CHAPTER VII. 



THE ORIGINAL OR PRIMITIVE STRUCTURE OF THE MOLAR TEETH 

 IN THE DIFFERENT ORDERS OF MAMMALS. 



MAJOR CLASSIFICATION OF THE MAMMALIA. 



THE Table of Classification on pages 11-16 naturally introduces the brief 

 review of the various orders of mammals with reference to the structure 

 of their grinding teeth which forms the subject of this chapter. 



The reptilian superorder Therapsida, including the order Cynodontia, 

 is the only one of the numerous orders of reptiles in which striking 

 resemblances to the mammals are observed in the skeleton as well as 

 in the teeth (see p. 91). These animals appear to stand nearest the 

 reptilian stock from which the mammals sprang, and to most nearly 

 fulfil the hypothetical groups, Hypotlieria, Promammalia, Sauromammalia 

 proposed respectively by Huxley, Haeckel, and Baur. 



The two grand divisions of the Mammalia proper, the (1) Prototheria 

 or egg-laying mammals, and (2) Eutheria or viviparous mammals, 

 include fourteen extinct and eighteen living orders, or thirty-two orders 

 altogether. It will be shown that in thirty of these orders tritubercular 

 evolution can either be directly demonstrated or is rendered in a high 

 degree probable by analogy. In three orders only the Monotrernata, 

 Allotheria (Multituberculata), and Cetacea, does there remain considerable 

 doubt as to the mode of dental evolution. 



In the two great divisions of the Eutheria, namely, the Didelphia 

 (Marsupialia) and Monodelphia (Placentalia), tritubercular evolution, 

 although proceeding independently, has arrived at closely similar adaptive 

 types, thus affording remarkable instances of parallel evolution. 



The Placentalia are conveniently divided into four great groups, 

 which it is true have a descriptive rather than strictly taxonomic value, 

 namely : 



1. The Unguiculata, including all the clawed animals of primarily 

 insectivorous (Insectivora, Cheiroptera), or carnivorous habits (Carnivora), 

 and secondarily of prevailing herbivorous habits (Rodentia, Edentata). 



