Preface. 



The investigation of the large Danish collections of hydroids from the Faroe Islands, Iceland, 

 and Greenland, and the researches into the interesting material brought home from the In go If 

 Expedition, have realised great results. In fact, several points of dispute as to the classification of 

 northern species have been settled. In the first place, the Danish collections contain original specimens 

 of some species which have been described as new several times after being originally recorded. In 

 other cases, the large number of specimens tend to bridge the division between species which have 

 hitherto been looked upon as "good" ones. It is, indeed, a matter of regret that deficiencies of diagnosis 

 and inaccurac}- of design have frequently put obstacles to the recognition of species previously recorded, 

 and that the literature has, consequently, been encumbered with synonyms whicli we should rather 

 have done without. This inconvenience, in fact, enforces the necessity of giving full and exhaustive 

 accounts of e\'er\' single species. The American investigators, indeed, on the pattern of Nutting- 

 have long tried to give brief diagnoses and drawings of all American species. But it must be observed 

 that, because of the impressionist way of drawing, the illustrations are, as a general rule, somewhat 

 wanting in accuracy, and to the brief diagnoses there is the objection that they are often too summary 

 to give exhaustive account of the distinguishing features. On the whole such weak points as appear 

 in the last works of Allman, have gone down to his epigones. Great difficulties, indeed, are in this 

 way given to students of the geographical distribution of the hydroids. No doubt, more species than 

 those which are at present pointed out by literature, are common to the European and the American 

 seas. But in general it proves impossible to form, on the ground of the brief diagnoses, a well-founded 

 opinion as to the virtual qualities of many species. As far as the European species are concerned, we 

 are fortunate in possessing the classic work of Hincks, A History of the British Zoophytes. However, 

 since the appearance of that work, plenty of fresh subjects have been added by descriptions of several 

 species and genera which can liardh' all be maintained, and publications have of late appeared in such 

 abundance that it j^roves difficult to students of this group of animals to find their way through the 

 crowded matter. 



These are the reasons why I have tried to gi\'e new and detailed diagnoses of every single 

 species in question. The diagnoses are essentially founded on the copious material occurring in the 

 Danish collections. Of synonyms I have only selected those of absolute neccssitx-. Detailed accounts 

 of synonyniN- are found — as far as earlier literature is concerned — in the excellent treatises of Be dot, 



Tlie Ingolf-lixpeililion. V. 6. I 



