218 On the Fossil Shells of the Paris Basin, 



ture of the tropics, correspond with the fossils in the se- 

 veral places we have mentioned. Now, considering the 

 number of species, and the great number of individuals 

 belonging to each of these, their development would sug- 

 gest the basin of the Gironde as the line of greatest inten- 

 sity of heat, where in former times an equatorial tempera- 

 ture prevailed ; it has necessarily been to this temperature 

 that we owe the present fossil remains of species, which 

 in former times inhabited our seas. It was necessary that 

 this increased temperature should also have been directed 

 continuously during a long series of cycle, in order that the 

 accumulated constituents of generations should form by their 

 remains a solid of such vast expanse. 



If, as we firmly believe, the basin of Gironde to have 

 been deposited under an equatorial temperature, it will be 

 sufficient to cast a look at a map in order to feel convinced 

 that the influence of this temperature has extended as far as 

 Poland, and to the middle of Russia in Europe. 



To determine the equatorial temperature of our second 

 tertiary period, we have compared nearly two hundred spe- 

 cies of the intertropical zone with the fossil species from the 

 upper strata at Bourdeaux and Dax, and other basins be- 

 longing to this second period ; but unfortunately one conclu- 

 sive element is deficient for the first tertiary period, that which 

 represents the Paris basin. In nearly fourteen hundred spe- 

 cies, thirty-eight only have living analogues ; it is true that 

 the greater number of these species are found throughout the 

 equatorial zone, but among them there appear to be some, 

 which are found not only in that zone, but which pass into 

 our temperate seas, and even wander to the North Sea. 



I must therefore abandon, in regard to the most important 

 tertiary period, those means for the estimation of its tempe- 

 rature, which have been employed with success in the case 

 of the two preceding periods. We endeavour in the mean 



