POSITION AND STRUCTURE OF HORSE 43 



which does not extend behind the extremities of 

 the outer case of the hoof, and is not applied to the 

 ground in walking. In the North African Gravy's 

 zebra, on the other hand, the frog is broader, with 

 scarcely any trace of the posterior groove, and its 

 hind-part touches the ground when the animal is 

 standing. In the kiang, and probably also in the 

 Asiatic onagers, the posterior development of the hoof 

 becomes much more marked, so that a considerable 

 portion projects behind the case of the hoof and 

 touches the ground, the cleft being deep and narrow. 

 Still greater development of the hind part of the 

 frog occurs in the ass, in which, as in some zebras, 

 it also becomes much thickened and somewhat 

 spongy in structure. In the extinct South Ameri- 

 can Onohippidium, the frog is somewhat inter- 

 mediate between that ot the horse and that of the 

 ass, being grooved, and not projecting behind the 

 case of the hoof, but being of considerable breadth 

 and thickness. Finally, in the bontequagga, and 

 probably the quagga, the medium-sized and slightly- 

 cleft frog is deeply sunk in the hoof, behind which 

 it projects to a small degree, not touching the 

 ground, except when the hoof is much worn down. 



These differences are probably correlated with 

 differences in the nature of the habitat of the various 

 species, and it is probable that species like the 

 horse, in which the frog is narrow, are adapted for 

 grassy or sandy plains ; while those in which it is 



