528 DEPTHS OF THE OCEAN 



Purely arctic three catcgories. The first category may be termed purely 

 forms. arctic, occurring in water having a low temperature all^the year 



round. ^ Allowing for slight variations it is safe to assert 

 that the majority of them require a temperature considerably 

 below what prevails in the deeper parts of the boreal region 

 (6° to 7' C), though a few coast and shallow-water forms are able 

 to exist at higher temperatures for a short portion of the year ; 

 this is particularly the case with those arctic forms that come 

 as far south as the Lofoten, Murman, and Finmark coasts. 

 Still even within the purely arctic areas we find faunal differ- 

 ences that are due to temperature. Some forms are never, or 

 very rarely, found in water having a temperature above o° C, 

 others appear to thrive impartially throughout the whole arctic 

 region in whatever temperatures prevail, while others again 



avoid the coldest water and 

 keep as much as possible to 

 temperatures slightly above 

 o° C. 



As regards horizontal dis- 

 tribution within the arctic 

 region we may assume that 

 most of the species are wide- 

 spread, even if they have not 

 yet been met with everywhere, 

 ^^^°- 37>3- for we are still only imper- 



1 oldia arctica. Gray. (After Stuxbers;. ) r i • i • i i 



tectly acquamted with the 

 fauna over a large portion of the arctic plateaus, especially that 

 off East Greenland. Some species, however, will undoubtedly 

 prove to be more or less local, judging from what we have 

 found in the boreal region. 



A few of the larger forms that characterise the arctic coasts 

 •and plateaus are given in the following list : " — 



Molluscs : Margarita cinerea, Onchidiopsis glacialis, Nat tea clausa, 

 Amauropsis islandica (rarely found on the Norwegian west coast), Nep- 

 tunea despecta, SipJio curtus, S. turgidulus, S. kfbyeri, S. glaber, Buccinum 

 glaciale, B. hydrophanuin, B. grdnlandicum, and a few other species of 

 Buccinum^ species of Beta, Sipho7todentalium vitreuni. Nucula tenuis var. 

 expansa, Yoldia hyperborea, Y. {Portlandia) arctica (see Fig. 370) and Y. 

 limatula, Area glacialis, Pecten gronlandicus, P. islandicus, Astarte 

 {Nicanid) banksi van, A. borealis, and A. crebricostata, Axinopsis 

 orbiculata, Axinus gquldi, Tellina calcarea (rarely found alive on the 

 Norwegian west coast, though extremely abundant in the arctic region), 



1 There are a few exceptions, for instance, Pecten islaitdkus, Ctenodiscus crispatus, 

 Onchidiopsis glacialis, which are more boreo-arctic than arctic (see p. 534). 



^ In this list I deal only with the molluscs, echinoderms, crustaceans, and ascidians. 



