ORDER UNGULATA. 



1367 



worn, the crown of each molar would carry two isolated fossettes 

 surrounded by enamel (fig. 1244). The worn crown-surface is trans- 

 versely ridged ; and there is a process projecting from the hinder 

 ridge into the middle valley termed the crochet, which is absent in 

 some species. The hinder premolars are as complex as the true 

 molars ; and the crowns of the cheek-teeth, though varying in 

 height, are never very tall, and their valleys are always open. In 

 the lower cheek-teeth the ridges form complete crescents, with their 

 concavity directed inwardly (fig. 1240). 

 The lower canines are always proclivous. 

 The skeleton and skull are very massive, 

 this feature being most marked in the more 

 specialised species. This genus may be 

 divided into several groups, of which the 

 Aceratherine is the most generalised. In 

 this group there is usually no horn, and 



, , -, /r N Fig. 1240. — 1 he third left lower 



the nasal bones (tig. I241) are COnse- true molar of Rhinoceros megar- 



quently small; cutting-teeth are always S^iSf° Cene ' Tw<>thirds 

 present, although there is some variation 



in their number, which may be expressed by the formula Z 



(0-2) 



C. 



In R. incisivus (which is the type of the so-called Acera- 



theriiwi) there are four digits in the manus ; but in many of the 

 North American forms (which on this account are separated by 



1241. — Cranium of Rhinoceros incisivus ; from the Lower Pliocene of Germany. 

 One-seventh natural size. (After Kaup.) 



Professor Cope under the name Aphelops, fig. 1242) the number 

 of digits was reduced to three ; and these forms were thus similar 

 to some female examples of the existing R. sondaicus, in which the 

 horn is absent. In Europe this group ranges from the Lower Mio- 

 vol. 11. 2 H 



