HYDROIDA 



39 



The species may be divided into two forms: 



Forma genuina, growing, in colonies more openly constructed, on stones and shells (^Mytilus); 

 it is delicately built and bluish or rose-coloured. 



Forma sqiian/afa forming clusters of polyps more brick-coloured or yellowish-red on the leaves 

 of Fucoideac\ its polyps are large and robustly built. 



Material: 



Iceland : Reykjavik 



Vestmaimey 

 The Faroe Islands: Sundelaget 



north of Kvalvik 



depth 3 — 4 fathoms, 

 on the shore (on Fucoidcae) 



on the shore (on Fucoideae) 



(There also occur specimens marked "Faero" without particular data). 



Clava iiin/ficoniis and Clava sqiiaiiiafa are recorded by most investigators of hydroids as two 

 separate species; the distinguishing characters, however, are rather vague, being made out by the 

 closer or opener occurrence of the polyps in the colony or by the colour of the colonies. A copious 

 material from various localities, in fact, presents all transitions possible, and it is virtually impossible 

 to draw any certain limit between the species. A closer inquiry soon makes clear to us that the 

 points of difference must be of biological nature, and the two species, therefore, have to be regarded 

 only as biologically determined "form.s" of a single species, which I, accordingly, denominate forma 

 genuina and forma sqiiiiiata. The occurrence of typical colonies of the forma gemiina, whicli arc, in 



Text-fig. K. The distribution of Clava midticornis in the Northern .\tlantic. 

 (In the hatched part of the Norwegian coastal region the occurrence is rather scarce). 



