5l4 UNGULATES. 



by a bell. Among the dun-coloured mules of the Punjab, dark stripes on the legs 

 are very common. 



There appear to be no authenticated instances of mules breeding among them- 

 selves ; although the female mule will occasionally produce offspring with the male 

 horse or ass. And it is somewhat remarkable that it does not appear that the 

 hybrids between any other members of the Equine family are mutually fertile. 



Fossil Horses. 



It has already been mentioned that remains, undistinguishable from the 

 existing horse, occur in the superficial deposits of Europe and Arctic America ; but 

 that those found in the corresponding formations of the United States and South 

 America appear to belong to extinct species of the genus Equus. In the upper 

 molar teeth of all these species the front inner pillar marked p in figure B on p. 

 487 is much elongated from front to back. In the figured tooth which belongs to an 

 extinct species (E. sivalensis) from the Siwalik Hills of India, that pillar is, how- 

 ever, shorter ; and in Steno's horse (E. stenonis), from the Pliocene deposits of 

 Europe, it is so much shortened as to be almost cylindrical. The same is the case 

 with certain extinct species from the later deposits of the United States and 

 Argentina, which, on account of the great length of the slit for the nose in the 

 skull, are separated as a distinct genus, under the name of Hippidium. All the 

 foregoing have but a single toe to each foot, but we now come to certain other 

 species in which there, were three distinct hoofs. One of these is the Protohippus 

 of the lower Pliocene strata of the United States, in which the upper molar teeth 

 approximate to the one represented in Fig. B on p. 487, but have shorter crowns. 

 The other is the European and Asiatic hipparion, or three-toed horse, of which an 

 upper molar tooth is represented in Fig. C of the page quoted. From that 

 figure it will be seen that the front inner pillar p is completely separated from 

 the portion pi. That the Protohippus was the ancestor of the true extinct horses 

 of America, there can be but little doubt ; but, from the separation of the inner 

 pillar of the molars, it is not so certain that the hipparion gave rise to the existing 

 European members of the family. 



Other Extinct Odd-Toed Ungulates. 



Ancestry of the The foregoing observations indicate that there is a complete 



Horse. transition from the modern single-toed horse to species with three 

 distinct toes to each foot, and with rather shorter-crowned and simpler molar teeth. 

 From these three-toed horses there is a further gradation to other extinct Ungulates, 

 which cannot be included in the Equine family, but some of which were doubtless 

 the direct progenitors thereof. One of these was the Miocene anchithere, common 

 to both Europe and the United States. From the figures given on p. 487, it will 

 be seen that the upper molar teeth of these animals, although formed on the general 

 plan of those of the horse, have very low crowns, with a simpler arrangement of 

 the pillars and ridges, and the intervening valleys perfectly open, owing to the 

 absence of cement ; and it may be added that other species show a complete 



