On the Fossil Shells of the Paris Basin. 213 



All marine animals are not true indicators of temperature ; 

 it is necessary to select for the purpose, those whose feeble 

 movements constrain them to depend for their sustenance 

 on the alternations of the seasons, and compel them to 

 limit their influence to the places where they were first pro- 

 duced. The greater number of the mollusca and zoophytes 

 supply these conditions. 



To arrive at the knowledge of the temperature of the 

 times anterior to the existence of man, the logical course to 

 pursue is, first, to search for some positive position to start 

 from, so as to assure ourselves of the real character of the 

 animals from which we derive our evidence, and then to 

 seize upon those conditions of their existence, with which 

 temperature has more or less to do. It is the principal part 

 which temperature plays in the distribution of the mollusca, 

 in advancing to the North or South, which we are now briefly 

 to explain; and for brevity's sake, shall speak only of those 

 which have been collected near Cape North, and in the 

 Gulf of Guinea. 



If the small number of species which live in the north 

 be separately considered, they can be divided into two 

 very distinct kinds: the one proper to the colder seas, 

 do not pass beyond the limits of these; the others, in 

 smaller number, coming to live in the temperate regions of 

 Germany, France, and England, with the species of these 

 seas. 



In examining the testaceous molluscs of the seas of the 

 temperate regions of Europe in which there exists a 

 greater number of species than in the seas of the north, it 

 is easy to separate them into three series : in the first of 

 these are comprised those which we have indicated, and 

 which return again to the seas of the north; the species 

 of the second series descend into the seas of the south; 

 lastly, those of the third series are proper to European 

 temperatures. If we now carry our observations to the 



2 F 



