TERTIARY AGE. 



507 



(3.) Pliocene. — Remains of the Pliocene Mammals that lived 

 about the Great Pliocene Lakes west of the Mississippi are found 

 on the Niobrara and the Loup Fork, the first from which localities 

 were gathered by Hayden, and described by Leidy. They include 

 true Camels ( Procamelus) ; Rhinoceroses, one as large as the Indian 

 Species ; several species of the Horse family of the genera Protohip- 

 pus, and Pliokippus (Plate X.) ; species of Deer, and Musk-Deer ; an 

 Elephant, larger than the modern, called Elephas Americanus, which 

 continued on into the Quarternary ; a Tiger, larger than the Bengal 



Fisr. 925. 



Oreodon gracilis. 



Tiger, besides other Carnivores. There was also the first known spe- 

 cies for America, of the genus Mastodon {Mastodon mirificiis Leidy). 



The succession of forms under the Horse type is illustrated on Plate 

 X. (from Marsh). This plate contains, in a series of seven columns, 

 figures of (1) the fore-foot ; (2) the hind-foot ; (3) the lower joint of 

 the fore-leg (made up of the radius and ulna) ; (4) the same of the 

 hind leg (tibia and fibula) ; and (5, 6, 7), others showing the length, 

 and the convolutions, of the teeth. Columns 1 and 2 illustrate the fact 

 of the diminishing number of toes, with the progress of the Tertiary, 

 until at last, in the modern kind, only No. 3 (or the middle toe) re- 

 mains, with rudiments either side of No. 2 and No. 4 (the splint bones), 

 while No. 3 becomes increasingly larger and longer. Columns 3 to 7 

 illustrate the changes in the limbs and teeth. 



Another important fact in the history of life as deduced by Lartet 

 from the early European Mammals, and by Marsh from the American, 

 is the increasing size of the brain with the progress of the Tertiary. 



