RHINOCKHi.iS 



365 



terior cleft into two half cones, with their fiat sides next each 

 other ; the convex side of each half cone is indented by two 

 angular vertical notches, bounding a strong intermediate pro- 

 minence. When their summits begin to be abraded, each lobe, 

 or pair of demicones, presents 

 a double trefoil of enamel 

 on the grinding surface, as 

 shown in fig. 120 ; when 

 attrition has proceeded to 

 the base of the half cones, 

 then the grinding surfaces 

 of each lobe presents a 

 quadrilobate figure. The 

 crown of the last molar tooth 



Fig. 120. 

 Molar tooth, Hippopotamus. 



of the lower jaw is lengthened out by a fifth cone, developed 

 behind the two normal pairs of half cones, and smaller in all 

 its dimensions. 



The hippopotamus is first met with in pliocene strata. 

 The remains of H. major have hitherto been found only in 

 Europe ; they are common along the Mediterranean shore, 

 and do not occur north of the temperate zone. In Asia this 



Fig. 121. 



Lower jaw of Hippopotamus major (fresh-water Pliocene, Cromer, Norfolk). 



form of Pachyderm was represented, perhaps at an earlier 

 period, by the genus Hexaprotodon — essentially a hippopo- 

 tamus, with «ix incisor teeth, instead of four, in each jaw. 



