however, not so generally known that these
animals had formerly been abundant here, and that long before, in
tertiary time, near relatives of the horse, and probably his ancestors,
existed in the far West in countless numbers and in a marvellous variety
of forms. The remains of equine mammals, now known from the tertiary and
quaternary deposits of this country, already represent more than double
the number of genera and species hitherto found in the strata of the
eastern hemisphere, and hence afford most important aid in tracing out
the genealogy of the horses still existing.

"The animals of this group which lived in America during the three
diversions of the tertiary period were especially numerous in the Rocky
Mountain regions, and their remains are well preserved in the old lake
basins which then covered so much of that country. The most ancient
of these lakes--which extended over a considerable part of the present
territories of Wyoming and Utah--remained so long in eocene times that
the mud and sand, slowly deposited in it, accumulated to more than a
mile in vertical thickness. In these deposits vast numbers of tropical
animals were entombed, and here the oldest equine remains occur,
four species of which have been described. These belong to the genus
Orohippus (Marsh), and are all of a diminutive size, hardly bigger than
a fox. The skeletons of these animals resemble that of the horse in many
respects, much more indeed than any other existing species, but, instead
of the single toe on each foot, so characteristic of all modern equines,
the various species of Orohippus had four toes before and three behind,
all of which reached the ground. The skull, too, was proportionately
shorter, and the orbit was not enclosed behind by a bridge of bone.
There were fifty four teeth in all, and the premolars were larger than
the molars. The crowns of these teeth were very short. The canine teeth
were developed in both sexes, and the incisors did not have the "mark"
which indicates the age of the modern horse. The radius and ulna were
separate, and the latter was entire through the whole length. The tibia
and fibula were distinct. In the forefoot all the digits except the
pollex, or first, were well developed. The third digit is the largest,
and its close resemblance to that of the horse is clearly marked. The
terminal phalanx, or coffin-bone, has a shallow median bone in front,
as in many species of this group in the later terti