UNGULATES. 
5 1 4 
by a bell. Among the dun-coloureci 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 
