n6 



HYDROIDA II 



and thus belongs to the hydroids found farthest up in the fjords, where the salinity is greatly reduced 

 by the inflow of river water. The species has also a wide southerly distribution, and is very common 

 even in the Mediterranean. It is a character form in the boreal areas (fig. LXI) where it occurs in 

 quantities along the coasts. On the other hand, its occurrence in strictly arctic waters is very rare 

 and sporadic. From Greenland it has been recorded at several places along the west coast, and will 

 probably prove more frequent here than the data at present to hand would seem to show. 



loom. 6vom. looo/n. looom 



Fig. LXI. The ilistributiou of Dynamena pumila in the Northern Atlantic. 



Gen. Abietinaria (Kirchenpauer). 



Upright colonies with hydrothecae having their opening horizontally or slightly obliquely set; 

 the opening margin lacks distinct teeth, but has often an adcauline sinus in which the large single 

 opercular plate is fixed. The polyp has a well developed abcauline blind sack. 



On comparing this definition with that given by Levinsen for the sub-genus Abietinaria 

 (1913 p. 310) we at once find that there is a fundamental difference, the diagnosis here given being 

 based chiefly on the organisation of the polyp. This gives us a clear mark of distinction as against 

 Diphasia, and necessitates the separation of the groups as two different genera. There can be no 

 doubt that Diphasia, as defined by Levinsen, formed a diphyletic group, and that two separate lines 

 of development lead from the Sertularella to the two groups. The relation in point of opercular formation 

 must here doubtless be taken as a secondary phenomenon of convergence, in comparison with the 

 organisation of the polyp. 



