32 P. T. CLEVE. THE PLANKTON OF THE NORTH SEA, THE ENGLISH CHANNEL, AND THE SKAGERAK. 



C. Fol'ms tliat occur in the spring above the 100 metre plateau and In the snramer or 



autumu above the 50 metre plateau. 



1. Occurring in the summer above the 50 metre plateau: 



Southern : Northern : 



Åcartia Clausii, Acartia longiremis, 



Oithona similis, Pseiidocalanus elongatus, 



Sagitta bipunctata. Temora longicornis. 



2. Occurring in the winter above the 50 metre plateau: 



Southern: Northern: 



Biddulphia mohilensis, Chcetoceros decipiens, 



Coscinodiscus concinnus, C. diadema. 

 Streptotheca thamesis. 



Those of the 1st group have probably migrated from the north towards the south. 

 The forms of the 2nd group arrived probably through the E. Channel. 



D. Forms, wliich occur cMefly In the eastern part of the North Sea. 



1. Winter and spring: 



Fritillaria horealis, Coscinodiscus oculus iridis, 



Parathemisto oblivia, Leptocylindrus danicus, 



Biddulphia aurita, JSfitzscliia seriata, 



Chcetoceros atlanticus, Thalassiosira Nordenskiöldii, 



C. constrictus, Thalassiothrix Frauenfeldii, 



C. criophilus, T. longissima. 



C. curvisetus, 

 C. scolopendra, 

 C. socialis, 

 C. teres. 



All, with the exception of Chcetoceros curvisetus, are northern species and it may 

 be assumed that they arrived in the winter through the deep Norwegian fissure or in 

 the spring from the Skagerak. 



2. Summer: 



Acanthometron elasticum, 

 Litholophus ligurinus, 

 Bacteriastrum delicatulum. 



All are southern. There was found also Tintinnus minutus, which belongs to 

 arctic regions. 



