Introduction. 



In the present paper as also in one or two more to be published later on, the Mednsse from 

 the northern Atlantic and adjacent waters will be dealt with. Owing to various casual circumstances 

 the group of the Leptomedusae has been worked up as the first. The geographical area, dealt with 

 in the said works, comprises: the Atlantic Ocean north of Lat. 50" N., the Davis vStrait, the western 

 part of the North vSea, the Norwegian Sea, and the Polar Sea as far eastwards as the Kara Sea. The 

 choice of tlie southern limit is rather gratuitous. The medusa; from the British Channel are going to 

 be mentioned in the "Report on the Danish Oceanographical Expeditions 1908 — 1909 to the Medi- 

 terranean and adjacent Waters", dealing with the fauna of the Mediterranean and the Atlantic from 

 the coast of Morocco to the British Channel. Moreover I am preparing a work on the medusx- of the 

 Danish waters; in the present paper, therefore, I have only included some few summary remarks on 

 the medusa: from that area. 



The present paper is based particularly on the collections in the Zoological Museum of the 

 University of Copenhagen. Besides I have made use of a smaller collection of medusic belonging to 

 the Plankton Department of the Danish Commission for Investigation of the Sea. P"or the admission 

 to that collection I owe my best thanks to Professor C. H. Ostenfeld, the Director of the Plankton 

 Laboratory. Moreover, during a stay in Bergen in the summer of 1916 I studied the exceedingly 

 interesting material of medusse collected during the first cruise of the M. S. "Armauer Hansen" in the 

 Atlantic west of Rockall in 1913; some of the specimens were sent to me to Copenhagen for further 

 investigation. With the permission of Professor A. Brink maun the results of my studies have been 

 included in the present paper. I wish to express my best thanks to Professor Br in km an n for that 

 permission. 



Some years ago I was commissioned to work up the Anthomedusa: and Leptomcdusje from 

 the "Michael Sars" North Atlantic Deep-Sea Expedition 1910. The paper was fini.shcd in 1915, and the 

 printing was nearly accomplished, when the great fire in Bergen on January 15"'' 1916 destro\ed the 

 matter in type; the printing has not yet been resumed, and as certain parts of the material dealt with 

 in that paper are of considerable interest with regard to the problems discussed in the present work, 

 I have made use of that material to such an extent as I found suitable. 



The Ingolf-Expediiioii. \^ 8. ^ 



