MOLLUSC A. GASTEROPODA. 165 



the Shetland Islands ; the Celtic, including the coasts of 

 Southern Sweden, the Baltic, Denmark, Northern France 

 and the British Isles; and the Lusitanian, 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. Mya, Lutraria, Scrobicularia) 

 being found especially on muddy bottoms, others (e.g. 

 Natica, Turritella, Cyprcea, Cardium) on sandy, and others 

 (e.g. Buccinum, Littorina, Patella, Area) on rocky. There 

 is also a distribution according to depth, caused by 

 temperature (which as a whole decreases with the depth), 

 and by the absence 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, Trochus, 

 Patella, Hydrobia, Haliotis, Fissurella, Solen, Mya, 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 mollusks. Some of the commonest genera met 

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

 are also very numerous. 



(3) Zone of Nullipores or Corallines, from 14 to about 

 35 fathoms, where the calcareous algae (Nullipores) are 

 very abundant. It is characterised by the abundance of 

 Pleurotoma, Fusus, Buccinum, Natica, Eulima, Venus, 

 Dosinia, Astarte, Nucula, Area, Lima, and Pecten. 



(4) Zone of Brachiopods and deep-sea Corals, from 

 35 to about 230 fathoms; off Europe Oculina is the 

 common coral; brachiopods and Polyzoa are also abundant. 



