KONGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 32. N:0 8. 11 



to convey into the Skagcrak such forms as Chcetoceros danicus, Ditylum Brightwellii, 

 Guinardia flaccida and Proto j)edata, which latter is not rare in December. 

 The northern forms are principally diatoms, such as: 



Chcetoceros borealis, Chcetoceros similis, 



C. constrictus, C. teres, 



C. contortus, Coscinodiscus polychordus, 



C. dehilis, Rhizosolenia setigera, 



C. diadema, Skeletoneina costatum, 



C. laciniosus, Thalassiosira gelatinosa, 



C. scolopendra, Thalassiothrix Frauenfeldii. 



The seasonal changes in the plankton of the Skagerak were in 1898 07i the whole 

 similar to those in 1895, 1896 and 1897. 



January Tp, Ns, JSfh, 



February— April Si, Ns, (T), C, 



May, June Nm «,^ 



July, August Tp, Nm or, 



September, October . . . J)?, Nm, 



November, December . . Tp, Ns, Nm. 



Seasonal distribution of the plankton-organisms in the 



North Sea and the Skagerak. 



Åscldiacea. 



Fritillaria borealis Lohm. — January: rare in the Skagerak. Marek: rare at Måseskär. 



Oikoplenra dioica Fol. — January: rare in the Skagerak and at Måseskär. March: 

 in the North Sea at 57° 18' N. 8° 27' E. and 5;)°43'N. 7° 29' E. June: sparingly in the 

 Skagerak. July, August: from the E. Channel to west of Skagen, common in the Skagerak 

 and along the coast, remains more or less abundant to November. 



Ampliipoda. 



Parathemisto oblivia Kköyer. — September: rare iu the Skagerak. November: rare 

 SW. of Norway (58° 12'N. 5° 30' E.). 



^ I propose to designate as Nm a the plankton in which Rhizosolenia gracillima occurs abundantly. 



