10 Estimate of Temperature during the Tertiary Period, 



ture, to ascertain if the life of those animals concerning, which 

 we are about to enquire, be connected more or less intimately 

 with circumstances among which climate may be the most 

 essential. This induces me briefly to state some facts relating 

 to the distribution of Mollusca, proceeding from north to 

 south ; and, in order to shorten it, I will only speak of those 

 which extend from Cape North to the Gulf of Guinea. If we 

 take all together the small number of species which inhabit the 

 north, we may divide them into two very distinct groups: first, 

 those peculiar to the icy regions, and which never pass their 

 limits ; and, secondly, a smaller number, which exist also in 

 the temperate seas of Germany, France, and England, with the 

 species belonging to those seas. 



In examining the Mollusca of our temperate seas, in which 

 there exists a greater number of species than in those of the 

 north, it is easy to separate them into three series. In the 

 first are comprised the species which I have just pointed out 

 as being common to the temperate and northern seas ; the 

 species of the second series extend also as far as the south 

 seas ; those of the third are peculiar to the temperate seas. 



Let us now turn to the region between the tropics, and we 

 shall observe similar phenomena: we shall find there a greater 

 number of species than in the two preceding regions; and, if 

 some amongst them are also found in the temperate zones, a 

 great many are peculiar to the equatorial seas. 



These are general facts; and we may already draw from 

 them this general inference, that each collection of species 

 represents the mean temperature of the region it belongs to. 

 But there are certain species more localised than others, and 

 some more widely distributed. Thus the Buccinum undatum, 

 for example, is found from the North Cape to Senegal, modi- 

 fying itself according to the temperature as it advances ; so 

 that it is easy to distinguish the varieties peculiar to the three 

 or four principal conditions of climate. This species is not 

 the only one thus extended ; but at present I am acquainted 

 with but a small number having, like this, the property of ex- 

 isting in temperatures so different. 



Other species, more sensible, as it appears, to the influence 

 of climate, are much more localised, and it is these which it 

 is most important to know. I will specify some of them : — 



1. Buccinum glacial e. 2. Cardium groenlandicum. These 

 two species do not pass the polar circle, and are found in 

 Norway and Greenland. 3. Terebratula psittacea. It in- 

 habits between the 65th and 75th degree. In my opinion, 

 these species, and some others, represent the mean temper- 

 ature of the north of Norway. 



