The Tertiary Mammalia of North America at present described belong to from eighty to one hundred genera, while those of Europe are nearly double that number; yet only eighteen genera are common to the two faunas, and of these eight are living and belong chiefly to the Pliocene period. Taking first, the genera which in America do not go back beyond the Pliocene period (ten in number), we find that eight of them in Europe go back to the Upper Miocene. These are _Felis_, _Pseudælurus_, _Hipparion_, _Cervus_, _Mastodon_, _Elephas_ (in India), _Castor_ and _Hystrix_; while another, _Canis_, goes back to the Upper Eocene and the tenth, _Equus_, confined to the newer Pliocene or perhaps to the Post-Pliocene in America, extends back to the older Pliocene in Europe. Of the seven European genera which are confined to the Miocene period in America, three, _Hyænodon_, _Anchitherium_, and _Lophiodon_ go back to the Eocene in Europe; three others, _Machairodus_, _Rhinoceros_, and _Aceratherium_, are also of Miocene age in Europe; _Amphicyon_ goes back to the Lower Miocene of Europe. _Lophiotherium_ belongs to the Eocene of both countries. If we turn now to families instead of genera, we find that the same general rule prevails. Mustelidæ (weasels), Ursidæ (bears), true Equidæ (horses), and Bovidæ (oxen &c.), go no further back in America than the Pliocene, while they all go back to the Miocene in Europe. Suidæ (swine) and Anoplotheridæ (extinct) are found in the American Miocene and in the European Eocene. Anchitheridæ (extinct) reach the Upper Eocene in America, while in Europe they range through Upper, Middle, and Lower Eocene. Cervidæ (deer) alone are Miocene in both countries. There remain two families in which America has the preeminence. Camelidæ (camels) were wonderfully developed in {142}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 Anthracotheridæ (extinct), found only in the Upper Miocene of France and India, reach even the Lower Eocene in America.