FAM. 1. HIPPOPOTAMIDAE.--The family Hippopotamidae contains of existing genera only _Hippopotamus_, for the Liberian dwarf Hippopotamus is not now regarded, as it was formerly, as the type of another genus, _Choeropsis_. The reasons for its former separation were the loss of the outer pair of incisors and the different proportions of various parts of the skull. This little Liberian animal has, however, been shown by Sir W. Flower[183] to possess the missing incisors occasionally; and as to the proportions of the skull, it is exceedingly common for small animals to vary from larger relatives in this way. Hence, considering the characteristic features of the Hippopotamus and the fewness of species, it seems unnecessary to divide it up further. We shall therefore only recognise one genus. The Hippopotamus at present is African in range, and confined to that continent. But quite recently it inhabited Madagascar; and further back still in time the existing African species, _H. amphibius_, ranged into Europe; there were also Indian forms, which were contemporary with the Stone-age man. The Common Hippopotamus is a great thick-skinned beast with but few hairs. It has four toes on each foot, a complex stomach, but no caecum. The strong incisors continue growing through life, as do the great canines. The number of incisors is two on each side of each jaw. Some of the extinct species had six in each {274} jaw, and they were distinguished as a genus _Hexaprotodon_, contrasting with _Tetraprotodon_, until intermediate conditions were observed. _Choeropsis_, as already observed, was a still further reduction of the tetraprotodont type. The molars (the formula is Pm 4/4 M 3/3) when worn show a double trefoil pattern. The orbital cavity is encircled by bone. As with many other aquatic mammals the kidneys are lobulated. [Illustration] FIG. 140.--Hippopotamus. _Hippopotamus amphibius._ x 1/40. A very singular fact about the Hippopotamus is the production of a "bloody sweat," a carmine-coloured secretion, containing small crystals and corpuscles, from the skin. This coloured fluid has of course nothing to do with blood.[184]