FOURFOLD DIVISION. 79 
nine marine formations, and the English Crag ; 
and to the Newer Pliocene, the more recent 
marine deposits of Sicily, Ischia, and Tuscany.* 
Alternating with these four great marine for- 
mations above the chalk, there intervenes a 
fourfold series of other strata, containing shells 
wdiich show them to have been formed in fresh 
water, accompanied by the bones of many terres- 
trial 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 enjoy- 
ment as the cognate mollusks of living species. 
As the examination of these shells would dis- 
close nearly the same arrangements and adap- 
tations that prevail in living species, it will 
be more important to investigate the extinct 
* The total number of known fossi! shells in the tertiary series 
is 3,036. Of these 1,238 are found in the Eocene ; 1,021 in the 
Miocene ; and 777 in the Older, and Newer Pliocene divisions. 
The numerical proportions of recent to extinct species may be 
thus expressed. — In the 
Newer Pliocene period. ... 90 to 95 " 
Older Pliocene period .... 35 to 50 Per cent, are of 
Miocene period 18 I recent species. 
Eocene period , 3^ 
— Lyell's Geology, 4. Ed. vol. iii. p. 308. 
