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. Repos., vol. XV, p. 306; type Rhinoc¬ 
eros hicornis. 
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 y 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, 
