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HYDROIDA II 



One risk attaching to the insertion of the mentioned boreo-arctic and Lusitanian mixed areas 

 lies on the one hand in the fact that by such further division, the whole arrangement is rendered 

 less easy to survey and handle. If, however, other and more essential advantages were procured by 

 so doing, we should naturally not hesitate to accept the biogeographical sub-areas in question But 

 here a very serious difficulty makes itself felt, more particularly on considering the group of hydroids 

 as a whole, to wit, the question of how far up in the boreal area — sensu laterior — the intermix- 

 ture of southern elements takes place to such a degree that the waters concerned should be regarded 

 as a mixed area. Bv way of illustration we may take the distribution of the families Plumulariida 



boom. 



looo m. 



Fig. XCIII. Finds of Plumulariidae in the Northern Atlantic. 



and Aglaopheniidce, the members of which without exception have their chief occurrence in warmer 

 and more southerly waters, and must in the boreal region be regarded as southern visitors, even though 

 some few of them, such as Nemertesia antennina, and Kirchenpaueria pinnata, may here and there 

 occur in considerable numbers. We should note then, that Plumulariidm (fig. XCIII) which save for 

 one or two exceptions belong to the upper 300 metres, are found - - and found very frequently — in 

 the North Sea and along the west coast of Norway, including the Trondhjem Fjord, round the Faroe 

 Islands, along the east, south and west coasts of Iceland; indeed, several species have even been met 

 with several times on the boreo-arctic Lille Hellefiskebanke in Davis Strait. Judging from the family 

 Plumulariidm then, we come to the result that the Lusitanian element plays a very prominent part 

 in the heart of those areas which we have hitherto designated as the boreal; so much so indeed, that 

 it would be necessary to place the greater part of the tracts concerned in the boreo-lusitanian mixed 

 area if we were to follow Appellofs principles to their logical conclusion. Taking now again the 





