HOOK-LIPPED OR BLACK RHINOCEROS 639 



layer, the crowns solid and rectangular in shape, the 

 valleys being filled with cement; first premolar shed 

 early, the cheek-teeth in the adult being six on each 

 side; base of first horn square in front 



Ceratotherium 



Black Rhinoceros 



Diceros 



Diceros Gray, 1821, London Med. Rapes., vol. XV, p. 306; type Rhinoc- 

 eros bicornis. 



The black rhinoceros differs so widely in many impor- 

 tant details of its structure from the other living forms that 

 it has been found necessary to separate it generically from 

 them. It has been the custom of naturalists to include all 

 the living forms in one genus, Rhinoceros, owing to the small 

 number of species. This has been done merely as a matter 

 of convenience, but we feel that the more logical course is 

 to classify the various forms on the merits of their struc- 

 tural differences or affinities so as to balance them with 

 other groups. Such a division into several genera will also 

 facilitate the tracing of their relationships with the numer- 

 ous fossil forms. In conformity with the white and the 

 Sumatran, it carries two dermal horns on the snout, the 

 rear one being situated directly behind the front one and 

 usually is much smaller and compressed laterally into a 

 blade-like knob. The genus Diceros, of which the black 

 rhinoceros is the type, differs almost as radically from the 

 other African genus, Ceratotherium, or white rhinoceros, as 

 from either the single-horned Indian or the two-horned 

 Sumatran rhinoceroses. It differs from the white by having 

 a short head which is deeply concave in profile on the top 

 owing to the great elevation of the occipital part. In these 

 two characters it resembles the Asiatic one-horned and two- 

 horned genera, but differs from them by its want of incisor 

 teeth and the distinctness of the post-tympanic process. 

 The genus is much less specialized than Ceratotherium; its 

 short skull and the simple structure of its short-crowned 

 teeth ally it much more closely to the remote ancestral 

 forms. The black rhinoceros in its dentition still shows 

 traces of the incisor teeth, and occasionally also of canines. 



