CHAPTKE VI. 

 CHRONOLOGICAL OR GEOLOGICAL SUCCESSION OK MOLAR TYPES. 



A REVIEW of the dental types as observed in the successive geological periods 

 presents us with the ideas of successive stages belonging to these periods, 

 of the enormously long era of time required for the transformation of the 

 teeth, and of the very significant fact that only two fundamentally distinct 

 dental types have thus far been discovered ; first, the (i) tubercular, and 

 its derivative, the nmltitubercular, the origin of which is still a matter of 

 hypothesis, and second the great (n) haplodont, protodont, triconodont, 

 trityhercular sequence, among the manifold offsprings of which we find 

 again arising secondarily the haplodont, triconodont, tubercular and 

 mulci tubercular. 



1. EEPTILIAX ANCESTORS OF MAMMALS IN THE TRIAS. 



It is now generally believed that the Theriodontia, an order of reptiles 

 found chiefly in South Africa, are, as the name indicates, not far from the 



i. a. 



FIG. 44. Lateral view of the Skull of (.'/inoijnuthiis rrnti >-onotiis, showing the five simple pre- 

 rnolars and triconodont (protodont) molars with grooved fangs. After Seeley. 



actual ancestors of the mammalia. It is true they exhibit a very large 

 number of reptilian characters : but mingled with these are features of the 



