190 The Palaeontological Record. I. Animals 
From their gregarious habits and individual abundance, the 
history of many hoofed animals is preserved with especial clearness. 
So well known as to have become a commonplace, is the phylogeny 
of the horses, which, contrary to all that would have been expected, 
ran the greater part of its course in North America. So far as it has 
yet been traced, the line begins in the lower Eocene with the genus 
Eohippus, a little creature not much Jarger than a cat, which has 
a short neck, relatively short limbs, and, in particular, short feet, 
with four functional digits and a splint-like rudiment in the fore-foot, 
three functional digits and a rudiment in the hind-foot. The fore- 
arm bones (ulna and radius) are complete and separate, as are also 
the bones of the lower leg (fibula and tibia). The skull has a short 
face, with the orbit, or eye-socket, incompletely enclosed with bone, 
and the brain-case is slender and of small capacity. The teeth are 
short-crowned, the incisors without “mark,” or enamel pit, on the 
cutting edge; the premolars are all smaller and simpler than the 
molars. The pattern of the upper molars is so entirely different 
from that seen in the modern horses that, without the intermediate 
connecting steps, no one would have ventured to derive the later 
from the earlier plan. This pattern is quadritubercular, with four 
principal, conical cusps arranged in two transverse pairs, forming 
a square, and two minute cuspules between each transverse pair, 
a tooth which is much more pig-like than horse-like. In the lower 
molars the cusps have already united to form two crescents, one 
behind the other, forming a pattern which is extremely common 
in the early representatives of many different families, both of the 
Perissodactyla and the Artiodactyla. In spite of the manifold 
differences in all parts of the skeleton between Hohippus and the 
recent horses, the former has stamped upon it an equine character 
which is unmistakable, though it can hardly be expressed in words. 
Each one of the different Eocene and Oligocene horizons has its 
characteristic genus of horses, showing a slow, steady progress in 
a definite direction, all parts of the structure participating in the 
advance. It is not necessary to follow each of these successive steps 
of change, but it should be emphasised that the changes are gradual 
and uninterrupted. The genus Mesohippus, of the middle Oligocene, 
may be selected as a kind of half-way stage in the long progression. 
Comparing Mesohippus with Eohippus, we observe that the former 
is much larger, some species attaining the size of a sheep, and has 
a relatively longer neck, longer limbs and much more elongate feet, 
which are tridactyl, and the middle toe is so enlarged that it bears 
most of the weight, while the lateral digits are very much more 
slender. The fore-arm bones have begun to co-ossify and the ulna 
is greatly reduced, while the fibula, though still complete, is hardly 
