100 



PALAEONTOLOGY. 



with their flat sides next each other ; the convex side of each 

 half cone is indented by two angular vertical notches, bound- 

 ing a strong intermediate prominence. When their summits 



begin to be abraded, each 

 lobe, or pair of clemicones, 

 presents a double trefoil of 

 enamel on the grinding sur- 

 face, as shewn in fig. 153; 

 when attrition has proceeded 

 to the base of the half cones, 

 then the grinding surfaces of 

 each lobe presents a quadrilo- 

 bate figure. The crown of the 

 last molar tooth 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. 153. 

 Molar tooth, Hippopotamus 



Fig. 154. 

 Lower jaw of Hippopotamus major (fresh-water Post-pliocene, Norfolk). 



form of Pachyderm was represented, perhaps at an earlier 

 period, by the genus Hexaprotodon — essentially a hippopo- 

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

 Suiclw. — The extinct Ghceropotqmus, Antliracotherium, Hyo- 



