28 Hjalmar Ostergren. [No. 9 



Cape God, 42^ K, as has been stated (cf. G. 0. Sårs, 1879), con- 

 stitutes a ratlier sharp border between this and the boreal.^) That 

 many species of Holothurians (Cucumaria minuta, C. calcigera, 

 Fsoliis fabricii, Chiridota lævis), which on the European side appear 

 as purely arctic, here go down to a latitude of 46°— 42° N., is 

 thus only a confirmation of the recogiiized fact, that the south border 

 of the arctic coast fauna is on the American east coast situated 

 at lat. 42° N. Such species (to them betongs, as above seen, also 

 Myriotrochus rinJcii) do not therefore cease to be purely arctic. 



If one, like LuDwia, makes the Arctic circle the border of the 

 arctic fauna, then most of the arctic species will also become boreal. 

 The really existing differences between the arctic and the boreal 

 fauna will thus not at all be rightly represented.-j 



On the contrary, if one when limiting the region of the arctic 

 fauna takes hydrography and the sea fauna itself into consideration, 

 one comes to quite a different result. The species of Holothurians 

 common to the arctic and boreal fauna are comparatively few. Each 

 region is characterized by its special fauna. ' Even if the isolated 

 cases of the occurrence of representatives of one fauna within the 

 region of the other are not tåken into account, the limit is certainly 

 not sharply delined. But, in any case, w^e have no right to fix 

 the Arctic circle or any other latitude as a boundary line, for this 

 will only bring confusion into the science about the distribution of 

 the animals. 



Unless one has a very detailed knowledge about the components 

 of the fauna, it is better to let the limit be decided by the distri- 



1) Compare also Ortmann, 1896 p. 48. 



-) Still more sharply, than in the case of the Holothurians, this is shown 

 in LuDWiG's treatise on Asteriodea in „Fauna Arctica". There all species, which 

 live on both sides of the Arctic circle, are represented as both arctic and boreal 

 (subarctic), even e. g. Brisivga coroyiata^ which has its northern border in the 

 prononnced boreal district at Lofoten, where at greater depths the temperature 

 never goes lower than -]- 6^ C. In this manner nearly every species, which is 

 spread over a little wider area in the North Atlantic, becomes a member of both 

 these categories. Of course, Lugwig has not overlooked that great differences, 

 regarding distribution, between all these species are met Avith. He di vides them 

 therefore into chiefly subarctic and chiefiy arctic. But as he continues to take the 

 latitude and not the conditions of the sea into consideration, the genuine arctic 

 Pteraster (Hexaster) ohsciirus, for instance, which had been more often found, 

 still at that time, on the Newfoundland Bank than north of the Arctic circle, 

 is reckoned to the class of chieflv subarctic. 



