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POPULAR SCIENCE REVIEW. 
ever, it has been ascertained that the region in which the 
stupendous mountain ranges of western North America have 
since been elevated were tenanted by animals of similar form 
and characters, and quite as varied in species and as numerous 
in individuals as those which at a corresponding period of time 
ranged through the marshes and forests of the Paris and London 
basins. 
None of these appear to be identical specifically with the 
European forms ; and even the generic indications, being often 
founded on very limited portions of the organisation only, as 
a few teeth, must be regarded as provisional. Many were un- 
doubtedly quite distinct from any which we know from the 
Eastern world. It would be useless here to give a catalogue of 
the generic and specific names which have been given to the 
animals of this group already discovered. A brief mention of 
the most important and interesting will suffice. The two best 
known genera are those named by Leidy Hyrachyus and 
Palceosyops ; the former is allied to the Lophiodons and Tapirs, 
the latter to the Palseotheriums. They both embrace animals 
in size varying from that of a small rhinoceros down to a 
peccary. The numerous modifications and combinations of 
characters found in forms apparently allied to them, which are 
little known to us at present except by the names given to them 
by their discoverer, will doubtless afford for a long time to 
come materials for the minutest scrutiny. Some appear to be 
allied to the European Lophiodon and Hyracotherium , one of 
which Orohippus (Marsh), seems to connect these forms through 
Miohippus and Mesohippus with the horse-like Anchitherium , 
and thus fill a link wanting in European formations in the 
pedigree of the Equine family. This animal, like so many 
other of the Eocene Perissodactyles, resembles the modern 
tapirs in retaining the fifth digit on the fore-foot, though, a3 
in all known members of the group, the first was wanting, and 
both first and fifth were wanting to the hind-foot. Several 
species are described, but none larger than a common fox. One 
form only, Dicer atherium (Marsh), is rhinocerotic. It is found 
in the uppermost Eocene strata of Utah, and gives the earliest 
indication of this group yet known. It seems, according to 
Marsh, to be connected with the lower Eocene Hyrachyus on 
the one hand, and the Miocene Hyracodon on the other. 
In the Miocene period in North America the Perissodactyles 
attained a great development of form, variety, and size, the 
groups became more distinctly separated from each other, and 
some of them possessed remarkably specialised characters. 
True tapirs have not yet been met with in this period, which 
is rather remarkable, taken in conjunction with the present 
geographical distribution of that group. The Palseotheroid 
