162 Bulletin American Museum of Natural History. [XoX. XXVII, 



sternum, and reptilian shoulder girdle heavy, the limbs stout and provided 

 with powerfid muscles, the feet fully plantigrade, armed with heavy claws 

 and well adapted for both digging and swimming. No trace whatever 

 of the primary syndactyly which is retained to some extent even in highly 

 specialized Marsupials (p. 215) is observed in the pes of Monotremes, nor are 

 there any evidences of former marked divergence of the pollex and hallex. 

 The general conclusion is that the Monotremes began to separate from 

 the mammalian stock before the Marsupials acquired arboreal habits (p. 226). 



CHAPTER III. GENETIC RELATIONS OF THE MESOZOIC 



ORDERS: PROTODONTA, MULTITUBERCULATA, TRI- 



CONODONTA AND TRITUBERCULATA. 



Analysis. 



Page 



The Protodonta 163 



The Allotheria, or Multituberculata 164 



Historical development of the ordinal classification 164 



Affinities of Tritj/Iodon and Microlestes 166 



Origin of the Multituberculate molar 167 



Marsupial affinities of the order 169 



The orders Triconodonta and Tritubercvilata 170 



Historical development of the ordinal classification 170 



Special interest of these orders 172 



The Triconodonta 172 



Naturalness of the group 173 



Peralestes probably not a member of the order 174 



Relations of the Trituberculata to the Triconodonta 176 



Conclusions: the Triconodonta and their bearing on the origin of the 



mammalian Molar 177 



The Trituberculata 177 



Generalized characters retained among Trituberculates 177 



Importance of Amphitheriutn 179 



Origin of the Tritubercular Molar 181 



The Premolar Analogy Theory 182 



The Theoiy of Trituberculy correct in its basal postulate, the originally re- 

 versed relations of trigon and trigonid 184 



Mechanics of molar evolution in the Jurassic 185 



Mechanics of Tritubercular evolution in the Cretaceous and Basal Eocene 189 

 Functions and spatial relations of the parts of the upper and lower molars 



in a generalized tritubercvilar tlentition 190 



Conclusions in regard to Trituberculy 191 



Additional notes on the Trituberculata 194 



Summary of the genetic relations of the Trituberculata 195 



