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
A. Atlantic
and Circumpolar		 1. Arctic.
2. Boreal.
3. Celtic.
4. Lusitanian.
5. West African.
6. South African.
B. Indo-Pacific 1. Indo-Pacific.
2. Japanese.
C. Australian 1. Australian.
2. Neozealanian.
D. American 1. Aleutian.
2. Californian.
3. Panamic.
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.

(2) The Boreal Sub-region may be subdivided into two provinces, the European and the American. The former includes the entire coast-line of Norway, the Färoe Islands, and Iceland (except perhaps the northern coast), and possibly the Shetland Islands; the latter the American coasts from the Gulf of St. Lawrence to Cape Cod (lat. 42°). Thus the Boreal American province does not extend nearly so far south as the Boreal European, the reason being that on the American coasts the cold Labrador current, which hugs the land, bars back the advance of southern genera, but allows Boreal genera to spread southwards, while on the European side the warmer conditions produced by the Gulf Stream keep the Boreal species back, and allow more southern forms to spread northwards.

Many of the Boreal species occur on both sides of the Atlantic, and thus support the theory of a more continuous fringe of continental land once existing along the north of the Atlantic. Among the prominent genera, besides several of those mentioned under the Arctic Sub-region, are Purpura, Chenopus, Littorina, Gibbula, Natica, Patella, Tectura, Chiton, Doris, Aeolis, Tellina, Thracia.

(3) The Celtic Sub-region includes the British Islands (excepting perhaps the Shetland Islands), the coasts of the North Sea and the Baltic, with N. France to Cape Ushant. The absence of any cold or warm current exerting direct influence upon the coast-line of this sub-region causes a very gradual change in the conditions of life as we move either southward or northward. The fauna of the British seas contains a decided mixture of northern and southern forms. The following are among the common Boreal species which attain their southward range on our coasts: Tectura testudinalis Müll. (to Dublin Bay and Scarborough), Trichotropis borealis Brod. (to the Dogger Bank), Margarita helicina Fabr. (to Yorkshire and Dublin Bay), M. groenlandica Chem. (western Scotland), Natica montacuti Forb. (to Cornwall), Trophon truncatus Str. (to Tenby), Chiton marmoreus Fabr. (to Dublin Bay and Scarborough). Buccinum undatum and Littorina littorea become very scarce on our extreme south-western coasts. Among Lusitanian species which reach our coasts are Gibbula magus L. (to Orkney and Shetland Islands), Phasianella pullus L. (to Caithness), Galerus chinensis L. (to Milford Haven), Galeomma Turtoni Turt. (to Weymouth), Cardium aculeatum L. (to Isle of Man), Solen vagina L. (to north Ireland).