xxx. i-2 FEEDING 723 



The feeding meehanism, though it has been the basis of the success 

 of the perissodactyls, is in several ways less specialized than that of 

 artiodactyls. The incisors are preserved and used for cropping, having 

 a pit on the free surface, so that sharp edges are presented as the tooth 

 wears away (incidentally allowing the age of the animal to be deter- 

 mined). The canine may be reduced or absent, and there is often a 

 diastema. The molars of many of the earlier types remained bunodont 

 and low-crowned, but those of the later rhinoceroses and horses 

 developed an elaborate grinding surface. This was achieved by forma- 

 tion of a longitudinal ectoloph along the outer edge of the upper molar 

 and parallel transverse ridges, the protoloph and metaloph (Fig. 465). 

 Even with the secondary complications of the latest forms these teeth 

 remain recognizably of quadritubercular pattern, and the same might 

 be said of the lower molars. The premolars come to resemble the 

 molars, giving a long battery of teeth. The gut shows less specializa- 

 tion than in artiodactyls, the stomach being undivided, but in horses 

 there is a large cardiac area of non-glandular, oesophageal structure. 

 Digestion of cellulose takes place in the caecum and large intestine, 

 which may be greatly developed. The brain of Perissodactyla is 

 relatively small, especially in the earlier forms, such as the tapirs. It 

 is of macrosmatic type and the sensory portion of the nose is highly 

 developed. 



The reproductive system also shows primitive features. The uterus 

 is bicornuate and the placenta of the diffuse epitheliochorial type, with 

 a large allantoic sac. The yolk sac grows to a large size and forms a 

 yolk-sac placenta during the early part of the development. 



2. Classification 



Superorder 4. Mesaxonia 

 Order Perissodactyla 

 Suborder 1. Hippomorpha 

 *Family 1. Palaeotheriidae. Eocene-Oligocene. Eurasia 



*Palaeotherium 

 Family 2. Equidae. Horses. Eocene-Recent 



*Hyrac other ium (= *Eohippus), Lower Eocene, Holarctic; 

 *Orohippns, Eocene, N. America; *Epihippus, Upper 

 Eocene, N. America; *Mesohippus, Oligocene, N. America; 

 *Miohippm, Oligocene-Miocene, N. America; *Anchi- 

 therium, Miocene, Holarctic; *Parahippns, Miocene, N. 

 America; *Merychippus, Miocene, N. America; *Hipparion, 

 Pliocene, Holarctic, Africa; *Pliohippus, Pliocene, N. 



