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 number 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: 1. 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 tertaceous ani- 

 mals. 



Miocene is derived from " meion," less, and " kai- 

 nos," 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. 



