728 PERISSODACTYLS xxx. 4- 



the ancestors of all Perissodactyla. There are four digits in the fore- 

 foot and three in the hind; the ulna and fibula are complete and dis- 

 tinct. The tooth row is complete (42 teeth in tapirs), but the pre- 

 molars are molariform, though of a simple square pattern, with low 

 crowns (Fig. 482). The nose has developed into a short trunk, with 

 characteristic shortening of the nasal bones. The stomach is like that 

 of horses and there is a large caecum. The placenta is diffuse. The 

 brain is relatively smaller than in horses. 



Fossil tapirs, very similar to modern forms, are found back to the 

 Oligocene, and somewhat more primitive Eocene related forms 



Fig. 483. Indian rhinoceros (Rhinoceros). 



(*Homogalax) might have been close to the ancestors of *Hyraco- 

 therium. We may say therefore with some confidence that the modern 

 tapirs show us with little change the condition of the perissodactyl 

 stock in late Eocene or Oligocene times, perhaps 40 million years ago. 

 Fossils almost exactly similar to the existing genus occur back to the 

 Miocene, say, 20 million years ago, and the tapirs were then of wide- 

 spread distribution. We may notice once again the important fact that, 

 given suitable environments, types persist with little change even when 

 their relatives, moving into other conditions, become greatly changed. 



5. Rhinoceroses 



The rhinoceroses (Fig. 483) are the only surviving large perissodac- 

 tyls ; they show the graviportal type of body form that was adopted by 

 many of the extinct forms (brontotheres, &c.) and also by many other 

 large eutherians. Dinocerates, elephants, hippopotamuses, and rhino- 

 ceroses all show the same type of skeleton. The vertebral column (Fig. 

 484) has long neural spines above the fore-legs, there are many ribs, 

 reaching back nearly to the pelvis. The whole column thus makes a 

 girder balanced on the fore-legs, and the head, being very heavy, 



