502 



CENOZOIC EHA— AGE OF MAMMALS. 



though not all, of the mammalian species are extinct, but nearly all 

 (ninety-five or more per cent) of the invertebrate species are living. 

 These facts are graphically represented in the following diagram, in 



CRETACEOUS 



T E R T I A J? Y | QUATERNARY \\ RECENT 



EOCENE | MIOCENE | PLIOCENE \\ feL AC \ CHAM TCP | RECENT 



Fig. 842.— Diagram illustrating the Kelative Duration of Lower and Higher Species. 



which the curved ascending lines are the lines of appearance of living 

 species, and of extinction of extinct species of Foraminifera, of mol- 

 luscous shells, and of mammals. In each case the lower shaded space 

 represents living species appearing in small numbers, and increasing 

 with the progress of time ; and the upper unshaded or less shaded 

 space, previous species gradually dying out and becoming extinct. It 

 is seen that living species of Foraminifera commenced in the Creta- 

 ceous, and very steadily increased in number; those of shells com- 

 menced in the earliest Tertiary, and increased somewhat more rapidly ; 

 while those of mammals commenced only in the Quaternary, and in- 

 creased correspondingly rapidly. Also the relative proportion of living 

 and extinct at any time is shown by comparing the amount of space 

 above and below the line at that time. Also the relative range in time 

 of low and high species, and the amount of overlapping of successive 

 faunae, are shown. 



The mammalian class probably culminated near the end of the Ter- 

 tiary or during the Quaternary period. 



Section 1. — Tertiary Period. 



Subdivisions. — We have already stated that the general differential 

 characteristic of this period, as compared with the next, is that all the 

 mammals, and most of the invertebrates, are extinct ; but of the latter 

 a percentage, small at first but increasing with the progress of time, 

 are still living. It is upon this percentage of living shells that Lyell 

 has based his division of the Tertiary period into three epochs — a 

 Lower, Middle, and Upper Tertiary, or Eocene, Miocene, and Pliocene. 



f Pliocene epoch, or Upper Tertiary = 50-90 per cent living shells. 

 Tertiary period. -{ Miocene epoch, or Middle Tertiary = 30 per cent living shells. 

 ^ Eocene epoch, or Lower Tertiary = 5-10 per cent living shells. 



