214 On the Fossil Shells of the Paris Basin. 



intertropical regions, we meet with similar phenomena ; we 

 meet with a greater number of species than in the two 

 preceding regions, and amidst these, some are proper to 

 the temperate region ; a great number also proper to the 

 equatorial seas. 



These are general facts, and we can already draw from them 

 this general conclusion, that each assemblage of species re- 

 presents the mean temperature. But there are some species 

 more locally, and others more generally distributed. Thus 

 the Buccinum undatum, for example, is found from Cape 

 North to Senegal, slightly modified by temperature ; thus it 

 is easy enough to distinguish in it the varieties produced by 

 three or four principal conditions of temperature. This 

 species is not the only one thus distributed, but we are al- 

 ready acquainted with a very considerable number, having 

 with this the property of living in different temperatures. 



Other species more sensible of the influences of tem- 

 peratures, are much more local, and are those which it is 

 important to understand. I here enumerate some of them : — 



1. Buccinum glaciale. It does not extend beyond the 

 polar circle, and is found in Norway and Greenland. 



2. Cardium grcenlandicum. With the preceding. 



3. Terebratula psittacia. Between 65° and the 75° ; these 

 species, and many others which it would be too numer- 

 ous to mention, represent the mean temperature of the 

 north of Norway. 



1. Tellina baltica. 



2. Patella noachina. 



3. Natica clausa. 



4. Many species of the genus Astarte. 



5. Patella testudinalis, etc. 



These and other species represent the mean temperature 

 of the north of England, south of Sweden and of Denmark. 

 In the British Channel, on the coasts of France and England, 

 there also exist many species peculiar to our temperature. 



