90 PAUL LØYNING. M.-N. Kl. 



SUMMARY. 



The Drobak Sound is the narrowest part of the Kristianiatjord and 

 is characterized by strong currents and only slightly 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 in\'estigations, and since 1895 the 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 Acoliciidac being 

 previously described from the Kristianiafjord, viz. Corvphcl/a pclliicidn (A. 

 and H.l, Favoriiiiis braucliialis (Muller) and Cnf/ioiia coi/ciiiiia i A. 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 1919 to the au- 

 tumn 1921, 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 Lophohclia which are partly o\ergrown with hydroids (Tubularia) and 

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

 are very strong around the Drobak bank. 



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

 langs-poll ma}' 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 lieen fully treated. The 

 work is however proceeding also with the other families. 



' -See "Merknad" in the Intro<Uiction. 



