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280 280 DISTRIBUTION [ANIMAL. of West America, since about 60 genera are common io both. Several important genera appear to be restricted to the north temperate zone, which should perhaps form a distinct region. About 30 important genera are confined to. the Judo-Pacific region; and nearly 20 are peculiarly tropical. The Atlantic coasts have few peculiar genera of importance, and the west coast of America hardly any, its difference from the Atlantic fauna on the one side and the Pacific on the other being chiefly specific. There is said to be not a single species common to he east and west coasts of tropical South America ; while the corresponding coasts of North America have more than 50 species in common, and many others so closely representative as to be almost equivalent to identical species. The shells of the Mediterranean were once supposed to be very peculiar, but recent drcdgings have proved that most of them exist also in the Atlantic, and it is now doubted if any are really confined to that sea. A small number (about 70 or 80 species) are identical with Red Sea shells. The marine-shells of Australia and New Zealand are exceedingly unlike those of Britain, exactly the reverse of what obtains among the Crustacea. The influence of temperature on the distribution of Mollusca is very marked, the warmer regions presenting a greater variety of forms, with a greater proportion of large and finely-coloured species. Yet in some cases the largest species are extra-tropical, a striking example being found among the volutes, which abound in tropical seas, yet attain their largest size in New Zealand. In temperate and even in Arctic seas Mollusca arc perhaps as abundant individually as in the tropics, but the number of species and genera is far less, and they are generally dwarfed in size, and of obscure colours. Till a comparatively recent period very few shells had been obtained at a greater depth than 200 fathoms. These were mostly of small size and obscure colours, belonging to groups of wide range and great geological antiquity. Dentalium, Crt/ptodon, Leda, and Area were the most important, and it was generally believed that the zero of Molluscan, if not of all but the very lowest forms of life, would be found at a depth of 300 or 400 fathoms. The recent development of deep-sea dredging has, however, completely changed our ideas on this subject, and the Mollusca as well as most other groups of Invertebrata are found to be capable of existing in the profoundest depths of the ocean. Off the coast of Norway Professor Sars has obtained Mollusca from a depth of 450 fathoms ; and near Florida, Agassiz found them at 500 fathoms. During the cruise of the &quot; Porcupine &quot; Professor Wyville Thomson obtained species of Pleurotoma and Dentalium from the enormous depth of 2500 fathoms in the Bay of Biscay. During the recent voyage of the &quot; Challenger &quot; many other groups have been discovered in the deepest seas. In the North Atlantic, 500 miles west of Teneriffe, three small living Mollusca belonging to the genera Area, Limopsis, and Leda were dredged from a depth of 2740 fathoms; while in the Central Pacific, from a depth of 2425 fathoms, species of Area, Necera, Pleuronectia, Trochus, Fusus, Dentalium, and some others have been obtained. All are small obscurely-colour el forms, resembling Arctic rather than tropical shells, due no doubt to the fact that in these profound depths tha water is permanently at a temperature very little differing from that of melting ice. Equally interesting is the fact ascertained by the naturalists of the &quot; Challenger,&quot; that the waters of the ocean down to the enormous depth of 1500 fathoms abound with true pelagic Mollusca belonging to the orders Pteropoda, Heteropoda, and Gasteropoda, while below this limit they appear to be absent (Woodward s Manual of the Mollusca; &quot; Challenger Reports, &quot; Proc. Royal Society, vol. xxiv. pp. 536, 578 ; Letters of Sir Wyville Thomson, in Nature, vols. 7-10.) Fishes. The distribution of marine fishes appears to agree generally with that of the Mollusca and Crustacea, their greater powers of locomotion, leading to a wider dis persal, being to some extent compensated by the more recent origin of most of the species, genera, and families. There are about 80 families of marine fishes, and no less than 50 of these are almost universally distributed. Several other families range over all tropical seas. About 5 families are found only in the Arctic and temperate seas of both hemispheres, while two (Discoboli and Accipenseridce) are found in the north temperate seas only. The best marked region is undoubtedly (as in the Mollusca and Crustacea) that which extends from the Red Sea and east coast of Africa to the Sandwich Islands and Australia. About ten families are confined to this region (which may be termed the Oriental or Lido-Pacific marine region) ; many genera of other families equally characterize it, while there are even a number of species which range over the greater part of its vast area. On the other hand, nu family of importance seems to be confined to the Atlantic, or to the coasts of Eastern or Western America, the differ ences of these provinces, as of the European and American shores of the Atlantic, being confined to generic and still more largely to specific forms. Many species of fish have enormous ranges, extending from the North Atlantic to Australia, from the Red Sea to the Sandwich Islands, and from New Zealand to South America. Some species range over almost the whole Atlantic Ocean, and a considerable proportion of those inhabiting the Atlantic and Pacific coasts of Central America have been found by Dr Giinthei 1 to be identical. It has long been known that a considerable number of fishes inhabit very deep water, never coming to the surface when alive ; but the researches of the &quot; Challenger &quot; expe dition have added greatly to our knowledge of these curious forms. A large number of genera and species, many of them new, and belonging to ten distinct families, have been obtained in the nets and trawls from a depth of 200 to 2400 fathoms in all the great oceans. These often come up greatly inflated by the expansion of the internal gases ; some were transparent, several were blind, and some had curious phosphorescent organs on the head. These deep- sea forms generally have a wide range. The greater num ber of the new and remarkable forms obtained during the voyage of the &quot; Challenger &quot; belong to the Scopelidae, a family known to inhabit the deep waters of all the warmer seas. The size attained by marine fishes appears to have no relation to latitude or temperature. (Giinther s British Museum Catalogue of Fishes ; Mr J. Murray s &quot; Preliminary Report on Vertebrates collected by the Challenger, &quot; Pro. Roy. Soc., vol. xxiv. p. 537.) Marine Turtles. These reptiles, forming the family Cheloniidcc, are too few in number and too widely distri buted to afford any indications as to oceanic regions. Cdacea. The whales and dolphins form the only group ot truly oceanic Mammalia. They are very widely distri buted, but their classification is too unsettled, and their history too imperfectly known, to throw much light on the general question of oceanic distribution. Two of the families Baloemdce and Balccnopteridaz seem to be con- lined to the cold and temperate seas of both hemispheres. The Catodontidce (comprising the sperm whales), on the other hand, are more especially tropical and sub-tropical. The Hypcroodontidce, or beaked-whales, are widely distri buted in northern, southern, and tropical seas ; while the largest family tli&Delphinida:- are nniversdly distributed. The largest whales inhabit the cold northern and southern seas. (Dr J. E. Gray s British Museum Catalogue of