IN LIMBS. 51 



which are symmetrical to one another, and tend to 

 develop at the expense of the others ; while in the odd- 

 toed group it is the third or middle digit which is 

 symmetrical in itself and undergoes an ultra development. 

 Now there are certain extinct creatures, which, while 

 having claws instead of hoofs at the ends of their toes, 

 yet are so closely related to the hoofed mammals that 



FIG. 18. Front and side views of the Hind Foot of Artionyx. 

 (After Osborn.) 



their separation therefrom is almost impossible. In one 

 of these, which has long been known in Europe as the 

 chalicothere, the third digit, as in the odd-toed hoofed 

 mammals, is symmetrical in itself, and larger than 

 either of the others ; whereas in the newly -described 

 animal known as Artionyx the third and fourth digits 

 resemble those of the even-toed division of the hoofed 

 mammals in being larger than the others and symmetrical 

 to a line drawn between them. While, therefore, we have 

 clearly a parallel development on different lines between 

 the odd and even-toed hoofed animals in the development 

 of a cannon-bone, these extinct clawed, hoof-like mammals 

 (for want of a better expression) show a parallel parallel- 

 ism (to coin another expression) which, if it had continued, 

 might have resulted in the development of a two-clawed 



E 2 



