Theories on the geographical distribution of marine Mollusca have been revolutionised by the discoveries of recent exploring expeditions. The principal have been those of Torell (Swedish) (1859–61) on the coasts of Greenland and Spitzbergen; of the _Lightning_ and _Porcupine_ (British) in 1868–70, in the N.E. Atlantic, off the Scotch, Irish, French, and Portuguese coasts, and in the Mediterranean; of the _Challenger_ (British), under Sir C. Wyville Thomson, in 1873–76, in which all the great ocean basins were dredged or sounded; of the _Blake_ (American), under Alexander Agassiz, in 1877–80, in the West Atlantic, Gulf of Mexico, and Caribbean Seas; of the _Travailleur_ (French) in 1880–83, off the west coasts of France, Portugal, and Morocco, Madeira, the Canaries, and the Golfe du Lion; of the _Talisman_ (French) in 1882, off the west coast of Africa from Tangier to Senegal, the Atlantic Islands, and the Sargasso Sea; of the _Albatross_ (American) in 1891, off the west coast of tropical America; of several other vessels belonging to the U.S. Fish Commission and Coast Survey, off east American shores; and of the Prince of Monaco in the _Hirondelle_ and _Princesse Alice_ at the present time, in the N. Atlantic and Mediterranean. The general result of these explorations has been to show that the marine fauna of very deep water is much the same all the world over, and that identical species occur at points as far removed as possible from one another. The ocean floor, in fact, with its uniform similarity of temperature, food, station, and general conditions of life, contains no effectual barrier to the almost indefinite spread of species.[383] To give a few instances. The _Challenger_ dredged _Silenia Sarsii_ in 1950 fath., 1100 miles south-west of Australia, and also in 2650 fath. off the mouth of the Rio de la Plata; _Semele profundorum_ in 1125 fath. near the Canaries, and in 2900 fath. mid N. Pacific; _Verticordia deshayesiana_ in 155 fath. near Cape York, and in 350 fath. off Pernambuco; _Arca pteroessa_ in 2050 fath. mid N. Pacific, in 1000–1675 fath. west of the Azores, and in 390 fath. off the West Indies; _Arca corpulenta_ in 1400 fath. off N.E. Australia, in 2425 fath. mid-Pacific, and in 1375 fath. near Juan Fernandez; _Lima goliath_ in 775 fath. off S. Japan, and in 245 fath. off S. Patagonia; _Pleurotoma engonia_ in 700 fath. north-east of New Zealand, and in 345 fath. off Inoshima. A surprising range was occasionally found even in shallow-water species; thus _Petricola lapicida_ was discovered by the same expedition in the West Indies and N. Australia, _Cardita calyculata_ off Teneriffe and in Bass Strait, _Arca imbricata_ off Cape York and in the West Indies, _Modiolaria cuneata_ at Port Jackson and Cape of Good Hope, _Lima squamosa_ at Teneriffe and the Philippines. In these latter cases it is not improbable that the species lives in deep water as well, from which it has not yet been dredged.