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



* Temora longicornis, Rhizosolenia delicatula, 



* Evadne Nordmannii, R. gracillima. 



* Podon Leuckartii. 



The sign * denotes that species marked thus occur as far into the Baltic as at least 

 to Åland. 



Whether these forms are indigenous in the Baltic or enter there from the Cattegatt by 

 nieans of undercurrents, cannot as yet be dt^cided, as they usually appear simultaneously 

 in tlie Baltic and along the west coast of Sweden. Cerataulinn Bercjonii comes no doubt 

 through the E. Channel, but Rhizosolenia gracillima, although common in the styli-plankton 

 of the Atlantic, does not occur to any extent in the North Sea. The new species Rh. deli- 

 catula occurs in the southern part of the North Sea. 



In the Skagerak, at some distance from the coast, tripos-plankton occurs from the 

 middle of May onwards, associated with some more saline or Atlantic species such as Acartia 

 Clausii, Calanus jinmarchicus and Oithona siinilis or euryhaline ones such as Pseudocalanus 

 elongatus and Evadne spinifera; these become abundant along the coast during the next period. 



Period IV. July to the end of August. 



The prevailing plankton type is now tripos-planhton, but with an admixture of species 

 remaining from the last period, and of southern neritic plankton. 

 Among the organisms we note the following: 



Änimals. Planta. 



Oikopleura dioica, Ceratium tripos, 



Acartia Clausii, C. macroceros, 



Calanus Jinmarchicus, Rhizosolenia gracillima. 



Oithona similis. 

 Paracalanus parvus, 

 Evadne spinifera, 

 Sagitta bipunctata. 



Period V. From the end of August to the end of October. 



The tripos-plankton continues as before, but now the didymus-plankton appears with 

 the three species Chcetoceros curvisetus, C. didymus and C. Schiittii, all at the same time 

 and abundantly. From the beginning of September Centropages typicus was common and 

 together with this copepod appeared also Anomalocera Patersonii, Lahidocera Wollastonii, 

 Acanthometron quadrifolium and Limacina halea, all probably, with the exception perhaps 

 of Labidocera, from the North of Scotland. 



Period Yl. From the end of Octotaer to the end of December. 



The tripos-plankton remains throughout, but in this period Halosphcera appears. 

 A number of northern forms begin to develop, and the southern species, formerly so abundant, 

 decrease in frcquency and die sooner or låter. The currents from the south still continue 



