362 RECENT EXPLORING EXPEDITIONS chap. 



number of genera as ' shallow-water ' and ' deep-water ' respec- 

 tively, while a still larger number occupy an intermediate 

 position. Among shallow-water genera may be named Patella^ 

 Littorina^ Nassa^ Purpura^ Strombus, Haliotis^ Mytilus, Oardium, 

 Solen; while among deep-water genera are Pleurotoma^ Scissw- 

 rella^ Seguenzia^ Dentalium, Oadulus, Limopsis^ Nucula^ Leda^ 

 Lima^ and Axinus. 



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 Golf e 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 Medi- 

 terranean. 



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.^ To give a few instances. 

 The Challenger dredged Silenia Sarsii in 1950 fath., 1100 



1 A break in this uniformity may be found underneath the course of a great 

 oceanic current like the Gulf Stream, which rains upon the bottom a large amount 

 of food. A. Agassiz {Bull. Miis. C. Z. Harv. xxi. p. 185 f.) explains in this way 

 the richness of the fauna of the Gulf of Mexico as compared with that of the 

 west coast of tropical America. 



