ZOOLOG Y. 73 
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 appear- 
ance 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 vertebra), 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 sys- 
tem 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 Paleo- 
therium 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 ances- 
tors 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- 
