The North American Cervidce. 131 



hippopotami, hogs, camels, musk-deer, deer, giraffe, and the Eovidez, 

 in which stand the cattle, sheep, and antelopes. The order is divided 

 into Perrissodactyla and Artiodactyla, or odd and even toed ungu- 

 lates. The distinction between the living representatives of these 

 two groups is well marked, and the division is a convenient one ; but 

 it is probable that the future discovery of fossil forms of ungulates 

 will show that the artiodactyles and perrissodactyles grade into one 

 another, so that it will be impossible accurately to define the terms. 

 Although these groups are loosely called odd-toed and even-toed 

 ungulates, the fundamental difference between them does not consist 

 in the number of digits on the foot, but in the fact that in the perris- 

 sodactyles the development of the foot takes place in the line of the 

 middle toe, which is usually symmetrical in itself, whereas in the 

 artiodactyles the third and fourth digits share equally in this develop- 

 ment, and together form a symmetrical pair. 



To make this clear, it is perhaps necessary to go a little further, 

 and, by explaining the manner of progression of two of the more 

 familiar forms of the order, to give an idea of the construction of 

 these two types of ungulate foot. The horse is a perrissodactyle, 

 having a single visible toe, which is symmetrical in itself. He walks 

 upon the extremity of this toe, the hoof corresponding to the nail of 

 the third or middle finger in man. Comparing it with the human 

 hand and arm, it will be seen that the fore leg of the horse, from the 

 hoof to the fetlock joint, corresponds to the middle finger, and the 

 portion between the fetlock and what is lsually termed the knee, to 

 the middle metacarpal — the bone which lies between the knuckle of 

 the third finger and the wrist. Following the leg up toward the 

 body, it appears that the " knee " of the horse is really its wrist ; that 

 the elbow is high up close to the body, while the humerus — the bone 

 between the elbow and the shoulder — lies within the body, and out 

 of sight. In the hind leg a similar sequence will be found. The 

 animal walks on the toe corresponding to the third digit in the hu- 

 man foot, the hock is the ankle, while the true knee is close to the 

 body. The horse, therefore, supports his weight on the third digit 

 of each foot. His thumb and little finger, and the first and fifth digits 

 of the foot, have been wholly lost ; but in the fore and hind foot the 

 metacarpals and metatarsals of the second and fourth digits still per- 

 sist in the form of the slender, sharp-pointed bones, called by horse- 



