252 MOLLUSCA. GASTEROPODA 



region the chief provinces 1 are : the Arctic, which includes 

 the polar seas and extends as far south as the north of 

 Iceland and the North Cape ; the Boreal, extending from 

 the last down to near the southern end of Norway and 

 including the Shetland Islands ; the Celtic, including the 

 coasts of Southern Sweden, the Baltic, Denmark, Northern 

 France and the British Isles ; and the Lusitanian, com- 

 prising the coasts of the Bay of Biscay, Portugal, the 

 Mediterranean, and North-west Africa. In each province 

 the character of the molluscan fauna also varies with the 

 nature of the sea-bottom, some genera {e.g. My a, Lutraria, 

 Scrobicularia) being found especially on muddy bottoms, 

 others {e.g. Natica, Turritella, Cyprcea, Cardium) on sandy, 

 and yet others {e.g. Buccinam, Littorina, Patella, Area) on 

 rocky. 



The distribution of the Mollusca varies also with the 

 depth of the sea, owing to the accompanying change 

 of temperature and pressure, and to the gradual dis- 

 appearance of light ; in this respect five zones have been 

 traced : — 



(1) Littoral Zone, extending between high and low 

 water marks; in Europe this is especially characterised 

 by the abundance of the genera Littorina, Troclius, 

 Patella, Hydrobia, Haliotis, Fissurella, Solen, My a, 

 Donax, and Cardium. 



(2) Laminarian Zone, ranging from low water down 

 to about 14 fathoms ; in it algae {Laminaria, etc.) are 

 particularly luxuriant, and afford food for numerous phyto- 

 phagous molluscs. Some of the commonest genera met 

 with are Trochus, Nassa, Ilissoa, and Ostrea. Nudibranchs 

 are also very numerous. 



1 For a map of the provinces see Woodward's Manual of the Mollusca, 

 or Fischer's Manuel de Conchyliologie. 



