The Testacea, of which so great a variety of species occurs in the sea, are a class of animals of peculiar importance to the geologist; because their remains are found in strata of all ages, and generally in a higher state of preservation than those of other organic beings. Climate has a decided influence on the geographical distribution of species in this class; but as there is much greater uniformity of temperature in the waters of the ocean, than in the atmosphere which invests the land, the diffusion of marine mollusks is on the whole more extensive.
Some forms attain their fullest development in warm latitudes; and are often exclusively confined to the torrid zone, as Nautilus, Harpa, Terebellum, Pyramidella, Delphinula, Aspergillum, Tridacna, Cucullæa, Crassatella, Corbis, Perna, and Plicatula. Other forms are limited to one region of the sea, as the Trigonia to parts of Australia, and the Concholepas to the western coast of South America. The marine species inhabiting the ocean on the opposite sides of the narrow isthmus of Panama, are found to differ almost entirely, as we might have anticipated, since a West Indian mollusk cannot enter the Pacific without coasting round South America, and passing through the inclement climate of Cape Horn. The continuity of the existing lines of continent from north to south, prevents any one species from belting the globe, or from following the direction of the isothermal lines.
Currents also flowing permanently in certain directions, and the influx at certain points of great bodies of fresh water, limit the extension of many species. Those which love deep water are arrested by shoals; others, fitted for shallow seas, cannot migrate across unfathomable abysses. The nature also of the ground has an important influence on the testaceous fauna, both on the land and beneath the waters. Certain species prefer a sandy, others a gravelly, and some a muddy sea-bottom. On the land, limestone is of all rocks the most favourable to the number and propagation of species of the genera Helix, Clausilia, Bulimus, and others. Professor E. Forbes has shown as the result of his labours in dredging in the Ægean Sea, that there are eight well-marked regions of depth, each characterized by its peculiar testaceous fauna. The first of these, called the littoral zone, extends to a depth of two fathoms only; but this narrow belt is inhabited by more than one hundred species. The second region, of which ten fathoms is the inferior limit, is almost equally populous; and a copious list of species is given as characteristic of each region down to the seventh, which lies between the depths of 80 and 105 fathoms, all the inhabited space below this being included in the eighth province, where no less than 65 species of Testacea have been taken. The majority of the shells in this lowest zone are white or transparent. Only two species of Mollusca are common to all the eight regions, namely, Arca lactea and Cerithium lima.[916]
Great range of some provinces and species.—In Europe conchologists distinguish between the arctic fauna, the southern boundary of which corresponds with the isothermal line of 32° F., and the Celtic, which, commencing with that limit as its northern frontier, extends southwards to the mouth of the English Channel and Cape Finisterre, in France. From that point begins the Lusitanian fauna, which, according to the recent observations of Mr. M'Andrew (1852), ranges to the Canary Islands. The Mediterranean province is distinct from all those above enumerated, although it has some species in common with each.
The Indo-Pacific region is by far the most extensive of all. It reaches from the Red Sea and the eastern coast of Africa, to the Indian Archipelago, and adjoining parts of the Pacific Ocean. To the geologist it furnishes a fact of no small interest, by teaching us that one group of living species of mollusca may prevail throughout an area exceeding in magnitude the utmost limits we can as yet assign to any assemblage of contemporaneous fossil species. Mr. Cuming obtained more than a hundred species of shells from the eastern coast of Africa identical with those collected by himself at the Philippines and in the eastern coral islands of the Pacific Ocean, a distance equal to that from pole to pole.[917]
Certain species of the genus Ianthina have a very wide range, being common to seas north and south of the equator. They are all provided with a beautifully contrived float, which renders them buoyant, facilitating their dispersion, and enabling them to become active agents in disseminating other species. Captain King took a specimen of Ianthina fragilis, alive, a little north of the equator, so loaded with barnacles (Pentelasmis) and their ova that the upper part of its shell was invisible. The "Rock Whelk" (Purpura lapillus), a well-known British univalve, inhabits both the North Atlantic and North Pacific.
Helix putris (Succinea putris, Lam.), so common in Europe, where it reaches from Norway to Italy, is also said to occur in the United States and in Newfoundland. As this animal inhabits constantly the borders of pools and streams where there is much moisture, it is not impossible that different water-fowl have been the agents of spreading some of its minute eggs, which may have been entangled in their feathers. The freshwater snail, Lymneus palustris, so abundant in English ponds, ranges uninterruptedly from Europe to Cashmere, and thence to the eastern parts of Asia. Helix aspersa, one of the commonest of our larger land-shells, is found in St. Helena and other distant countries. Some conchologists have conjectured that it was accidentally imported into St. Helena in some ship; for it is an eatable species, and these animals are capable of retaining life during long voyages, without air or nourishment.[918]
Perhaps no species has a better claim to be called cosmopolite than one of our British bivalves, Saxicava rugosa. It is spread over all the north-polar seas, and ranges in one direction through Europe to Senegal, occurring on both sides of the Atlantic; while in another it finds its way into the North Pacific, and thence to the Indian Ocean. Nor do its migrations cease till it reaches the Australian seas.
A British brachiopod, named Terebratula caput-serpentis, is common, according to Professor E. Forbes, to both sides of the North Atlantic, and to the South African and Chinese seas.
Confined range of other species.—Mr. Lowe, in a memoir published in the Cambridge Transactions in 1834, enumerates seventy-one species of land Mollusca, collected by him in the islands of Madeira and Porto Santo, sixty of which belonged to the genus Helix alone, including as sub-genera Bulimus and Achatina, and excluding Vitrina and Clausilia; forty-four of these are new. It is remarkable that very few of the above-mentioned species are common to the neighbouring archipelago of the Canaries; but it is a still more striking fact, that of the sixty species of the three genera above mentioned, thirty-one are natives of Porto Santo; whereas, in Madeira, which contains ten times the superficies, were found but twenty-nine. Of these only four were common to the two islands, which are separated by a distance of only twelve leagues; and two even of these four (namely Helix rhodostoma and H. ventrosa) are species of general diffusion, common to Madeira, the Canaries, and the south of Europe.[919]