_Rodentia._--This order is represented in the Pliocene by a beaver, a porcupine, and an American mouse (_Hesperomys_), all extinct species of living genera, the _Hystrix_ being an Old World type; and Professor Cope has recently described _Panolax_, a new genus of hares from the Pliocene of New Mexico. The Miocene deposits have furnished an extinct genus allied to the hares--_Palæolagus_; one of the squirrel family--_Ischyromys_; a small extinct form of beaver--_Palæocastor_; and an extinct mouse--_Eumys_. The Eocene strata of Wyoming have lately furnished two extinct forms of squirrel, _Paramys_ and _Sciuravus_; and another of the Muridæ (or mouse family), _Mysops_. _Cetacea._--Numerous remains of dolphins and whales, belonging to no less than twelve genera, mostly extinct, have been found in the Miocene deposits of the Atlantic and Gulf States, from New Jersey to South Carolina and Louisiana; while seven genera of the extinct family, Zeuglodontidæ, have been found in Miocene and Eocene beds of the same districts. Some remains associated with these are doubtfully referred to the Seal family (Phocidæ) among the Carnivora. _Edentata._--Till quite recently no remains of this order have occurred in any North American deposits below the Post-Pliocene; but in 1874 Prof. Marsh described some remains allied to _Megalonyx_ and _Mylodon_, from the Pliocene beds of California and Idaho, and forming a new genus, _Morotherium_. As these remains have only occurred to the west of the Rocky Mountains, and in Pliocene deposits whose exact age is not ascertained, they hardly affect the remarkable absence of this group from the whole of the exceedingly rich Tertiary deposits in all other parts of North America. _General Relations of the extinct Tertiary Mammalia of North America and Europe._--Having now given a sketch of the extinct Mammalia which inhabited Europe and North America during the Tertiary period, we are enabled by comparing them, {141}to ascertain their relations to each other, and to see how far they elucidate the problem of the birth-place and subsequent migrations of the several families and genera. We have already pointed out the remarkable features of the Quaternary (or Post-Pliocene) fauna of North America, and now proceed to discuss that of the various Tertiary periods, which is closely connected with the extinct fauna of Europe.