262 GEOGRAPHICAL AND GEOLOGICAL DISTRIBUTION. 



have an almost world-wide extension, many of the species even 

 appearing in widely separated parts of two or more continents. 

 Still, we know of no truly cosmopolitan species of these groups, 

 although through artificial transport many of the forms have been 

 made to spread far beyond the limits of their natural domain. 

 Helix similaris, indigenous to Eastern Asia, is now largely found in 

 Malaysia, Polynesia, Australia, the Seychelles, South Africa, the 

 Antilles, and Brazil, and as nearly an extensive range is claimed 

 for Ennea bicolor. The common garden-snail of Europe (Helix 

 aspersa) has been naturalised in Algeria, the Azores, Brazil, and 

 California, and human agency has, doubtless, been largely involved 

 in the dissemination of the small moss-inhabiting Helix pulchella, 

 which now inhabits the greater part of the continent of Europe, 

 the Caucasus, Madeira, the region of the Cape of Good Plope, and 

 nearly all Northern North America. 



Bathymetrical and Hypsometrical Distribution. There are 

 five zones of distribution usually recognised in the oceanic waters, 

 as follows : 1, the Littoral Zone, or that existing between tide-marks 

 the habitat of the periwinkle, limpets, sand-clam, cockle, mussel, 

 and barnacle ; 2, the Laminarian Zone, extending from low- water 

 mark to about fifteen fathoms, and characterised by a dense growth 

 in many places of sea-weed and tangle the haunt of the vegetable- 

 feeding Testacea and of various nudibranchs, and the home of the 

 oyster ; 3> the Coralline Zone (from fifteen to forty or fifty fathoms), 

 the zone of the encrusting Algae (nullipores), and of the large car- 

 nivorous Gasteropoda (Buccinum, Fusus, Pleurotoma, Natica, &c.); 

 4, the Deep-sea Zone, or that of the Brachiopoda and deep-sea corals 

 (from fifty to two hundred and fifty or three hundred fathoms) ; 

 and, 5, the Abyssal Zone, from three hundred fathoms to the oceanic 

 floor, throughout which the shells are generally of small size, trans- 

 lucent (thin), and white or but feebly coloured. The visual organs 

 are here exceptionally devoid of pigment, and blindness has been 

 noted in a few species of normally seeing gasteropods. It might be 

 doubted, however, whether the last two divisions ought not more 

 properly to constitute a single division, considering the large pro- 

 portion of genera and species which pass from the shallower to the 

 deeper parts. Dall, from the data furnished by the dredgings 

 made in the Gulf of Mexico by the steamer " Blake" (1877-78), 

 affirms that fully twenty per cent, of the molluscan species obtained 



