HYDROIDA II 



183 



water species of the Atlantic, found here and there in the waters investigated (fig. XCII). Myriothela 

 phrygia is apparently an easterly arctic species which penetrates out to the southern limit of the 

 cold area; it has in a single instance been taken on the southern, deep slope of the Wyville-Thomson 

 Ridge, brought thither probably by larval transportation. Corymorpha grocnlandica moves out every- 

 where to the boundary, and has once passed beyond it, in Davis Strait, on the Store Hellefiskebanke, 

 and between Iceland and the Faroe Islands, probably likewise as a result of larval transportation. 

 Both species are, moreover, met with in a single isolated spot far up in the boreal region, to wit, iu 

 Trondhjem Fjord, where they must probably be regarded as relicts — The opposite group, the exo- 

 tics, will be represented by Graviiiian'a confcrta, Zygophylax biarmata^ Polyplumaria profunda, and 

 ScrtnlareUa amphorifera. One or two of them may in southern localities exhibit a shallower occur- 

 rence; here in the north, however, save for a single find of PolypluDiaria profunda up in Danmark 

 Strait, they are not met with bejond the 600 metre curve, nor within the true boreal region. 



A study of the hydroids leads then to the result, as seen from the instances given, that the 

 limits of the boreal zone as against northerly or arctic waters must be drawn along the 0° isotherm; 

 to the south b\' the southern side of the ridges along the 600 metre curve. The boreal region ex- 

 tends thus from far ujj in Davis Strait, where, however, it extends only in a very few places in to 

 the coast of West Greenland, across the highest part of the submarine ridge into Danmark Strait, 

 round the coasts of Iceland, including also the plateau between Iceland and the Faroe Islands, the 

 Faroe Islands Banks, the western Scottish waters, and the North Sea and the Norwegian coast pla- 

 teaus (vide charts). Practicalh' speaking, the region embraces all the coast banks of the waters in- 

 vestigated, down to 600 metres, save for the upper 200 metres of the Greenland banks. 



On comparing the results here arrived at with the Hues laid down in Appel lot's' and v. 

 Hofsten's^, 3 fundamental works on Crustacea and Echinoderms, we find that they agree very closely 

 therewith. We have, however, here disregarded the mixed areas, which in the works referred to are 

 strongly emphasised. The "boreo-arctic" area, where arctic and boreal elements appear in equal pro- 

 portions, should strictly speaking doubtless comprise the upper parts of Store and Lille Hellefiske- 

 banke, possibly also F}lla Banke, West Greenland, while the north Icelandic coastal region likewise 

 seems to belong to the same. The .second mixed area, which according to A p pell of includes the 

 southern parts of the North Sea, the English Channel, and the Irish Channel, should be characterised 

 by a very marked intermixture of southern, Lusitaniau or Mediterranean species. 



It has previously been pointed out that hydroids only penetrate in small numbers down beyond 

 600 metres depth. It might therefore be objected that the southern boundary of the boreal region as 

 against the Atlantic will in the case of other animal groups be found to follow a deeper curve than 

 here indicated. In comparison therefore, we should also include arctic-boreal species of other animal 

 groups which are more eurybathic. It will here suffice to refer to v. H of s ten's charts for Ophiocten 

 sericeuni and Spiroiitocaris Gaimardii; they show the same features as the corresponding hydroid spe- 

 cies, and the same southern limits for the boreal region at about 600 metres' depth. 



I 1906, Die dekapoden Crustaceen. .Meeresfauna von Bergen, Heft 2 und 3. Bergen. 



= 1915, Die Echinoderaien des Eisfjordes. Zoologische Ergebnisse der schwedischen Expedition nach Spitzbergen 

 190S. Teil II. Stockholm. 



3 1916, Die decapoden Crustaceen des Eisfjordes. Ibid. Teil II. Stockholm. 



