MARTNE PROVINCES. 55 



Buccinum. Velutina. *Crenella. 



*Chrys(Xlomua. Lacuna. *Yol(lia. 



*Trophon. ^Margarita. *Astarte. 



Admete. Cj-prina. 



*Trichotropis. *Rhj-iichonella. Glycimeris. 



Tlie following have been thought peculiar to the warmer re- 

 gions of the sea : 



Nautilus, Conus. Columbella. Pema. 



Kostellaria. Harpa. Cypraea. Vulsella. 



Triton. Oliva. Nerita. Tridacna. 



Cancellaria. Voluta. Spondylus. Crassatella. 



Terebra. Marginella. Plicatula. Sanguinolara. 



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 geo- 

 logical period. Rhynchonella and ^sforie were formerly " tropi- 

 cal 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 Yolutes range to the extremity of South America, 

 and the largest of all inhabits New Zealand. 



The tropical and sub-tropical provinces might be naturally 

 grouped in three principal divisions, viz., the Atlantic, the 

 Indo-PaciJBic, and the "West- American, — divisions which are 

 bounded by meridians of longitude, not by parallels of latitude. 

 Tho Arctic province is comparatively small and exceptional ; 

 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 mollusca 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 Pauna consists of a numbjr ot 

 peculiar species, properly, more than half;' and of a smaller 

 number which are common to some'other provinces. By ascer- 

 taining the direction of the tides and currents, and the circum- 

 stances under which the species occur, it may be possible to 

 detenx »ne to which province these more widely diffused mollusca 



