326 



OEDERS OF MAMMALIA. 



There is thus good reason for separating Acerotherium gener- 

 ically from lihinoceros. 



The typical Ehinoceroses all possess immensely-developed 

 nasal bones, to serve for the support of a horn or liorns, this 

 purpose being occasionally further subserved by the partial 

 or complete ossification of the septum between the nostrils. 

 We may therefore divide the species of Ehinoceros, in 

 accordance with this character, into three groups : — 



1. Those in which the nasal septum remains unossified 

 (EJnnoccros iiugarhinus). 



2. Those in which the nasal bones are strengthened by 

 the partial ossification of the nasal septum, giving rise to an 

 incomplete bony partition or " cloison " between the nostrils 

 {R. Etruscus and B. hemitce-clius). 



3. Those in which the nasal septum is completely ossified, 

 and the nostrils are thus separated by an unbroken partition 

 or " cloison." This condition is coincident with the great 

 development of the horns, and is especially well seen in the 

 Woolly Rhinoceros {B. tichorhimis, fig. 630). 



Fig. 630.— Skull of the Ticliorhine Rliinoperos, the lioni.s being wanting. One-tentli of tlie 

 natural size. Post-Pliocene deposits of Europe and Asia. 



The horns of the Ehinoceroses are, as has been mentioned, 

 epidermic structures, and are totally distinct in their nature 

 from the horns of the Hollow-horned Euminants or of the 

 Deer. In the so-called " unicorn " Ehinoceroses there exists 

 but a single horn placed upon the nasal bones in the median 

 line of the head. In the " bicorn " Ehinoceroses, on the 



