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Show lOS GEOGRAPHICAL DISTRIBUTION AND [Ch. VII. doubt, however, that the boundaries in this case, both of latitude and longitude, will be found in general well defined. The continuous lines of continents, stretching from north to south, prevent a particular species from belting the globe, and following the direction of the isothermal lines. The inhabitants of the West Indian seas, for example, cannot enter the Pacific without passing round through the inclement climate of Cape Horn. 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 whjch love deep water are arrested by shoals ; others, fitted for shallow seas, cannot migrate across unfathomable abysses. Some few species, however, have an immense range, as the Bulla ape·rta for example, which is found in almost all zones. The habitation of the Bulla st·riata extends from the shores of Egypt to the coasts of England and France, and it recurs again in the seas of Senegal, Brazil, and the West Indies. The Turbo pefr(J]Us inhabits the seas of England, Guadaloupe, and the Cape of Good Hope*, and many instances of a similar kind might be enumerated. The Ianthina fragilis has wandered into almost every sea both tropical and temperate. This '' common oceanic snail" derives its buoyancy from an admirably contrived float, which has enabled it not only to disperse itself so universally, but to become an active agent in disseminating other species which attach themselves, or their ova, to its shell t. It is evident that among the testacea, as in plants and the higher orders of animals, there are species which have a power of enduring a wide range of temperature, whereas others can· not resist a considerable change of climate. Among the fresh- * Fer. Art. Geogr. Phys. Diet. Class. d'Hist. Nat. t Mr. Broderip possesses specimens of Iantltinafragilis, bearing more than one species of barnacle (Pentelasmis), presented to him by Captain King and Lieu· tenant Graves. One of these specimens, taken alive by Captain King far at sen, and a little north of the equator, is so loaded with those cirrhipeds1 and with numerous ova, that all the upper vart of its shell is invisible. Ch.VIJ.] MIGRATIONS OF TESTACEA. 109 water molluscs, and those which breathe air, Ferrussac mentions a few instances of species of almost universal diffusion. 'rhe Helix putris (Succinea putris, Lam.) so common in Europe, where it reaches from Norway to Italy, is also found in Egypt, in the United States, in Newfoundland Jamaica Tranquebar, and, it is even said, in the Marianne 'I sles. 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. Helix aspersa, one of the commonest of our larger land-shells, is found in South America, at the foot of Chimborazo, as also in Cayenne. Some conchologists have conjectured, that it was accidentally imported in some ship; for it is an eatable species, and these animals are capable of retaining life, during long voyages, without air or nourishment *. Mr. Lowe, in a memoir just published in the Cambridge 'rransactions, 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 subgenera 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 • Four inclividuals of a large species of Bulimus, from Valparaiso, were b~ought to E~?'land by Lieutenant Graves, who accompanied Captain King in his late oxped1hon to the Straits of Magellan. They had been packed up in a bo; and enveloped in cotton, two for a space of thirteen, one for seventeen, and : 0 ourth for upwards of ~wenty months; but on being exposed, by Mr. Broderip, the w~rmth of a fire m London, and provided with tepid water and leaves they revlVed, and are now living in_Mr. Loddiges' palm-house. ' |