58 NUMERICAL PROPORTION OF RECENT SHELLS [Ch. V. 



three similar subdivisions, both of the Miocene and Pliocene 

 epochs. In that case, the formations of the middle period 

 must be considered as the types from which the assemblage of 

 organic remains in the groups immediately antecedent or sub- 

 sequent will diverge. 



The Recent strata form a common point of departure in all 

 countries. We derive one great advantage from beginning our 

 classification of formations by a comparison of the fossils of the 

 more recent strata with the species now living, namclv, the ac- 

 quisition of a common point of departure in every region of the 

 globe. Thus, for example, if strata should be discovered in 

 India or South America, containing the same small proportion 

 of recent shells as are found in the Paris basin, they also might 

 be termed Eocene, and, on analogous data, an approximation 

 might be made to the relative dates of strata placed in the arctic 

 and tropical regions, or the comparative age ascertained of 

 European deposits, and those which are trodden by our anti- 

 podes. 



There might be no species common to the two groups; yet 

 we might infer their synchronous origin from the common 

 relation which they bear to the existing state of the animate 

 creation. We may afterwards avail ourselves of the dates 

 thus established, as eras to which the monuments of preceding 

 periods may be referred. 



Numerical proportion of recent shells in the different Ter- 

 tiary periods. There are seventeen species of shells discovered, 

 which are common to all the tertiary periods, thirteen of which 

 are still living, while four are extinct, or only known as fossil*. 

 These seventeen species show a connexion between all these 

 geological epochs, whilst we have seen that a much greater 

 number are common to the Eocene and Miocene periods, and a 

 still greater to the Miocene and Pliocene. 



We have already stated, that in the older tertiary formations, 

 we find a very small proportion of fossil species identical with 



* See the Talks of M. Jkshaycs in Appendix I. 



