KONGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 32. N:0 8. 7 



The plankton-type which prevails in the east, i. e. from Skagen to 4° E. and 55° N., 

 is the t7'ipos-2ylankton. From the 55th degree to the English Channel southern neritic 

 plankton is the ruling kind. The areas of the water with 33 to 34 p. m. salinity on the 

 hydrographical niap by Pettersson and Ekman contain chiefly tripos-plankton, as far at 

 least as the 55th degree of latitude, but south of it there is southern neritic plankton, which 

 is also the case with the area witli 34 to 35 p. m. salinity, north of the coast of Belgium. 



Some species from the preceding period, such as Oithona similis, Paracalanus parvus, 

 Pseudocalanus elongatus, Ceratium macroceros and Guinardia fiaccida have evidently increased 

 in abundancy, others, such as Evadne Nordmannii, E. spinifera and especially Rhizosolenia 

 styliformis have decreased. Podon intermedius and Oikopleura dioica begin to develop. 



4. The North Sea, November— December 1898. 



The prevailing plankton type is the tripos-plankton, which extends from the north- 

 east of Scotland to the south west of Norway and to Newcastle. From there the limit 

 can be traced to 53° N. 4° E. and from this point to Skagen. The tripos area corresponds 

 thus on the wliole with the area of 35 p. m. water on the hydrographical map by Ekman 

 and Pettersson. As the salinity of the tripos region is higher now than it was in 

 July — August, it is evident that a considerable amount of Atlantic water has entered 

 the North Sea, which also becomes apparent when the composition of the plankton is 

 considered in details. 



We note first that the Atlantic organism Halosphcera, not seen since the spring, 

 occurs now round Scotland, across the North Sea to Norway and Skagen, i. e. above the 

 100 metre plateau. The purely Atlantic species Rhizosolenia styliformis has about the 

 same distribution. Another remarkable feature in the plankton is the abundance of 

 Coscinodiscus concinnus and Biddulphia mobilensis, which occur in the E. Channel and spread 

 thence to Skagen, i. e. above the 50 metre plateau. 



Species of the tripos region in Novembei'. The tripos plankton contains the usual 

 forms, such as Ceratium trijios, C. macroceros (which has considerably increased in frequency), 

 C. furca and C. fusus; but there are besides a number of other forms, partly from the 

 southern and partly from the northern Atlantic, as follows: 



Southern species. Northern species. 



Centropages typicus c, Parathemisto oblivia, 



Microsetella atlantica, Calanus Jinmarchicus, 



Oithona plumifera rr, Metridia hibernica r, 



Paracalanus parvus cc, Pseudocalanus elongatus, 



Acanthochiasma fusiforme, Liinacina balea, 



Acanthometron quadrifolium, Plectophora arachnoides. 

 Gazelletta hexanema (r south of 



the Orkneys), 

 Pyrophacus horologium. 



