Alternating with these four great marine formations above the chalk, there intervenes a fourfold series of other strata, containing shells which show them to have been formed in fresh water, accompanied by the bones of many terrestrial and aquatic quadrupeds.
The greater number of shells, both in the fresh-water and marine formations of the tertiary series, are so nearly allied to existing genera, that we may conclude, the animals by which they were formed, to have discharged similar functions in the economy of nature, and to have been endowed with the same capacities of enjoyment as the cognate mullusks of living species. As the examination of these shells would disclose nearly the same arrangements and adaptations that prevail in living species, it will be more important to investigate the extinct genera of the higher orders of animals, which seem to have been constructed with a view to the temporary occupation of the earth, whilst the tertiary strata were in process of formation. Our globe was no longer tenanted by those gigantic reptiles, which had been its occupants during the secondary period; neither was it yet fit to receive the numerous tribes of terrestrial mammalia that are its actual inhabitants. A large proportion of the lands which had been raised above the sea, being covered with fresh water, was best adapted for the abode of fluviatile and lacustrine quadrupeds.
Our knowledge of these quadrupeds is derived solely from their fossil remains; and as these are found chiefly (but not exclusively)[1] in the fresh-water formations of the tertiary
The numerical proportions of, recent to extinct species may be thus expressed.—In the
| Newer Pliocene period | 90 | to | 95 | Per cent. are of recent species. | |
| Older Pliocene period | 35 | to | 50 | ||
| Miocene period | 18 | ||||
| Eocene period | 3½ |
—Lyell's Geology, 4 Ed. vol. iii. p. 308.
- ↑ The remains of Palæotherium occur, though very rarely, in the Calcaire Grossier of Paris. The bones of other terrestrial mammalia, occur
