HYDROIDA 



Materiaux pour servir a I'histoiie des Hydro'ides, and — regarding recent literature on northern 

 hydroids — in the groupings framed by Jaderholm (1909), Broch (1909), and Kramp (1914). As far 

 as possible, the various species are accompanied by maps illustrative of the geographical data in the 

 northern Atlantic. Besides the collections the recent literature has served as basis. In this respect 

 the groupings occurring in the works of Bonnevie (1899), Jaderholm (1909), Broch (1909), Sce- 

 mundsson (1911), and Kramp (1914), have proved particularly available. 



I have made it a point to define precisely the limits of genera and families by full diagnoses, 

 and at the same time I have tried to account for the leading principles I hold to for the purpose of 

 division and classification. The various authors have maintained various opinions as to the systematic 

 principles of classification ; many of them have disregarded phylogeny and allowed biological considera- 

 tions to play a predominant part; consequently the circumscription of genera has been practised in 

 most various ways. To leave no opening for misunderstanding I ha\e thought it necessary to give 

 a detailed account of each species in question, even if the work should be delayed and grow a little 

 broader than I wanted. However, I hope that in this way the extensive and methodical Danish col- 

 lections will tell better and to fuller advantage than otherwise. 



Trondhjevi the 22nd Aitgii-sf IQ15. 



