434 THE GENERAL PRINCIPLES OF ZOOLOGY 



close of the Miocene period we find the first appearance of 

 the modern genus Equus, with one digit on each foot. In this 

 series the first digit to disappear was the first, or innermost, then 

 the fifth, or outermost, and finally the second and fourth, leaving 

 the third, which forms the foot of the present horse. In the 

 Artiodactyla there is likewise a progressive reduction in the 

 lateral digits and a progressive development of tusks and of 

 horns. The antlers of the deer furnish an interesting example. 

 It is a familiar fact that in the true deer antlers are borne by 

 the male only, except in the case of the reindeer, where both 

 sexes bear horns ; and the antlers are yearly growths, falling 

 off and being replaced by new each year. The second year 

 after birth the male develops a pair of horns which are un- 

 branched ; these are replaced the next year by a pair with two 

 prongs or points. The following year horns develop with three 

 or more points, and this process goes on in many deer, so that an 

 old male will have very complicated antlers with a large number 

 of points. In the Miocene deer, however, we find genera with 

 antlers which never had more than two points; in many of the 

 Pliocene deer there were only three points ; in the Post-tertiary 

 the antlers become as complex as in many existing genera. 



Of the Proboscidea we find the elephant in the Miocene of 

 India and in the Pliocene of Europe, Asia, and North America; 

 the extinct mastodon, which differed from the elephant chiefly 

 in the form of its teeth, is found in the Miocene of Europe and 

 Asia and in the Pliocene of North America. One of the most 

 interesting of the Tertiary Proboscidea is the extinct genus 

 Dinotherium (Fig. 413) of the Miocene, whose huge skull 

 measured about a meter in length. The tusks were directed 

 downward and came from the lower jaw. The extinct genus 

 Dinoceras, found only in the Eocene of North America, exhibits 

 resemblances to both the Perissodactyla and the Proboscidea ; 

 it was about as large as an elephant and is peculiarly interesting 

 because of the size of the brain, which was smaller in proportion 

 to the rest of the bodv than the brain of any other mammal. 

 There are several other orders of extinct ungulates found in the 

 Tertiary, for the most part much more primitive in strueture 

 than existing genera, but not of sufficient interest to be con- 

 sidered here. 



