 



Ml 



sial teeth. 



Condyle with neck. 

 Sigmoid notch deep. 

 Angle rarely or but slightly in- 

 flected. 



Brain case broader. 

 Zygomata projecting less. 



Sutures close earlier. 



Nasals broader anteriorly. 



Malar does not reach glenoid fossa. 



Palate not fenestrated. 



Before passing to the actual description of marsupial denti- 

 tions, it is opportune to refer to an interpretation of tooth 

 evolution which has not yet been mentioned, namely that de- 

 veloped by the late Dr. Ameghino and known as the Plexo- 

 dont or Progressive Simplification Theory. The oldest 

 Mammal known which approximates the modern type, Pro- 

 teodidelphys, from the Cretaceous of South America presents, 

 as would be expected from its Trituberculate ancestry, a tuber- 

 culo-sectorial type of molar. We have already noted that this 

 type of molar when first encountered is fairly fully developed 

 and nothing is known of how it evolved. 



According to Ameghino it is probable that the molar type 

 developed rapidly and to do this must have resulted from the 

 fusion of a number of teeth each presenting a simple conical 

 crown and a single root. Furthermore according to this 

 theory, as in the Polybuny Theory previously discussed, all 

 simpler forms of teeth found among Mammals today have re- 

 sulted from progressive simplification. A little reflection upon 

 the features of the reptilian and early mammalian teeth pre- 

 sented in the foregoing pages will convince the reader that 

 the basal assumptions of this theory are not warranted. It 

 may therefore be dismissed as untenable. 



