506 the tertiary period 



which are believed to represent the lower Bridger ( Wind River 

 substage) in time. These shales have yielded no mammalian 

 remains, but great numbers of plants, insects, and fishes occur in 

 them. 



The Bridger mammals represent a steady advance upon those 

 of the Wasatch, but there is no such complete change as followed 

 the Puerco. The Perissodactyls may be said to culminate in the 

 Bridger ; for though they afterwards reached much higher stages 

 of development, they never again had the same relative impor- 

 tance. Horses, Tapirs, Rhinoceroses, and Titanotheres (Palceo- 

 syops) are extraordinarily abundant. Coryphodon has vanished, 

 but the wonderful elephantine, six-horned Uintatherium takes its 

 place in North America, though not in Europe. Artiodactyls, 

 Creodonts, Rodents, Tillodonts, and Lemurs were more diver- 

 sified than ever, and Bats are found here for the first time. 



In the Uinta the mammals continue to advance along the same 

 lines, gaining in size and advancing in structure. Noteworthy 

 is the appearance of the true Carnivores (Cynodictis) , and the 

 great development of the Artiodactyls, which in Europe display 

 wonderfully manifold types of structure. In America Oreodonts 

 \Protoreodoii) and primitive Camels {Leptotragulus) are the com- 

 monest forms. Creodonts, Lemurs, and Tillodonts decline. 



In the Upper Eocene seas great whale-like forms (Zeuglodon) 

 of extraordinary appearance and structure had grown abundant. 



Volcanic eruptions continued in the Rocky Mountain region 

 during the Eocene. The Yellowstone Park was a centre of great 

 activity, with numerous cones ejecting acid lavas and tuffs. 



The climate of the Eocene was very much the same as in the 

 Cretaceous, mild and equable all over the Northern Hemisphere, 

 at least, as is shown by the character of the vegetation and the 

 marine shells, though probably there was already a beginning of 

 climatic zones. 



The Oligocene Epoch 



American. — This term (derived from the Greek oligos, little, 

 and kainos, recent) is seldom used in this country, but it is impor- 



