incisor (front) teeth, like the Sloth and Ant-eaters, which have no teeth at all, seem to make a fourth stem.
Without attempting a detailed account of these orders, we will try to call attention to the most important peculiarities connected with their organization and possible origin. The group of odd-toed (Perissodactyla) is so called from its representatives having an uneven number of toes, the Rhinoceros three, the Tapir three (at least in hind foot), the Horse one. These animals, however different in appearance externally, agree further in the structure of the skull and teeth, the number of pieces in the backbone (not less than twenty-two dorso-lumbar vertebrae), the simple stomach, and the peculiar character of the intestine (caecum). These animals are linked together by fossil forms, the whole series forming a very natural group, the odd-toed, of which the Paleotherium (Fig. 151) is the oldest. The Artiodactyle or even-toed group—the Hippopotamus, etc., having four toes, the Cow, Sheep, Deer, etc., two—agree in the structure of the skull and teeth, and in the number of dorso-lumbar vertebrae (nineteen), while some of them in the complex digestive system form the sub-group of the Ruminants, in which there exist three or four stomachs, one of which serves to hold the food until it is chewed a second time, while in the Camel and Llama the second stomach is modified to hold water. The living even-toed animals, linked together by extinct forms, make the second natural order of the Ungulata. The oldest even-toed is the Anoplotherium (Fig. 152). In the age preceding that in which the Anoplotherium and Paleotherium appeared there lived the Lophiodon, Coryphodon, Pliolophus, etc., animals which, in their dentition, seem to have combined the peculiarities of both the even- and the odd-toed orders. They are considered to be the common ancestors of the Ungulata, and the posterity of the Diprotodon and Nototherium, animals allied to the browsing Kangaroo. The line of descent would be: Marsupials like the Diproto-
