We shall now survey the principal divisions of the ocean; the line of demarcation being drawn, not by latitude or longitude, but by genera and species.
The Mollusca of the Arctic seas are well known to show considerable analogy with those of the later Tertiary periods of Europe. Hence the great interest connected with their comparison, as it affords,—provided we are satisfied with this line of argument,—a proof that an Arctic climate formerly existed in temperate regions. It is the northern Drift of which we are speaking. Even when species are found living in Britain identical with those of the Arctic regions, still there is often a difference in the form or size of British and Arctic specimens; certain species, such as Cyprina Islandica, being comparatively small in the south of Britain, larger in Shetland, and attaining their greatest size in Iceland.
The countries included in the Arctic molluscan province are Lapland, Iceland, Greenland, the west coast of Davis' Straits, and Behring's Straits. About two hundred species are enumerated by the various Arctic voyagers, as found in these seas; of these about one-half are peculiar to these seas, and the other half are either found living in the temperate regions of Europe, or in their so-called glacial strata.
The Boreal province includes the North Atlantic, from Nova Scotia to Iceland, and from thence to Faroe, Shetland, and the Norway coast.
The number of species is very large; and more than one-half are common both to Scandinavia and the North American coast, while a great number also are found on the British coast.
The province called Celtic by Professor Edward Forbes embraces the coasts of Britain, Sweden, and Denmark.
Our British mollusca are about seven hundred in number; those bearing shells are above five hundred. Of these about thirty are peculiar to Britain. The shells of the Baltic are identical with those of this province.
The Lusitanian province stretches from Madeira and the Canaries to the coasts of Spain and Portugal, and includes also the Mediterranean. But as one might expect, on close examination, the Mollusca in so large an area differ so widely that we are forced to admit the existence of great divisions.
The number of species found on the coast of Madeira by Mr. McAndrew was one hundred and fifty-six, of which forty-four per cent. were identical with British species, and eighty-three found near the Canaries.