142 DISTRIBUTION OF EXTINCT ANIMALS. [PART 11. 
the American Pliocene and Miocene periods, abounding in 
genera and species; whereas in Europe the group only exists in 
the Post-Pliocene or Lower Pliocene, with one Upper Miocene 
species of Camelus in N. India. The Anthracotheridz (extinct), 
found only in the Upper Miocene of France and India, reach 
even the Lower Eocene in America. 
These facts may be due, in part, toa want of strict co-ordina- 
tion between the Tertiary deposits of Europe and North America, 
—in part to the imperfection of the record in the latter country. 
Yet it does not seem probable that they are altogether due to 
these causes, because the Miocene beds, which are by far the 
best known in America as in Europe, exhibit deficiencies of the 
same kind as the less known Eocene deposits. The fossil fauna 
of both countries is so rich, that we can hardly impute great and 
well marked differences to imperfect knowledge; yet we find 
such important families as the Civets, Hyznas, Giraffes, and 
Hippopotami absent from America, with the Weasels, and Ante- 
lopes almost so ; while America possesses almost all the Camelide, 
two peculiar orders, Dinocerata and Tillodontia, and four remark- 
ably peculiar families, Limnotheride, Lemuravide, Oreodontide 
and Brontotheride. If then the facts at present known represent 
approximately the real time-relations of the groups in question 
on the two continents, they render it probable that weasels, bears, 
true horses, swine, oxen, sheep and antelopes, originated on the 
Old World continent, and were transmitted to America during 
some part of the Miocene period; while camels originated in the 
New World, and somewhere about the same time passed over to 
Europe. Of the extinct families common to the two hemi- 
spheres, the Anthracotheride alone seem to have had an 
American origin. Of the genera common to the two countries, 
almost all seem to have bad a European origin, the only genera 
of equal date being the two rhinoceroses and three Anchi- 
theride ; but if the Brontotheride are allied to the Rhinocerotide, 
these latter may have originated in America, although now an 
exclusively Old World type. These conclusions are not im- 
probable when we consider the much greater size of the Old 
World continents, extending far into the tropics and probably 
