296 HISTORICAL PALEONTOLOGY. 



II. The Miocene formation (Gr. meion, less; kainos, new), 

 with more recent species than the Eocene, but less than the suc- 

 ceeding formation, and less than one-half the total number in the 

 formation. As before, only the Mollusca are taken into account, 

 and about 17 per cent of these agree with existing species. 



III. The Pliocene formation (Gr. Pleion, more; kainos, new) 

 with generally more than half the species of shells identical with 

 existing species the proportion of these varying from 35 to 

 50 per cent in the lower beds of this division, up to 90 or 95 

 per cent in its higher portion. 



IV. The Post-Tertiary Formations, in which all the shells 

 belong to existing species. This, in turn, is divided into two 

 minor groups the Post-Pliocene and Recent Formations. In 

 the Post-Pliocene formations, while all the Mollusca belong to 

 existing species, most of the Mammals belong to extinct 

 species. In the Recent period, the quadrupeds, as well as the 

 shells, belong to living species. 



The above, with some modifications, was the original classi- 

 fication proposed by Sir Charles Lyell for the Tertiary rocks, 

 and now universally accepted. More recent researches, it is 

 true, have somewhat altered the proportions of existing species 

 to extinct, as stated above. The general principle, however, 

 of an increase in the number of living species, still holds good; 

 and this is as yet the only satisfactory basis upon which it has 

 been proposed to arrange the Tertiary deposits. 



EOCENE FORMATION. 



The Eocene rocks are the lowest of the Tertiary series, and 

 comprise all those Tertiary deposits in which there is only a 

 small proportion of existing Mollusca from three and a half 

 to five per cent. The Eocene rocks occur in several basins in 

 Britain, France, the Netherlands, and other parts of Europe, 

 and in the United States. The subdivisions which have been 

 established are extremely numerous, and it is often impossible 

 to parallel those of one basin with those of another. It will 

 be sufficient, therefore, to accept the division of the Eocene 

 formation into three great groups Lower, Middle, and Upper 



