2 4 HIPPOPOTAMUS CHAP. 
jaw, and they were distinguished as a genus Hexaprotodon, con- 
trasting with Zetraprotodon, until intermediate conditions were 
observed. Choeropsis, as already observed, was a still further 
reduction of the tetraprotodont type. The molars (the formula is 
Pm # M8) when worn show a double trefoil pattern. The 
orbital cavity is encircled by bone. As with many other aquatic 
mammals the kidneys are lobulated. 
Fia, 140.—Hippopotamus. Hippopotamus amphibius. x 3. 
A very singular fact about the Hippopotamus is the pro- 
duction of a “bloody sweat,’ a carmine-coloured secretion, con- 
taining small crystals and corpuscles, from the skin. This 
coloured fluid has of course nothing to do with blood." 
The animal grows to a length of at any rate 14 feet. The 
limbs and the tail are short. Like other aquatic animals the 
nostrils are on the surface of the head, and can be closed when 
the animal is under water. When it reaches the surface of the 
water after a prolonged immersion, it spouts like a Whale. Sir 
Samuel Baker says that ten minutes is the longest time that the 
Hippopotamus can remain below the water. It is frequently a 
dangerous animal to encounter, as 1t will capsize boats, and even 
bite large pieces out of their bottoms; with its huge teeth it 
1 Tomes, Proc. Zool. Soc. 1850, p. 160. 
