138 



HYDROIDA II 



Hydrallmaniafalcata has its chief occurrence in the littoral region of the boreal seas, and may 

 exceptionally penetrate deeper down into the abyssal, where it has, however, once or twice been ob- 

 served at 2000 metres depth. The species can move right up into arctic waters, and has even been 

 met with in the Kara Sea; otherwise, its occurrence coincides with the presence of warmer currents 

 in the high arctic areas. It may also penetrate some distance to the southward, as is shown by its 

 more sporadic occurrence in the Bay of Biscay. In the North Atlantic also, it has been found off the 

 east coast of America. It is thus the more surprising that it should never have been recorded from 

 Greenland (fig. LXXIV) as it is one of the most common of all species in Iceland waters; on the west 

 and east coast especially, the finds were so numerous that it was impossible to note them all on tlie 



Fij;. LXXIV. The tlistribution of Hydrallinania falcata in the Northern Atlantic. 

 In the hatched regions the hterature notes a common occurrence. 



chart. On the north coast of Iceland, Hydrallmania falcata is quite sporadic in its occurrence, and is 

 not particularly numerous on llie south coast. Between Iceland and the Faroe Islands, and on the 

 northern side of the latter, it is again fairly numerous, while in the North Sea area and along the 

 coast of Norway it appears as a character form. 



Gen. Thujaria (Fleming). 



Upright colonies with imbedded hydrothecse. The hydrothecae are of bilateral structure; the 

 aperture is furnished with an abcaulinally fixed single opercular plate or membrane, attached to the margin 

 of the hydrotheca, as a rule in a more or less pronounced sinus. The polyp has an abcauliue strongly 

 developed blind sack. 



