FOSSIL VERTEBRATES—MAMMALIA 1 65 



appears and permits the cement below to appear at the surface. 

 Now, as the cement is softer than the enamel, the wear of the 

 tooth will serve to keep this inner part lower than the enamel 

 walls, and there will always be sharp edges to serve for grinding 

 up the grass and other herbage that forms the food of the animal. 



Protohippus and Merychippus from the Pliocene of North 

 America are the next forms in the series. The two lateral digits 

 of the feet, two and four, are shortened and do not reach the 

 ground, though they still retain all the phalanges. The ulna and 

 the fibula are reduced to short splints, confined to the proximal 

 ends of the radius and the tibia. The molars are very horse- 

 like in the fact that all the tubercles have lost the enamel from 

 the upper surfaces, and there are great "lakes" of cement, bor- 

 dered by sharp enamel edges, for grinding the food. 



Hippariou is a form somewhat off the line of the horses, but 

 differs little from the Protohippus . It was very common in the 

 Miocene time both in North America and in Europe. It extended 

 up into the Pliocene in Africa and Asia. 



Plioliippus and Equus are from the Pliocene deposits of most of 

 the world, and from there upward the deposits of the Pleisto- 

 cene and the recent times show their presence, except that Plio- 

 liippus is absent in the Recent. It is of interest to note that 

 though the continent of North America was undoubtedly the 

 original home of the horses and the theater of their greatest 

 development, that at the end of the Pleistocene time they seem 

 to have disappeared from the continent, and were only reintro- 

 duced by man when the Spaniards brought them over to aid in 

 the conquest of Mexico. 



The last two genera are distinguished by the reduction of the 

 two lateral digits to two splint bones on the proximal end of the 

 cannon bone and the nearly complete loss of the ulna and the 

 fibula ; the latter remains only as the olecranon process. 



The phylogeny of the horse series has thus been arranged by 

 Mr. Farr, a late writer on the subject. 

 Pliocene to Recent Equus 



Hippidium 



