Introduction, 



The present work has been prepared on the basis of the very complete collec- 

 tions of northern and arctic mollusca, which the Swedish State Museum has acquired 

 during the process of time and which have hitherto been only partially dealt with 

 by scientists. The geographical area from which they have been gathered comprises 

 the following waters: the Baltic Sea, the Kattegatt, the Skagerrack and the North 

 Sea, the Atlantic and the Arctic Oceans off the coast of Norway, the Kara and the 

 White Seas, the Arctic Ocean off Siberia and to some extent the Behring Sea, the 

 Coasts of Spitzbergen, Iceland and Greenland, and the northern parts of the Atlantic 

 together with Davis Strait and Baffin Bay. 



A survey of the expeditions which brought the collections home is given by 

 Professor Hj. Theel in K. Sv. Vet.-Akacl. Handl. Bd. 39, H. 1 ; that survey is here 

 referred to when dates are annexed to the localities given. 



In order to complete the fauna of that area those forms are also included, 

 which are found within it but which are not represented in the Museum collections. 

 All these latter are printed in brevier type for the sake of distinction. The speci- 

 mens that I have myself seen, that is to say the collections in the State Museum 

 and those in the Museums at Uppsala and Gothenburg, are put under the heading 

 of » Habitat ». The localities of »General Distribution» are quoted from several 

 works; the works quoted from are arranged chronologically in an enclosed survey of 

 literature. 



In the first part of this paper there is given a systematic synopsis of the 

 forms. This is intended to constitute a graphic representation of their reciprocal 

 relationship, so far as this has been hitherto established, and to give a clue for the 

 determination of forms met with. The system is worked out with the guidance of 

 Fischer's, Bergh's and Pelseneer's works on Opistobranchs. 



To synonyms no very close study has been devoted, only the most essential 

 ones have been mentioned. 



The index contains all the names met with in the paper, arranged in alpha- 

 betical order, and, in addition, the generic names, under which the species are de- 

 scribed. 



