IV. VERTEBRATA: MAMMALIA, PROBOSCIDIA. 643 



the prong horn, which sheds its horns, and Hoploceras montanus* the 

 Kocky mountain sheep. 



Section III. TYLOPODA, stomach without manyplies, no frontal 

 horns, diffuse placenta. Camelus, the camels of the Old World; C. drome- 

 darius, one hump; C. bactrianus, two humps. Auchenia lama, A. alpaca 

 of South America. 



Paleontology of the Ungulata. 



Extensive paleontological material, especially from the tertiary rocks 

 of our western states, has cleared up many lines of ungulate descent 

 and has rendered it probable that the CONDYLARTHRA of the eocene, 

 with five-toed plantigrade feet, well-developed ulna and fibula, and an 

 omnivorous dentition, formed the stock from which descended the artio- 

 dactyles and perissodactyles, and possibly carnivores and primates as well, 

 the ungulate line extending through the Amblypoda. From one group of 

 these (the PHENACODONTID^E) the lines of rhinoceros and tapir have come, 

 and in an almost complete series we know the ancestry of the horse. 

 Hyracotherium (Eohippus) and Orohippus of the eocene had the fore feet 

 four-toed (fig. 666. 1) ; Pafaotherium and Mesohippus (2} of the lower 



FIG. 666. Evolution of fore foot of horse. (From Wiedersheim.) 1, Orohippus 

 (eocene): 2, Mesohippus (lower miocene); 3, Miohippus (miocene); It, Protohippun 

 (upper pliocene); 5, Pliohippus (pleistocene); 0, Equus. 



miocene and Miohippus of the later miocene were three-toed, while Mery- 

 hippus and Hipparion (Pliohippus, 4) of the pliocene were near the horse 

 in tooth structure. The single-toed horses appeared in the pleistocene 

 with Pliohippus (5) and then Equus itself (6). It is a peculiar fact that 

 the horse entirely died out in America, although the chief part of its his- 

 tory was enacted here. 



The AMBLYPODA, mentioned above, were semi-plantigrade penta- 

 dactyle forms, appearing in the lowest eocene, and reaching, in Uinta- 

 therium (Dinocerus) an elephantine size. The TOXODONTIA of the 

 South American tertiaries combined perissodactyle, rodent, hyracoid, and 

 proboscidian features, while the TILLODONTIA of the eocene recall both 

 carnivores and rodents. 



Order VI. Proboscidia. 



The elephants and their allies, with their hoofs and herbivorous 

 dentition, are closely related to the ungulates. They are charac- 

 terized by their thick skin ( pachyderm '), the large, massive, 

 five-toed legs, and especially by the nose drawn out into a 



