I 366 CLASS MAMMALIA. 



are Rhinocerotic in structure, but are extremely narrow in a trans- 

 verse direction, and the ridges of the lower cheek-teeth are imper- 

 fectly crescentoid. Apparently nearly related to the preceding is 

 Homalodontotherium, from Tertiary strata of unknown age in Pata- 

 gonia ; the dental formula is the typical one, and there is no diastema, 

 but the skeleton is unknown. We now come to the consideration 

 of those animals which we may term true Rhinoceroses — a group in 

 which very diverse views as to the limits of generic terms are pre- 

 valent among zoologists and paleontologists. By some writers the 

 five existing species are referred to at least three distinct genera, 

 and if this view be adopted, it will be necessary to make a large 

 number of genera for the extinct forms ; the English school, how- 

 ever, now generally include all the living species in one genus, and 

 from this point of view there seems no good reasons for generically 

 separating any of the extinct species, which form a series so inti- 

 mately connected that it would be very difficult to define all the 

 genera into which they are divided by the American school. 1 Using, 

 then, the term Rhinoceros in its widest sense, the variations in the 



Iq 2) n 



number of teeth may be expressed by the formula I. ) -, C. 



(O-I)' (O-I), 



Pm. -, M. - : the absence of upper canines is a distinctive feature ; 



4 3 



the upper true molars (fig. 1239) have their crowns relatively wide, 

 their transverse ridges well developed, the hinder lobe of the last 



Fig. 1239. — The left upper true molars of Rhinoceros megarkinus ; from the Pleistocene 

 of England. One-half natural size. 



tooth partially aborted, and frequently a more or less distinct but- 

 tress at their antero-external angle. The teeth represented in fig. 

 1239 are the most generalised type; and it is evident that, when more 



1 In this and other instances the number of generic divisions which we may be 

 disposed to adopt is solely a matter of convenience. From the writer's point of 

 view the multiplication of generic terms, which as our knowledge advances must 

 become less and less susceptible of exact definition, tends to drown the science in 

 a sea of names, which form a great burden to the memory, and thus tend to de- 

 stroy the very object of classification. 



