12 P. T. CLEVE, PLANKTON-RESEARCHKS IN 1897. 



The chceto-plankton is represented by Chcetoceros decipiens and Phceocystis Pouchetii. 

 As is seen from the report on the North Sea both the cha^toplankton and the northcrn 

 plankton entered the North Sea round the north of Scotland, the water with cht\3to- 

 plankton pushing before it that ^vith arctic and northei-n forms, which could be traced 

 to the south side of Iceland. 



At the end of March a small amount of southern nei'itic plankton sets in, represen- 

 ted by rare specimens of Cerataulina Beiyonii, Chcetoceros curvisetun and C. danicu-s. Of 

 these Ch. curvisetus rearches its maximum from the end of September. 



lll*^ Period. April S*'' to May 29*'\ This period is characterized by the gradual 

 disappeai-ence of the arctic and northern species as well as by the development of the 

 southern neritic plankton and of the tripos-plankton. Still, some apparently northern 

 species, as Chcetoceros constrictus, C. contortits and Leptocylindrus danicus, reach their 

 maximum during this epoch. 



Of southern species Acartia Clausii was found as early as in the beginning of 

 April, the following in the end of April and the beginning of May: 



Oithona similis. 



Evadne Nordmannii. 



Podon Leuckartii. 



Ceratium fusus. 



Rhizosolenia gracillima. 



lyth Period. June 19*'^ to October 9*^'. The arctic and northern forms are now 

 gone, almost completely, and the predorainant plankton-types are the tripos-plankton and 

 the southern neritic plankton. 



In this period the following species occur more or less abundantly: 



A. From the end of May: 



Acartia Clausii rearches its maximum in the middle of August. 

 Oithona similis, commou during the entire period. 

 Evadne Nordmannii, also common during the whole period. 

 Rhizosolenia f/racillima, also common during the whole period. 



B. From the middle and end of June: 

 Paracalanus parvus, the whole period. 

 Tintinnopsis campanula, the whole period. 

 Tintinnus subiilatus rearches its maximum in August. 



C. From August: 



Oihopleura dioica ] 



Centropages typncns 



Isias clavipes 



Sagitta hipunctaia 



Ceratium macroceros \ i • , , 



71 .7. . ,. ( occur through the whole period. 



Pendimum divergens ] *= ^ 



Noctiluca, iniliaris has its maximum in September. 



reach their maximum at the end of the inonth. 



