ABYSSAL BENTHAL AND PELAGIAL 265 



bottom is even, without holes or cracks, since there are no modeling 

 forces in play. With environmental barriers wanting, the deep-sea 

 fauna is characterized by great uniformity over extremely wide areas, 

 and it was even supposed at one time that it was essentially uniform 

 throughout the oceans. The increase in our knowledge of deep-sea life 

 has shown that the uniformity, though great, is not world-wide. 

 Faunistic differences are less marked in the abyssal benthal and pela- 

 gial than in the lighted benthal or even in the lighted pelagial, and the 

 great number of animals of various groups which are found in the 

 depths of all oceans makes any faunal division of the abyssal oceans 

 more or less vague. It is predominantly the bathypelagic forms which 

 exhibit such wide distributions, but widespread benthic forms are not 

 wanting. The medusae Atolla and Periphylla are known from all seas. 

 Numerous sea urchins of great depths are common to the Atlantic and 

 Indo-Pacific. 28 The deep-sea ostracod Gigantocypris is known from the 

 Atlantic and Indian oceans. Ortmann 29 enumerates 49 species of world- 

 wide, deep-sea, decapod crustaceans belonging both to the pelagic and 

 benthic habitats. A number of abyssal lamellibranchs such as Silenia 

 sarsii and Semele profunda have a very wide distribution. The cepha- 

 lopods Calliteuthis reversa, Mastigoteuthis flam?nea, Toxeuma belone, 

 and others, are known from the Atlantic and Indian oceans; Spirula 

 from the Pacific as well. Of 130 bathypelagic genera of fishes a fourth 

 are known from three oceans and another fourth from two. Numerous 

 species also have the same distribution, as Chimaera monstrosa or 

 Cyclothone microdon, in all seas, and Malacosteus indicus, Cyema 

 atrum, Melanocetus krechi, and others, from the Atlantic and Indian 

 oceans. It is furthermore to be expected that the number of the deep- 

 sea forms known to be widespread will be increased as our knowledge 

 of the deep sea grows. 



There are, on the other hand, faunal differences which may appear 

 even without demonstrable barriers. Of the 272 species taken by the 

 Challenger near Kerguelen, from depths greater than 2300 m., 60% 

 were unknown from other regions. Of 523 species dredged from depths 

 of more than 1800 m. south of the tropic of Capricorn, only 36% were 

 known from other regions. 30 The Sunda Archipelago on one hand and 

 the Gulf of Bengal and Oman on the other exhibit two distinct deep-sea 

 faunae of holothurians, though their genera and families are much the 

 same. 31 The 21 North Atlantic species of sea urchins and the 28 West 

 African forms have only 10 species in common. The east and west 

 Atlantic in the abyssal zone have a total of 74 species of sea urchins, 

 but only 24 are common to both areas. 32 The decapod crustaceans 

 exhibit similar differences in the North and subtropical Atlantic. 32 



