304 UNGULATA 
previously unsuspected extinct fauna in the American continent of 
the Tertiary period, as interpreted by Leidy, Cope, Marsh, and 
others, has thrown a flood of light upon the early history of this 
family, and upon its relations to other mammals. 
There have been found in these regions many Camel-like 
animals exhibiting different generic modifications; and, what is 
more interesting, a gradual series of changes, coinciding with the 
antiquity of the deposits in which they are found, have been traced 
from the thoroughly differentiated species of the modern epoch 
down through the Pliocene to the early Miocene beds, where, their 
characters having become by degrees more generalised, they have 
lost all that specially distinguishes them as Camelidw, and are 
merged into forms common to the ancestral type of all the other 
sections of the Artiodactyles. Hitherto none of these annectant 
forms have been found in any of the fossiliferous strata of the Old 
World; and it may therefore be fairly surmised (according to 
the evidence at present before us) that America was the original 
home of the Tylopoda, and that the true Camels have passed over into 
the Old World, probably by way of the north of Asia, where we 
have every reason to believe there was formerly a free communica- 
tion between the continents, and then, gradually driven southward, 
perhaps by changes of climate, having become isolated, have under- 
gone some further special modifications; while those members of 
the family that remained in their original birthplace have become, 
through causes not clearly understood, restricted solely to the 
southern or most distant part of the continent. The occurrence 
in the dentition of the fossil Siwalik Camels of a feature now 
found only in Auchenia is especially interesting from this point 
of view. 
Briefly referring to some of these fossil types, we may note 
that Pliauchenia, of the Loup Fork beds (Lower Pliocene) of 
the United States, has three lower premolars, while in Procamelus 
there were four of these teeth. In Protolabis of the Miocene 
we have a more generalised form, in which the dental formula 
isi 2, c4,p 4, m3; and from this type a transition may be 
traced to Poébrotherium, which, while having the same dental 
formula, was no larger than a Fox, and had the third and fourth - 
metacarpals separate, with rudiments of the fourth and fifth. The 
earliest undoubted representative of the group is Leptotragulus, of 
the Uinta Eocene, which appears to have been closely allied to 
Poebrotherium. It is, however, probable that the first lower pre- 
molar was wanting; while the other premolars of the mandible 
were much shorter antero-posteriorly than in the last-named genus. 
The manus, moreover, appears to have been less reduced, the second 
metacarpal retaining its connection with the magnum. It is 
suggested that Leptotragulus may have been derived from the 
