110 TERTIARY FORMATION. 
This classification of the tertiary group was made 
by M. Deshayes. It is founded on the fact first 
particularly noticed by him, that the fossil shells of 
the oldest strata of the tertiary have much less re- 
semblance to the living species of the neighbouring 
seas than those of the newest group. In other 
"words, in proportion as the age of a tertiary forma- 
tion is more modern, so also is the resemblance 
greater of its fossil shells to the testaceous ani- 
mals of the present seas. In carrying out the plan 
of classification on this principle, Mons. D. has en- 
deavoured to determine the proportional numbe.\of 
shells, identical with living species, which belonged 
to each group. 
The result of a comparison of 3000 fossil shells 
found in the tertiary strata of Paris, with 5000 re- 
cent species, is, that in the oldest tertiary depos- 
ites, such as those about London and Paris, about 
three and a half per cent, are identical with recent 
species ; that about seventeen per cent, of the mid- 
dle tertiary period are identical with recent species ; 
and that the third division embraces from thirty- 
five to fifty per cent. ; and in formations still more 
modern, as in Sicily, the number of species identical 
with those now living is from ninety to ninety-five 
per cent. To these four groups Mr. Lyell has given 
the names: I. Eocene, 2. Miocene, 3* Older Plio- 
cene, 4. Newer Pliocene* The first is derived from 
" eos," dawn^ and " kainos," recent^ because the fos~ 
sil shells of this period contain a very small propor- 
tion of living species ; which may be considered as 
indicating the dawn of the recent testaceous ani- 
mals. 
Miocene is derived from melon," less^ and "kai- 
Bos," recent ; and 'pliocene^ from " pleion," more, and 
" kainos," recent ; expressing the more or less near 
approach which the deposites of these eras, when 
contrasted with each other, make to the existing 
species of molluscous animals. 
