It appears from the Mollusca of our Crag formations that at the time of their deposition the temperature of our seas must have been considerably warmer than it is now. Thus in the Crag we find many species and even genera (e.g. Mitra, Fossarus, Triton, Vermetus, Ringicula, Chama) which now occur no farther north than the southern coasts of the Channel, the west of France, and the Mediterranean.
The Baltic, a sea specially liable to violent changes of temperature, with a large admixture of fresh water at its eastern end, appears to possess only about 65 species in all. More than 50 genera occurring on the western coasts of Denmark do not enter the Sound. In the eastern portion of the Baltic marine and fresh-water species live together (p. 12).
(4) The Lusitanian Sub-region extends from Cape Ushant in the north to Cape Juby (lat. 28°) in the south, and includes the whole of the Mediterranean, as well as the Azores, Canaries, and Madeira groups.
The English Channel acts as an effectual barrier to the northward extension of many species; as many as 81 species which occur in western France do not reach British coasts (P. Fischer). At the same time, the western coasts of France are rather intermediate between the two sub-regions than distinctly Lusitanian, for between 50 and 60 Mediterranean genera do not occur on those coasts.
The Mediterranean itself is exceedingly rich in species, about 1200 in all (including deep-water species) being known. A certain number of these belong to tropical genera which here find their northern limit, e.g. Fasciolaria, Cancellaria, Sigaretus, Siliquaria, Chama, Spondylus. Here too occur Carinaria, Lobiger, Oxynoe, Pedicularia, Cypraea, Marginella, Mitra, Dolium, Cassis, Cassidaria, Pisania, Euthria, Vermetus, Argonauta, and many others. A few Celtic and even Boreal species, which occur on the western coasts of Morocco, do not enter the Mediterranean. Among these are Purpura lapillus, Helcion pellucidum, and Tellina balthica. Halia, a rare West African genus akin to Pleurotoma, is found in Cadiz Bay, and the West African Cymbium occurs on the Spanish coasts as far as Malaga.
The Black Sea, whose northern and western coasts are exceedingly cold, is comparatively poor in species. The Sea of Azof is chiefly characterised by forms of Cardium.
(5) The West African Sub-region extends from Cape Juby to a point probably not very far south of lat. 30° S., the cold current which sweeps up from the Pole probably limiting the southward extension of tropical species on this side of Africa, while the warm Mozambique current on the eastern side permits the spread of many Indo-Pacific species almost as far south as the Cape. Owing to its extreme unhealthiness, and the absence of harbours, the sub-region is very little known.
The principal genera are Cymbium, Pleurotoma, Marginella, Terebra, Mitra, Agaronia, Murex, Cancellaria, Purpura, Pseudoliva, Natica, Tellina, Lucina, Tugonia, Schizodesma, and Arca. Studer has enumerated as many as 55 species common to West Africa and the opposite American shores. The north and south equatorial currents, which circulate in this part of the Atlantic, probably transport the larvae from one coast to the other. Purpura coronata Lam., a characteristic West African species, is represented by a well-marked variety in Demerara.
The Mollusca of St. Helena (178 known species) most resemble those of the West Indies, 50 per cent being common, while 30 per cent are common to the Mediterranean. From Ascension Island only 33 species are known, which in their general relations resemble those of St. Helena.[385]
(6) The South African Sub-region extends along the coast from about lat. 30° on the west, to about East London on the east. Mr. G. B. Sowerby enumerates 740 species from ‘South Africa,’ but includes in this list Natal species, which more properly belong to the Indo-Pacific fauna. Of these 740, 323 are peculiar, while 67 also occur in European seas, some being familiar on our own shores. It is remarkable to find in a sub-region separated from ourselves by the whole width of the tropics, such well-known forms as Mangilia costata Don., M. septangularis Mont., Cylichna cylindracea Penn., Pholas dactylus L., Solen marginatus Pult., Cultellus pellucidus Penn., Ceratisolen legumen L., Lutraria oblonga Chem., Tellina fabula Gmel., T. tenuis Da C., Modiolaria discors L., and many others.