Ch. X.] FOSSIL SHELLS. 115 



and, secondly, what is probably a consequence of the former, the great 

 duration of species in this class, for they appear to have surpassed in 

 longevity the greater number of the mammalia and fish. Had each 

 species inhabited a very limited space, it could never, when imbedded in 

 strata, have enabled the geologist to identify deposits at distant points ; 

 or had they each lasted but for a brief period, they could have thrown 

 no light on the connection of rocks placed far from each other in the 

 chronological, or, as it is often termed, vertical series. 



Many authors have divided the European tertiary strata into three 

 groups — lower, middle, and upper ; the lower comprising the oldest 

 formations of Paris and London before-mentioned ; the middle those of 

 Bourdeaux and Touraine ; and the upper all those newer than the mid- 

 dle group. 



When engaged in 1828 in preparing my work on the Principles of 

 Geology, I conceived the idea of classing the whole series of tertiary 

 strata in four groups, and endeavoring to find characters for each, ex- 

 pressive of their different degrees of affinity to the living fauna. With 

 this view, I obtained information respecting the specific identity of many 

 tertiary and recent shells from several Italian naturalists, and among 

 others from Professors Bonelli, Guidotti, and Costa. Having in 1829 

 become acquainted with M. Deshayes, of Paris, already well known by 

 his conchological works, I learnt from him that he had arrived, by inde- 

 pendent researches, and by the study of a large collection of fossil and 

 recent shells, at very similar views respecting the arrangement of tertiary 

 formations. At my request he drew up, in a tabular form, lists of all 

 the shells known to him to occur both in some tertiary formation and in 

 a living state, for the express purpose of ascertaining the proportional 

 number of fossil species identical with the recent which characterized 

 successive groups ; and this table, planned by us in common, was pub- 

 fished by me in 1833.* The number of tertiary fossil shells examined 

 by M. Deshayes was about 3000 ; and the recent species with which they 

 had been compared about 5000. The result then arrived at was, that 

 in the lower tertiary strata, or those of London and Paris, there were 

 about 3£ per cent, of species identical with recent ; in the middle ter- 

 tiary of the Loire and Gironde about 17 per cent.; and in the upper 

 tertiary or Subapennine beds, from 35 to 50 per cent. In formations 

 still more modern, some of which I had particularly studied in Sicily, 

 where they attain a vast thickness and elevation above the sea, the num- 

 ber of species identical with those now living was believed to be from 

 90 to 95 per cent. For the sake of clearness and brevity, I proposed 

 to give short technical names to these four groups, or the periods to 

 which they respectively belonged. I called the first or oldest of them 

 Eocene, the second Miocene, the third Older Pliocene, and the last or 

 fourth Newer Pliocene. The first of the above terms, Eocene, is derived 

 from r t us, eos, dawn, and xaivog, cainos, recent, because the fossil shells of 



* See Princ. of GeoL vol. iii. 1st ed. 



