(6) The Argentinian[390] Sub-region extends from about Cape Melo in Patagonia to the neighbourhood of S. Caterina I. in South Brazil (lat. 28° S.). The sub-region stands in the same relation to the Magellanic, on the east coast, as the Peruvian sub-region on the west, but, owing to the influence of the warm Brazil current, which overpowers the colder water of the Falkland branch of the Cape Horn current, it reaches a point much farther south.
The Mollusca are not well known. The prevailing genera appear to be Oliva, Olivancillaria, Voluta, Bullia, Crepidula; Periploma, and Lyonsia.
(7) The Caribbean Sub-region extends from S. Caterina I. in the south to Florida in the north, and includes the shores of the Gulf of Mexico and the whole of the West Indies. The influence of the warm Brazil current (a branch of the South Equatorial) carries the range of the purely tropical species to a point much farther south than is reached by the tropical species on the west coast.
The sub-region is very rich in species, especially on the coral reefs of the Bahamas and N. Cuba, but the exceedingly small tide-fall makes shore collecting somewhat difficult beyond a certain point. The leading genera are Murex, Purpura, Melongena, Latirus, Marginella, Strombus, Triton, Cerithium, Littorina, Nerita, Scalaria; Tellina, Strigilla, Lucina, and Venus. Pleurotomaria, a genus long regarded as extinct, has been dredged alive off Tobago.
As compared with the tropical fauna of the Old World, that of the New World is poor in peculiar genera (compare p.[ 368]). The relations of this sub-region to the West African and the Panamic have been already dealt with (pp. [367] and [372]).
(8) The Transatlantic Sub-region extends from Florida to Cape Cod (see p. [364]). In the north the limits of the sub-region are distinctly marked, in the south Caribbean species intermingle. Gould and Binney, in their Invertebrata of Massachusetts, enumerate 275 species (Cephalopoda, 6; Gasteropoda, 159; Scaphopoda, 2; Pelecypoda, 108), of which 59 (Gasteropoda, 37; Pelecypoda, 22) are British.
Among the characteristic genera are Urosalpinx, Eupleura, Fulgur, Ptychatractus, Nassa, Crepidula; Solenomya, Mactra, Cypricia, Raëta, Astarte, and Yoldia. Our common Littorina littorea appears to have been introduced into Nova Scotian waters in about 1857, no previous trace of it occurring either in literature or shell-heaps. Since then it has spread rapidly into the Gulf of St. Lawrence, and also as far south as Newhaven, and is said to be driving out the indigenous L. palliata from New England shores.[391] The debt has been repaid by the introduction into British waters of the American clam (Venus mercenaria L.), which, according to the Manchester City News of 23rd March 1889, was first observed in the Humber in 1864, and has steadily increased up to the present time, when it bids fair to compete, in those waters, with the familiar Cardium edule.
Characteristics of Abyssal Mollusca.—Large shells appear to be rare in the great ocean depths, and are usually very fragile; even moderately-sized specimens are far from common. The only group in which species occur larger than the usual size is the Nudibranchs, which are represented by at least one form larger than an orange.
It would seem that abyssal molluscs are much less active and energetic than their brethren on the shores. This view is favoured by the looseness of their tissues, which seem ill adapted for prompt and vigorous action. The tenacious character of the mud on the ocean floor must make rapid motion very difficult. The shell itself is usually fragile and delicate, the upper layers of arragonite being thin as compared with shallow-water species, which makes the nacreous layer, when present, appear unusually conspicuous; in many cases the surface is characterised by a peculiar iridescence or sheen. The colour in the shell of deep-sea Mollusca is never very pronounced, and is often absent altogether. Light pink and salmon, pale yellow and brown, are not uncommon. If the colour is in pattern, it is usually in the form of necklaces of spots, which sometimes coalesce into bands. With regard to sculpture, stout knobs and powerfully buttressed varices, such as occur in the tropical Murex and Purpura, are not found in deep-sea species. But the ornamentation is frequently elaborate, and the sculpture rich and varied. There is an especial tendency towards strings of bead-like knobs, revolving striae, and delicate transverse waves, the sculpture being in many cases of a character which tends to strengthen the structure of the shell, like the ridges in corrugated iron.
A remarkable feature in some deep-sea Mollusca is their singular resemblance, in shape, and particularly in the possession of a strong green periostracum, to some of our common fresh-water species. According to Dr. Dall, the cause of this phenomenon is the same in both cases. The fresh-water Mollusca secrete a strong periostracum, in order to protect the shell against the corrosive influence of the carbonic acid gas with which the water is surcharged. The shells of deep-sea Mollusca, living, as they do, in water probably undisturbed by currents of any kind, have to protect themselves against the same eroding influence, and do so in the same way.[392]