PERISSODACTYLA. 



325 



or presence of horns and the variable development of these 

 appendages when present. In the Miocene period we meet 

 with various species of Rhinoceroses which are entirely 



Fig. 627. — Penultimate molar of the 

 lower jaw of Rhinoceros megarhinus, 

 two-thirds of the natural size. Post- 

 Pliocene. 



Fig. 628. — Penultimate molar of the 

 lower jaw of Rhinoceros tichorhinus, 

 two-thirds of the natural size. Post- 

 Pliocene. 



devoid of horns, and which are grouped together under the 

 generic or sub-seneric title of Acerotherium. In these forms 

 (fig. 629) the nasal bones are very greatly reduced in size, 

 though they vary in their dimensions in different species. 



Fig. 629. — Skull of Acerotherium {Rhinoceros} incisivum, one-seventh of the natural size. 

 Miocene Tertiary. (After Kaup.) 



It may be added here, that in addition to the absence of 

 horns the species of Acerotherium are distinguished by the 

 low crowns of the praemolar and molar teeth, the general 

 presence of two or more lower incisors additional to the 

 single pair normally present, and the fact that the fore-feet 

 are typically or always four-toed (as in the living Tapirs). 



