THE TERTIARY PERIOD 495 



monoids, Belemnites, and many curious Bivalves disappeared 

 almost entirely at the end of the Cretaceous, leaving only a few 

 stragglers here and there to persist into the older Tertiary. Even 

 more striking is the dwindling of the Reptiles ; the Ichthyosaurs, 

 Plesiosaurs, Pythonomorphs, Dinosaurs, and Pterosaurs, which had 

 given such a marked individuality to the Mesozoic fauna, have 

 become totally extinct, leaving only Lizards and Snakes, Turtles 

 and Crocodiles, to represent the class. But Cenozoic life is not 

 distinguished from Mesozoic merely by negative characters ; it has 

 its positive features as well. The plants and invertebrated ani- 

 mals nearly all belong to genera which are still living, and the 

 proportion of modern species steadily increases as we approximate 

 the present time. The Fishes, Amphibia, and Reptiles differ but 

 little from those of modern times, and the Birds take on the 

 diversity and relative importance which characterize them now. 

 Above all, the Mammals undergo a wonderful expansion and take 

 the place of the vanished reptiles, giving to Cenozoic time an alto- 

 gether different character from all that went before it. The great 

 geographical and climatic changes produced migrations of animals 

 and plants upon a great scale, from continent to continent and 

 from zone to zone, the result of which is the distribution of living 

 beings over the earth's surface as we find it to-day. 



There is some difference of usage regarding the subdivisions 

 of the Cenozoic group, though the difference is principally with 

 reference to the rank of those subdivisions. We shall follow the 

 usual American practice of dividing the group into two systems, 

 the Tertiary and Quaternary. 



THE TERTIARY PERIOD 



The names Tertiary and Quaternary are remnants of an old 

 geological nomenclature which has lost its significance, and was 

 proposed when the whole succession of strata was believed to be 

 divisible into three groups, called the Primary, Secondary, and Ter- 

 tiary, respectively. When it was learned that there were groups 

 and systems much older than the so-called Primary, the name Pa- 



