90 



FAUL LøYMNf.. M.-N. Kl. 



SUMMARY. 



The Drøbak Sound is the narrowest part of the Kristianiafjord and 

 is characterized by strong currents and only sHghtly pronounced tidal 

 movements. 



With regard to depths the reader is referred to the chart (p 61 on 

 which the numbers indicate the depth in metres. As will be seen, the sound 

 is on the whole shallow, the depth decreasing evenly towards the shores. 



Near the shores the bottom is practically everywhere covered with a 

 growth of algae, in some localities with Zostera. 



The fauna of the sound has for a long time been the object of partly 

 very thorough investigations, and since 1895 die government has enter- 

 tained a biological station at Drobak. No research, however, has had the 

 nudibranch-fauna as its special object, and that part of the fauna was there- 

 fore scarcely known until now, only 3 species of the family Acolididac being 

 previously described from the Kristianiafjord, viz. Coryphclla pclhicida (A. 

 and H.), Favoriiiiis hraiichialis (MCller) and Citthoua conciiDia W. and H.l. 

 The latter I have not found again'. 



My investigations of the nudibranch fauna have been carried on over 

 a period of a little more than 2 years from the summer 19 19 to the au- 

 tumn 1 92 1, and have comprised the area around the Biological Station. 

 The most important finding places for nudibranchs are marked on the chart 

 (Station i — 9I. 



Station i, the Drobak bank, is of special interest having proved to be 

 the locality containing the greatest number of species. It is covered with 

 dead LophoheUa which are pard}- overgrown with hydroids (Tubularia) and 

 red algae (Rhodophyseae). As a result of the shallow depth the currents 

 are very strong around the Drobak bank. 



As an interesting contrast to the Dmbak bank Station 5 in the Ha- 

 langs-poU may be pointed out with smooth water and the bottom covered 

 with Zostera. This station has been a favourite spawning place for several 

 species of aeolids. 



I have on the whole had a rich material of various nudibranchs, of 

 which as yet only the family of Acolididac has been fully treated. The 

 work is however proceeding also with the other families. 



' See "Merknad" in the Introduction. 



