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 
