kongl. sv. vet. akademiens handlingae. band 32. n:o 8. 9 



Florö in Norway (winter-time), from 56° 35' N. 20°19'W. (25th of September, C. Vogt), 

 and from 60° N. 7°"W. (the 29th of March 1893, Vanhöfeen). In the year 1898 it was 

 found by Ostenfeld in a sample from 60°12'N. 3° 53' W. (21th of August) and from 

 59° 59' N 5° 56' W. (14th of September). I found it in a collection from 60° 10' N. 

 3° 12' W. (the 6th of September 1898) and from 62° 12' N. 0° 37' E. (the 8th of September 

 1899). The abundancy and sudden appearance of this species över a wide area of the 

 Skagerak in 1898 proves better Ihan anything else that the plankton of the Skagerak is 

 of foreign origin. It is worthy of note that another species, not formerly observed in 

 the Skagerak, was found at the same time viz. Dictyocysta elegans, a species that is not 

 rare in the temperate Atlantic nor in the Färöe Channel. 



Period II. February — March and April, 



This period is characterized by the abundance of arctic and northern species {Si, 

 T, Ns). They had already appeared in the previous period, but did not reach such a 

 considerable development as now. In connection with these northern forms Chcetoceros 

 decipiens also appears, a species which, as stated above, was the dominant one in the 

 North Sea in March. 



The organisms which characterize this period are principally diatoms, animals and 

 dinoflagellates being as a rule rare. The diatoms belong to the boreal and arctic regions 

 of the Atlantic. Most of the southern species, which were abundant in the autumn, 

 have disappeared or are scarce, among them Halösphcera. 



The most decided arctic species, such as Thalassiosira Nordenskiöldii, Chcetoceros soci- 

 alis, continue during February only and have already disappeared by the middle of March. 

 Others, such as Ch(Btoceros dehilis, C. diadema, Nitzschia seriata, continue during April. The 

 most long-lived ones, such as Thalassiothrix Frauenfeldii, Leptocy lindrus danicus, Rhizosolenia 

 seinispina, Chcetoceros constrictus, C. coyitortus, C. hiemalis do not disappear before the end 

 of May or in June, by which time a number of southern species have become frequent. 



The most common diatoms during this period are the following: 



Chcetoceros constrictus, Chcetoceros socialis, 



C. contortus, Coscinodiscus oculus iridis, 



C. debilis, Thalassiosira gravida, 



C. decipiens, T. Nordenskiöldii, 



C. diadema, T. Frauenfeldii. 



Period III. May and June. 



This period is remarkable for the abundance of euryhaline species, such as the following: 



Änimals. Plants. 



* Acartia longiremis, Cerataulina Bergonii, 



* Centropages hamatus, * Chcetoceros danicus, 



K. Sv. Vet. Akad. Handl. Band 32. N:o 8. 2 



