b r. T. CLEVE. THE PLANKTON OF THE NORTH SEA AND THE SKAGERAK. 



Chcetoeeros borealis, Rhizosolenia semisjnna, 



C. contortus, B. f<etigera, 



C. teres. 



The animals occurred in some few samples only. The following were noted: 



Fritillaria horealis, Pseudocalanus elongatus, 



Caprella septentrionalis, 7'emora longicornis, 



Aeartia longiremis, Cyttarocylis denticulata, 



Centropages hamatus, Ptychocylis acuta, 



Tintinnopsis suhacuta. 



All these aniraals are boreal or arctic. 



At the station Vinga one sample of plankton Avas collected at the depth of 70 ni., 

 where the Avater had the temperature 4,9i and the salinity 34,52. The plankton was of 

 the same kind as in the superficial stratuni, but very poor. 



April 1900. 



A. The North Sea ^vas explored on several routes by steamers, and the results of 

 the examination of the samples of plankton have been registered in Table II. 



It appears from the hydrographical deterrainations that water with 34 — 35 p. m. 

 salinity extends from Scotland towards Skagerak and also E. and S. of the Dogger Bank. 



The plankton of this kind of water is totally diflferent from what it was in the 

 winter. Of the then prevailing tripos-plankton insignificant traces only remain. Now 

 the plankton eonsists chiefly of boreal, more or less neritic species, but intermingled with 

 comparatively rare specimens of a considerable number of southern forms. 



The plankton of the bank-water has a great number of species in commoii with 

 the 34-water, so it is difficnlt to decide what forms characterize the one and other kind 

 of water. In order to determine this question I noted the number of spöts at whieh 

 every species had been found in the one and other kind of water. If, with due consi- 

 deration for the relative number of samples collected in the water of 34 p. m. and 

 in the \vater of lower salinity, one species was found more frequently in one of these 

 kinds of water, I concluded that it belonged properly to that kind. 



The result of this investigation has been set forth in the following lists, in which 

 fi-equently recurring forms have been printed with larger types. 



Southern forms Boreal or arctic forms 



■ ■ Oikopleura dioica, ' '' Calanus hyperhoreiis, 



■ '■ ' Paracalanus jparvus, ■-'■'■'■ Phreorystis Pouchetii, 



-''•■'■-' 1 Codonella ventricosa, ' Gonyaulax spinifera, 



Halospkcera, Chietoceroti brevis, 



Ceratium biicephalum, '' •' C. debi/is, 



