FOURFOLD DIVISION. 69 



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 aUied 

 to existing genera, that we may conclude, the animals by 

 which they were formed, to have discharged similar func- 

 tions in the economy of nature, and to have been endowed 

 with the same capacities of enjoyment as the cognate mul- 

 lusks of Uving species. As the examination of these shells 

 would disclose nearly the same arrangements and adapta- 

 tions that prevail in living species, it will be more important 

 to investigate the extinct genera of the higher orders of ani- 

 mals, which seem to have been constructed with a view to 

 the temporary occupation of the earth, wliilst 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*) 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 'v 



Older Pliocene period . . 35 to 50 / Per cent, are of 



Miocene period 18 ^ recent species. 



Eocene period 3^ j 



— Lyell's Geology, 4 Ed. vol. iii. p. 308. 



* The remains of Palaeotherium occur, though very rarely, in the Cal- 

 caire Grossier of Paris. The bones of other terrestrial mammalia, occ ur 



