INVERTEBRATE BOTTOM FAUNA 527 



18 metres of water, and the same is the case with Ophiocten 

 sericeum ; Nymphon rohishmi, which even at depths of 2000 

 metres is the most characteristic pycnogonid of the Norwegian 

 Sea deep basin, can actually thrive at a depth of 6 metres in the 

 arctic littoral zone ; Gorgonocephahis eucneniis occurs in the Nor- 

 wegian Sea deep basin and yet finds itself at home in the arctic 

 littoral zone. Many similar examples could be adduced, but 

 special works on the different groups, indicating the depths at 

 which the various forms have been found, furnish the clearest 

 evidence. The character of the water in different arctic areas 

 must also be taken into consideration. Species which almost in- 

 variably live in water at a temperature below 0° C. will not be 

 met with in shallow depths except where truly polar water pre- 

 dominates ; thus on the west coast of Spitsbergen there are 

 echinoderms found only in deep water, which on the east side 

 occur very much nearer the surface, owing to the fact that on 

 the west side the Gulf Stream makes its influence felt to a con- 

 siderable depth, while on the east coast the water is everywhere 

 polar. I shall return to the influence of warm currents upon 

 animal life in arctic tracts. 



It must not be supposed, however, that the vertical distribu- 

 tion in arctic tracts is entirely devoid of system. No doubt there 

 are a great many forms with a far more extensive distribution than 

 would be possible in the boreal region, still the arctic plateaus 

 shelter numerous forms that do not descend into the Norwegian 

 Sea deep basin, and apparently therefore are unable to thrive in 

 such deep water. In their case it is evidently not temperature 

 but other factors that regulate distribution, and besides it is 

 actually possible to point to a purely littoral arctic fauna, although 

 its representatives are far from numerous. 



Hard bottom as well as soft are to be found in the 

 deeper parts of the arctic plateaus ; where the bottom is of 

 mud it differs from the brownish Globigerina (or Biloculina) 

 ooze of the Norwegian Sea deep basin, being of a grayish 

 colour like what we find in the Norwegian fjords and on 

 the boreal coast banks ; in the Barents Sea, however, we get 

 greenish-gray mud. The arctic mud, like the boreal, contains 

 many foraminifera, though the species differ to a certain extent.^ 



We may divide the species composing the arctic fauna into 



^ The species named by Kiier {Norwegian North Atlantic Expedition, Thalamophora, 

 p. 12) as characteristic of the gray mud in northern arctic areas are : Astrorhiza crassatina, 

 Lagena apiciilata, Ptdvinulina karsteni, Globigerina pacliyderma. Biloculina IcEvis, Globigerina 

 bulloides 3.nd G. pachy derma, Haplopliragmium latidorsatuiii , Truncatidina wullerstorji, Rotalina 

 orbicularis, and Lagena apiculata are common in the Globigerina (or BilocuHna) ooze of the 

 Norwegian Sea deep basin ; some of them belong also to boreal areas. 



