MAMMALIA. 11' 



which are protected or encased in a greatly expanded nail or hoof 

 (ungula). This apparently superficial character is but one element 

 in the structure of the ungulate foot, which structure is of the 

 ut«iost value in tracing relationship of fossil mammals, and the 

 study of it one of the most fascinating chapters in Comparative 

 Anatomy. 



The number of digits of full size never exceeds four, at least the 

 first digit being always obliterated. As the limbs ai'e used only for 

 locomotion, never for prehension, clavicles are useless and therefore 

 wanting. The molar teeth are massive, with broad crowns suited 

 for grinding vegetable food, although the primitive species were 

 probably omnivorous, like a few living forms. The brain is pro- 

 portionately small and the food-canal unusually long. 



The Ungulates were the most numerous and important mammals 

 of Tertiary time, and those ancient species were the ancestors of 

 the present specialized forms, the line of descent being clearly 

 traced in some cases, especially in the horse, the most highly differ- 

 entiated species. 



The division of this order, proposed by Owen, into Perissodactyla 

 {odd-toed), and Artiodactyla (even-toed), applies also to the extinct 

 forms, which as early as the lowest Eocene were thus separated, 

 notwithstanding their generalization in other respects. CorypJiodon, 

 found at the base of the Eocene in both Europe and America, hav- 

 ing a small bi-ain and five toes, possesses characters which point to 

 the primitive ungulate type. 



In the evolution of the present Ungulates there are two elements 

 of special interest. The primitive ancestors of the order were prob- 

 ably omnivorous, like the existing pig, with tuberculated (hunodoyit) 

 molars. But the specialized forms, as the horse of the odd-toed, 

 and the ruminants among even-toed, have developed molars better 

 fitted for grinding, which have the enamel disposed on the effective 

 surface in double crescents {selenodont dentition), whose convexity 

 is turned inwards in the upper teeth and outwards in the lower. 

 The other factor in the evolution was the relation of the small bones 

 of the wrists and ankles to the surviving digits. In the loss of the 

 side digits, and the enlargement of the central ones, it became neces- 

 sary for the latter to either appropriate the carpal or tarsal bones 

 belonging to the side digits, or for their own small bones to become 

 properly enlarged. Nature employed both methods; but it has been 

 shown by Kowalevsky, the Russian naturalist, that the first or 

 appropriative method was the better, and that all the species in 

 both sections in which the latter (inadaptive) plan occurred have 

 become extinct. 



