II. The _Miocene_ formation (Gr. _meion_, less; _kainos_, new), with more recent species than the Eocene, but _less_ than the succeeding formation, and less than one-half the total number in the formation. As before, only the _Mollusca_ are taken into account, and about 17 per cent of these agree with existing species. III. The _Pliocene_ formation (Gr. _pleion_, more; _kainos_, new), with generally _more_ than half the species of shells identical with existing species--the proportion of these varying from 35 to 50 per cent in the lower beds of this division, up to 90 or 95 per cent in its higher portion. IV. The _Post-Tertiary Formations_, in which all _the shells belong to existing species_. This, in turn, is divided into two minor groups--the _Post-Pliocene_ and _Recent Formations_. In the _Post-Pliocene_ formations, while all the _Mollusca_ belong to existing species, most of the _Mammals_ belong to extinct species. In the Recent period, the quadrupeds, as well as the shells, belong to living species. The above, with some modifications, was the original classification proposed by Sir Charles Lyell for the Tertiary rocks, and now universally accepted. More recent researches, it is true, have somewhat altered the proportions of existing species to extinct, as stated above. The general principle, however, of an increase in the number of living species, still holds good; and this is as yet the only satisfactory basis upon which it has been proposed to arrange the Tertiary deposits. EOCENE FORMATION. The Eocene rocks are the lowest of the Tertiary series, and comprise all those Tertiary deposits in which there is only a small proportion of existing _Mollusca_--from three and a half to five per cent. The Eocene rocks occur in several basins in Britain, France, the Netherlands, and other parts of Europe, and in the United States. The subdivisions which have been established are extremely numerous, and it is often impossible to parallel those of one basin with those of another. It will be sufficient, therefore, to accept the division of the Eocene formation into three great groups--Lower, Middle, and Upper Eocene--and to consider some of the more important beds comprised under these heads in Europe and in North America.