182 



HYDROIDA II 



Both species occur in Davis Strait, very commonly on both sides of the o° isotherm, but do not go 

 deeper down from the coastal banks. Both, however, are apparently entirely lacking along the east 

 coast of Greenland south of 76° N. This may be due to paucity of investigations, but it would seem 

 likely that the reason is rather to be sought in the fact that the water, owing to extensive interming- 

 ling of fresh and ice-cold glacier water is poor in suitable forms of nourishment, and unfavourable to 

 the growth of hydroids generally. Altogether, only a very few hydroids have been found along the 

 range in question, the finds in question invariably consisting of but few and small colonies. The 

 apparent absence of the mentioned species in North Iceland waters, on the other hand, would seem 



200 m. 600 m. 1000 m. 2ooom. 



Fig. XCII. Localities of the arctic deep sea species Myriothela phrygia □, and Corymorpha groenlandica o, 

 and of the southern, exotic guests Grammaria confer ta * , Zygophylax biarmata^, Polyplumaria profunda •. 

 Cladocarpus Diana a Sertularella amphorifera ▲, and Sertularia (?) titbuliformis Q- (The red line approxim- 

 ately indicates the limit of the cold area). 



rather to be due to lack of sufficient investigation; it is difficult to find any other explanation. The 

 frequent occurrence of the species round all the remainder of Iceland's coast shows that the range in 

 question must as a whole be included in the boreal area; there is no indication of warmer tendency 

 at any place on the true coastal banks here. The ridge between Iceland and the Faroe Islands, and 

 the waters round the latter down to 600 metres' depth must be taken as belonging to the boreal 

 region, as also the western Scottish waters. The North Sea also is boreal, and the Norwegian coast 

 to the extent shown on the chart. 



Before concluding this characterisation of the boreal area, two other geographical groups of 

 less frequently occurring hydroids should be considered. These are the deeper-living inhabitants of 

 the cold area, represented by Myriothela phrygia and Corymorpha groenlandica, and the exotic warm- 



