THE EVOLUTION OF THE MAMMALIA. 43 
and fibula) were entirely free and separate from one another. The 
neck is short, and the head relatively small ; the cranium is nar- 
row and of proportionately small capacity, and the brain is small, 
and only simply convoluted ; the orbit for the eye is but partially 
enclosed by bone, and is placed quite far forward over the molar 
teeth, so that the face is short. The teeth are very short 
crowned, early forming roots, and, at first sight, seem to be utterly 
different from those of existing horses ; the front teeth (incisors) 
are simple and chisel-shaped, not having the deep pit which horse- 
men call the " mark "5 of the grinding teeth, the anterior four 
(premolars) have a much less complex pattern than the posterior 
three (molars). The latter have a grinding surface made up of 
six small, and nearly conical tubercles, arranged in two transverse 
rows. Many other minor differences may be noted between 
these ancient horses, as they may fairly be called, and their 
modern descendants, but lack of space forbids a consideration of 
them here. 
In the Wind River, Bridger, and Uinta stages we find the horses 
gradually and steadily advancing, but we may pass over these 
genera and compare the White River representative of the series 
(Mesohtppus) directly with its Wasatch ancestor, always remem- 
bering, however, that the genera which have been omitted from 
consideration make a complete and perfect transition from the 
older to the newer form. In Mesohippus quite marked advances 
may be noted. The most obvious one is the increased stature, the 
White River genus being as large as a sheep (and some of its 
species still larger) ; it is thus quite double the size of its Wasatch 
predecessor. The grinding teeth have become decidedly more 
complex ; the tubercles have coalesced to form sharp crests, and 
all but the most anterior one have acquired the same pattern and 
size. These grinding teeth are, however, still very low crowned, 
and indicate that these animals, like their forerunners, still con- 
tinued to feed upon soft and succulent vegetation. A very inter- 
esting feature is the appearance of a shallow depression, or cup, 
upon the upper incisors, the first beginnings of the " mark." 
The face is somewhat lengthened, the orbits commencing to shift 
