OCEANIC DEPOSITS AND BOTTOM FAUNA 31 



the freezing-point of fresh water, and its range does not exceed 

 7 F., being constant throughout the year at any one locality. 



Sunlight and vegetable organisms are entirely absent. 

 Although most of the groups of marine organisms are present, 

 there is no great wealth either in the number of individuals or 

 species. Many of the species present archaic characters. 

 The inhabitants of the abyssal plain are of exceptional 

 interest. About two-thirds of the ocean floor is covered by 

 2,000 fathoms of water (58'4 per cent, between 2,000 and 3,000 

 fathoms, and 6' 7 over 3,000 fathoms deep), the whole forming 

 a plain over goj million square miles in extent, or nearly half 

 the surface of the earth. The amount of trawling and dredging 

 that has taken place over this extensive area is really very 

 slight, and it must not be forgotten that even when there is 

 evidence that the dredge or trawl has fished successfully on 

 the bottom, species of pelagic habit may be captured acci- 

 dentally while the dredge or trawl is being hauled inboard. 

 Some animals have certainly been captured on the bottom, 

 and these include worms, molluscs, holothurians (sea- 

 cucumbers), starfish, and corals. Fish and Crustacea may 

 have been caught at the bottom or at intermediate depths. In 

 some cases it is easy for an expert to say whether a given fish 

 or crustacean has been accidentally taken while the trawl is 

 being hauled in, but in the other cases it is difficult, if not 

 impossible, to say whether a captured fish or crustacean was 

 taken on the bottom or not. Many lists of so-called deep-sea 

 captures contain the names of species which are unquestionably 

 pelagic and not demersal forms. If the strictest tests be applied, 

 then the number of animals e.g., fish which are certainly 

 found below the 2,ooo-fathom line is, according to present 

 knowledge, quite small. 



Hjort has summarised (1912) the available information in 

 the case of fish. According to him there are only 35 individuals 

 belonging to 21 species and 6 families of fish which have really 

 been captured at these depths. Even of these he is doubtful 

 as to 12 forms, and he would only regard 23 individuals, 15 



