280 CNGULATA 
hair on the lips, on the sides of the head and neck, and at the 
extremity of the short compressed tail, the skin of the hippopotamus, 
some portions of which are two inches in thickness, is entirely desti- 
tute of covering. 
The common Hippopotamus (H. amphilius), widely distributed 
in the rivers and lakes of the African continent, is a huge bulky 
animal, characterised by having only two incisors on either side 
of each jaw; the central lower pair being very much larger than the 
outer ones. A male from the Upper Nile which lived for nearly 
thirty years in the gardens of the Zoological Society of London 
measured 12 feet along the back from the nose to the root of the tail. 
The Hippopotamus lives in herds of from twenty to forty 
individuals on the banks and in the beds of rivers, in the neighbour- 
hood of which it finds its food. This consists chiefly of grass and 
aquatic plants, of which it consumes enormous quantities, the 
stomach being capable of containing from 5 to 6 bushels. These 
animals feed principally by night, remaining in the water during the 
day, although in districts where they are undisturbed by man they 
are less exclusively aquatic. In such regions they put their heads 
boldly out of the water to blow, but when rendered suspicious by 
persecution, they become exceedingly cautious, only exposing their 
eyes and nostrils above the water, and even this they prefer 
doing amid the shelter of water plants. In spite of their enormous 
size and uncouth form, they are expert swimmers and divers, and 
can remain under the water from five to eight minutes. They 
are said to walk with considerable rapidity on the bottoms of 
rivers, beneath at least a foot of water. At nightfall they come 
on land to feed; and when, as often happens on the banks of 
the Nile, they reach cultivated ground, they do immense damage 
to growing crops, destroying by their ponderous tread even more 
than they devour. 
A much smaller species, known as the Pigmy Hippopotamus 
(Z. libertensis), inhabits some of the rivers of Western Africa, and 
is characterised by having only a single pair of lower incisors. 
Mainly on this account, it has been proposed to regard this species 
as representing a distinct genus, under the name of Cheropsis ; but 
since it agrees so essentially in other characters with the common 
form, and sometimes has two incisors on one side of the lower 
jaw, it appears preferable to include it in the type genus. The 
greater relative size of the brain-cavity as compared with the facial 
portion of the skull renders, indeed, the contour of the skull 
decidedly different from that of H. amphibius - but this is a feature 
generally found in young individuals of larger species, and also in 
the adults of allied smaller forms. 
Both the existing species are now exclusively confined to Africa, 
but in the Pleistocene and Pliocene periods the genus was widely 
