It follows from these considerations that any attempt to classify marine Mollusca under Regions and Provinces can only apply to Mollusca which occur at moderate depths. The most important factor in the environment, as determining distribution, is the _temperature of the water_, which is probably to be regarded as affecting not so much the adult Mollusca as their ova; for the adult might possibly support life under conditions in which the ova would perish. It appears that a sudden change of temperature is the most effective barrier to distribution,[384] and may bring the range of a species to an almost instantaneous stop, while a very gradual change will allow it to extend its range very widely. It has been usual to classify marine Mollusca from moderate depths under the following regions and sub-regions:-- Regions Sub-regions {1. Arctic. {2. Boreal. =A.= =Atlantic= and {3. Celtic. =Circumpolar= {4. Lusitanian. {5. West African. {6. South African. =B.= =Indo-Pacific= {1. Indo-Pacific. {2. Japanese. =C.= =Australian= {1. Australian. {2. Neozealanian. {1. Aleutian. {2. Californian. {3. Panamic. =D.= =American= {4. Peruvian. {5. Magellanic. {6. Argentinian. {7. Caribbean. {8. Transatlantic. =A. The Atlantic Region= includes the whole to the eastern shores of the Atlantic, from the extreme north of the Cape of Good Hope, together with the circumpolar seas, which may be regarded as roughly bounded by the Aleutian Islands and the coast of Newfoundland. (1) _The Arctic Sub-region_ includes the circumpolar seas, and is bounded in the N. Pacific by a line drawn between Cape Avinoff in Alaska, and Cape Lopatka in Kamchatka, so as to exclude the Aleutian Islands. On the western shores of the Atlantic the cold Labrador current brings it as far south as the coast of Newfoundland, but on the eastern shores the influence of the Gulf Stream has the contrary effect, so that the North Cape may be taken as its southern limit. The principal genera (many species of which are common to the whole sub-region) are _Volutomitra_, _Buccinum_, _Buccinopsis_, _Neptunea_, _Trophon_, _Bela_, _Admete_, _Velutina_, _Trichotropis_, _Lacuna_, _Margarita_, _Philine_, _Pecten_, _Leda_, _Yoldia_, _Astarte_, and _Mya_. The shells are generally unicoloured, and of a dead white or rather sombre tint.