THE MAMMALS 



441 



vertebrates. The toenails, which formed the claws of the carnivores, 

 are enlarged to produce the hoofs. 



The Odd-toed Ungulates. The horses are the most abundant of 

 this group. They use only one toe on each foot, which is greatly en- 

 larged. This is the third, or middle toe, but rudimentary second and 

 fourth toes can be seen higher up the leg, although they do not touch 

 the ground. There were ancient prehistoric horses that used four toes 

 and more recently those that used three toes. A fairly complete history 



i '■ -_ ■ 



. .. m 



Photo by Winchester 



Fig. 28.17. The American tapir, found in tropical South America, is one of the odd- 

 toed ungulates and a close relative of the horses. This family group from habitat 

 scene in the Chicago Natural History Museum. 



of the development of horses has been worked out from fossils. This is 

 discussed more fully in Chapter 34. The donkeys and zebras are 

 very similar in their structure to our modern horse. 



The tapirs are also placed in the odd-toed group, but they have four 

 toes on the front feet and three on the hind feet. 



The rhinoceros is a well-known ungulate that has three toes on both 

 front and hind feet. They have one or two "horns" coming out of the 

 median dorsal region of the head. These are not true horns, but are 

 formed by a massing and fusion of hairs in this region. 



The Even-toed Ungulates. Most of these ungulates use two of their 

 toes, the second and third, which are incompletely fused together to form 



