MARINE PROVINCES. 353 



ing may be taken as examples of the shells of high latitudes ; those marked * 

 being found in the southern, as well as in the northern hemisphere : — 



Buccinum. Velutina. *Crenella. 



*Cb]'ysoclomus. Lacuna. ' *Yoldia. 



*Trophon. *Margarita. *Astarte. 



Admete. Cyprina. 



*Trichotropis. *Ehyiiclionella. Glycimeris. 



The following have been thought peculiar to the wai-mer regions of the 



But it must not be inferred that these genera were always characteristic 

 of extreme climates. On the contrary, the whole of them have existed in the 

 British seas at no very remote geological period. RJiyncJionella and Astarte 

 were formerly " tropical shells ;" and since the period of the English chalk- 

 formation there have been living Nautili in the North Sea, and Cones and 

 Olives in the " London basin." It is not true that the same species have 

 been at one time tropical, at another temperate, but the genera have in many 

 instances enjoyed a much wider range than they exhibit now. Some of the 

 " tropical " forms are more abundant and extend farther in the Southern 

 hemisphere ; several large Volutes range to the extremity of South America, 

 and the largest of all inhabits New Zealand. 



The tropicd and sub-tropical provinces might be naturally grouped in 

 three principal divisions, viz., the Atlantic, the Tndo-Pacific, and the West- 

 American, — divisions which are bounded by meridians of longitude, not by 

 parallels of latitude. The Arctic province is comparatively small and excep- 

 tional ; and the three most southern Faunas of America, Africa, and Australia 

 differ extremely, but not on account of climate. 



If only a small extent of sea-coast is examined, the character of its 

 moUusca will be found to depend very much upon the nature of the shore, 

 the tides, depth, and local circumstances, which will be referred to again in 

 another page. But these peculiarities will disappear when the survey is 

 extended to a region sufficiently large to include every ordinary variety of 

 condition. 



It has been stated that each Fauna consists of a number of peculiar 

 species, properly, more than half; and of a smaller number which are com- 

 mon to some other provinces. By ascertaining the direction of the tides and 

 currents, aud the circumstances under which the species occur, it may be 

 possible to determine to which province these more widely diffused mollusca 

 originally belong-cl. And when species occur both recent and fossil it is easy 

 to perceive the diiection in which their migrations have taken place. 



