364 



HISTORICAL GEOLOGY. 



class, viz., mammals, reptiles must decline in number 

 and size, and thus seek safety in a subordinate position. 



Subdivisions. The Cenozoic era and Mammalian age 

 is divided into two periods Tertiary and Quaternary. 

 In the Tertiary all the mammalian species are extinct, 

 but many invertebrate species are still living, and the 

 percentage of living species increases with time. In the 

 Quaternary, on the contrary, nearly all the invertebrate 

 species, e. g., mollusks, still survive, and some of the 

 mammalian species also survive. These facts are shown 

 in the diagram (Fig. 325). The space above the lines 

 of mollusks and mammals shows proportion of extinct, 



FIG. 325. Diagram showing the relative number of species living and extinct. 



and below the line, of living, species. The dawn of liv- 

 ing species of shells is with the beginning of the Ter- 

 tiary ; the dawn of living mammalian species is in the 

 Quaternary. Both curves show increasing Dercentage 

 of living species with time. 



SECTION I. TERTIARY PERIOD. 



As already stated, the dawn of living molluscan species 

 is in the earliest Tertiary, and thenceforward the per- 

 centage of living species steadily increases ; but no living 

 mammalian species are found there. 



Subdivisions. The subdivisions of the Tertiary period 

 into epochs are founded on this percentage of living mol- 

 luscan species. It is thus divided into three epochs 

 Eocene, Miocene, and Pliocene. If we find a stratum 



