INVERTEBRATE BOTTOM FAUNA 549 



be seen in glacial deposits, though they no longer live in the 

 neighbourhood. Considerable numbers of the arctic species 

 have succeeded in adapting themselves to the altered conditions, 

 and constitute at the present day a regular portion of the 

 boreal fauna, being at the same time distributed throughout the 

 arctic region ; these are the arctic-boreal forms. 



The present-day fauna of the Norwegian Sea thus consists 

 of two elements of different origin: (i) an endemic arctic 

 element, and {2) a southern element derived from the littoral, 

 sub-littoral, and the deeper parts of the Atlantic and Medi- 

 terranean. Thus we may divide the present-day fauna into 

 groups, as follows : — 



(i) One group consists of two categories of endemic arctic 

 forms, viz. the purely arctic species, and the arctic-boreal species 

 widely distributed throughout both arctic and boreal waters. 

 Both categories existed everywhere in the Norwegian Sea 

 throughout the Glacial Age, but only species of the last-named 

 category have since been able to adapt themselves to the boreal 

 areas. These species, therefore, in contradistinction to the 

 remaining boreal forms, are of genuine arctic descent ; that is to 

 say, when a species occurs normally in both arctic and boreal 

 areas, it is as a rule arctic in its origin. 



The purely arctic species are not generally limited to the 

 arctic region of the Norwegian Sea, but are usually widely 

 distributed over the other arctic seas as well. Very frequently 

 they inhabit all the areas round the pole (European, Asiatic, 

 and American), and are in that case designated cir'ciimpolar 

 species. The arctic-boreal species have precisely the same 

 arctic distribution, but within the boreal region their southern 

 boundaries have strict limitations ; the bulk of them on the 

 European side never leave the Norwegian Sea, being absent 

 from the coast banks south of the British Islands and deeper 

 parts of the Atlantic,^ owing to the physical differences of the 

 sea-water. A great many of the arctic-boreal forms, in boreal 

 areas at any rate, inhabit the littoral or sub-littoral zone along 

 the coasts and in the North Sea, and it is precisely in these 

 zones to the south of the English Channel that the hydro- 

 graphical conditions (and especially the temperature) differ 



^ There are, however, a few exceptions to this rule in the case of archibenthal and abyssal 

 forms, some arctic-boreal deep-water species being distributed throughout the northern Atlantic 

 as far as the Azores, including among others the echinoderms Cribrdla sangidnolenta, Pontastef- 

 tenuispimis var. , and Ophiacantha bidentata. An explanation may perhaps be found in the 

 fact that the temperatures in the deeper boreal areas of the Norwegian Sea and Atlantic are fairly 

 alike and uniform. 



