218 Prof. R. Owen on the Occurrence 



becomes equal and similar in later Artiodactyles. But in 

 Coryphodon a superior size of the third digit coexists with a 

 three-trochantered femur. I therefore limit myself to tracing 

 the subsequent simplifications of the foot in the Perissodactyle 

 series. 



As these Ungulates approach the present time the feet gain 

 in length but lose in breadth ; and the latter loss is due not only 

 to proportions of the constituent bones of the fore and hind 

 feet, but to disappearance of digits. 



The first or innermost is always the first to go. 



Two series, however, may be traced, in which the tendency 

 to length over bi'eadth of foot is more marked in one than in 

 the other. The broader type is represented in the still living 

 series by the rhinoceros, the naiTOwer type by the horse. 



The earliest, now extinct, form of Rhinoceros, called, from 

 the non-development of the defensive weapon, Acerotherium, 

 exhibits the type of fore foot shown in fig. 4. 



Its diminutive congener {Uyrax), which escapes an enemy 

 by hiding in the cavities of rocks, is also hornless, like the old 

 Tertiary Acerothere, and retains a similar type of four-toed 

 fore foot. With the coming in of enemies in the later Miocene 

 and Pliocene periods the formidable horn is developed in the 

 larger beasts, single or two in number, and these one behind 

 the other, never in a pair; although elevations of the outer 

 table of the skull, simulating horns, do occur, in a symmetrical 

 pair, in some species of Acerothere. The contemporaries of the 

 tiger in India, and of the lion in Africa, superadd to their 

 weapon of attack defensive armour, in the thickness of their 

 folded hide. The foot of the modern rhinoceros is reduced, 

 as in PI. XI. fig. 5, to the tridactyle type ; but a rudiment 

 of the fifth metapodial (ib. v) is still retained. The diver- 

 gence from the pentadactyle type in the longer and narrower 

 form of foot can now be traced through a rich series of grada- 

 tions *, of which three are selected for the present illustration. 

 In PI. XI., fig. 6 represents the fore foot of the Orohippus, in 

 which the first digit alone is wanting. The relative size of 

 the third indicates the superior share it takes in station and 

 progression. The persistence of the fifth digit, though slender, 

 adds to the power which Orohippus possessed to pass over 

 swamps, in which the foot of the modern horse woiild sink. 



In the Miocene Hippothere (ib. fig. 7) the fifth digit has gone, 

 and the second and fourth are reduced, while the third is en- 

 larged. It is a form of foot better adapted than that of Orohippus 



* For a knowledge of which we are chiefly indebted to Prof. Marsh, 

 "Notice of new Equine Mammals from the Tertiary Formation," in Amer. 

 Journal of Arts and Sciences, yoI. vii. March 1874, 



