ANCIENT AND MODERN HIPPOPOTAMUSES 265 



to a simple type not unlike that now found in the pigs. 

 The molar-teeth of the hippopotamus, though of the same 

 general plan as those of the latter, have, however, their 

 four main columns, when partially worn, with a distinctly 

 trefoil-shaped pattern ; and it is quite evident that such 

 a tooth could never have given rise to the crescent-teeth 

 of the ruminants. The hippopotamus molar is, indeed, 

 quite peculiar, and its structure is so well marked and 

 characteristic that any person who has once seen a 

 specimen could immediately identify any example that 

 might come under his notice. 



As regards their front teeth, it may be mentioned that 

 hippopotamuses have an enormous pair of curved tusks or 

 canines in each jaw. In the common species, between 

 these huge tusks are two pairs of incisors, those of the 

 upper jaw being of nearly equal size, whereas in the lower 

 jaw, where these teeth are cylindrical and project nearly 

 horizontally forwards, the central ones are very much 

 larger than the lateral pair. If, however, we examine 

 the lower jaw of the pigmy Liberian species, we shall 

 find that normally there is but a single pair of incisors 

 between the tusks, which would lead to the conclusion 

 that this animal is a more specialised type than its larger 

 relative. The truth of this inference is curiously confirmed 

 by the circumstance that individuals of the Liberian hippo- 

 potamus are occasionally met with in which there are two 

 incisor-teeth on one side, while on the other there is but 

 the single tooth ; this being an excellent example of what 

 evolutionists term reversion or atavism. This, however, 

 by no means brings us to the end of the variation in the 

 number of these teeth obtaining in the group under 

 consideration ; but before proceeding farther it is necessary 

 to remark that, since in ordinary mammals the typical or 



