On the Fossil Shells of the Paris Basin* 



219 



time to supply by several indirect means, the want of direct 

 means of comparison which we here experience. 



In the frozen seas, there exists but a small number of 

 molluscs ; but some species are adapted to endure cold in pro- 

 portion as they advance from the warmer regions, and thus 

 there are but eight or ten, which subsist at the 80th degree, 

 while there are above nine hundred living species in the tro- 

 pical region of Senegal and Guinea. This augmentation of 

 species with temperature, sufficiently indicates the power- 

 ful agency of heat in the creation of these beings. But this 

 phenomenon is not alone seen in those parts of the terrestrial 

 globe which we have selected for example, it is repeated 

 from the sea of Behring to the isles of Sunda, and may be 

 traced inversely on each coast of South America. 



One important fact is elicited, and affords a new point 

 cT appui in the estimation of the temperatures of these last 

 tertiary periods; it is the proportion in the number of fossil 

 and of the living species. Thus in northern latitudes, few 

 species exist, and few of those which do, are found fossil ; of 

 tropical regions nearly seven hundred species are fossil, and 

 six hundred exist. It must be remembered, that this differ- 

 ence in the proportion of the living as compared with the 

 fossil species, is owing to a certain number belonging to lost 

 races. In short, the elevated temperature of the second period 

 may be regarded as settled with certainty, when we state, that 

 nearly one thousand species in the corresponding basins 

 have been examined and compared with nine hundred living 

 species from the intertropical seas of Africa. 



Since the number of species accords with the temperature ; 

 since, on one particular portion of the intertropical region 

 we find nearly nine hundred species, it appears to me 

 natural, that we should attribute to the first tertiary period 

 a temperature at least equatorial; for we are actually ac- 

 quainted with fourteen hundred species, of which twelve 

 hundred were collected in the Paris basin, that is to say, in 



