

194 



HYDROIDA II 



and again on to the deepest parts of the Faroe Bank, where they rise from the cold area at the 

 eastern slopes of the ground. 



The community which is characterised by the last-named species, and which further includes 

 Lafcea fruticosa forma grandis, Lafaa gracillima forma elegantula, and Scrtularella polyzonias forma 

 gigantea, has its home throughout the whole of the arctic regions; farther to the north, we find it in 

 shallower water, in the southern parts of the cold area in deeper; here also, it penetrates out to the 

 Wyville Thomson ridge in the Faroe Channel, where warm atlantic water-masses present a barrier 

 to its further progress. Only a single find of Myriothela phrygia has been made on the southern slope 

 of the Wyville Thomson ridge. These species thus characterise the Norwegian Sea Deep, and show 

 the marked difference between it and the Atlantic deep sea region. The limit of their occurrence here 

 in the south follows on the whole very closely the o isotherm. 



True these species may at times, in higher latitudes, occur in water layers of higher tempe- 

 rature, but this can have no effect on the limitation of the cold area as a whole against the boreal 

 and atlantic tracts. A biogeographical peculiarity is seen in the Trondhjem Fjord, where hydrogra- 

 phical conditions of an atlantic character prevail, nevertheless we here find Myriothela phrygia, Tu- 

 bularia regalis, Corymorpha groenlandica, and Stegopoma plicatile thriving excellently; indeed, Tubula- 

 ria regalis, and Stegopoma plicatile even appear as local character forms in the otherwise purely atlantic 

 Lophohelia-biocoenoze. This can apparently only be explained by regarding the species as relicts in 

 the fjord, which have been able to adapt themselves to altered conditions there. This exception, how- 

 ever, cannot alter their general character as arctic species. 



On the other hand, we find in the material also species which are not able to penetrate beyond 

 the limits of the warm atlantic water-layers, and which are thus entirely lacking in the boreal region. 

 These species emphasise still further the marked difference between the fauna of the Atlantic abyssal 

 region and the cold area. We may here in particular point out Grammaria conferta, Zygophylax 

 biarmata, Polyplumaria profunda, and Sertularella amphorifera, typical representatives of the warm 

 atlantic deep-sea fauna. The finding of these so far to the north shows us on the one hand how 

 uniform the fauna must be throughout the deep region of the Atlantic, but also, on the other hand, 

 how little we yet know as to the geographical distribution of the bottom fauna, and what great and 

 tempting tasks still await the investigator in thoroughly elucidating the question of bottom fauna 

 even here in the North Atlantic. 



