110 TERTIARY FORMATION. 



This classification of the tertiary group was made 

 by M. Deshayes. It is founded on tlie fact first 

 particular!}^ noticed by him, that the fossil shells of 

 the oldest strata of the tertiary have much less re- 

 semblance to the hving 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 resembhinctf 

 greater of its fossil shells to the testnceous 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, vv ith 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 wiih 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 testaceous ani. 

 mals. 



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

 nos," recent; and fliocene, from "pleion," ?72ore, and 

 "kainos," recent; expressing the wore or less near 

 approach which the deposites of these eras, when 

 contrasted with each other, make to the existing 

 species of molluscous animals. 



