10 



P. T- CLEVE, THE PLANKTON OF THE NORTH SEA, ENGLISH CHANNEL, ETC. 



Period IV. From October 20 th to December 24 th 1898. Some species from the period 

 I I. reappeared, as C/urtoceros densus, C. didymus, Eueampia zodiacus, but as a new and 

 important constituent of the plankton Rhizosolenia Stolterfothii occurred, and, in its company, 

 a number of southern forms, such as Euterpe acutifrons, Bacteria stmm varians, Belle- 

 rochea mallens and, in very great abundance, Cluetoceros curvisetus. 



Some boreal forms occurred rarely, as Biddulphia aurita, Chcetoceros decipiens, 

 Coscinodiscus oculus iridis and Thalassiosira gravida, showing a slight influence of water 

 from arctic regions. 



Period V. From January l st to Marek 15 th 1899. Chcetoceros curvisetus and C. didymus 

 continued to be common, and some other southern forms also remained, although more 

 or less scarce, as Chcetoceros densus, Eueampia, zodiacus, Rhizosolenia Shrubsolei and R. 

 Stolterfothii. On the contrary Bacteriastrum varians and Bellerochea had disappeared. 

 The following new, non-aretie, species appeared: 



Biddulphia mobilensis. Ditylum Brightirellii. 



Chcetoceros danicus. Streptotheca thamesis. 



Besides, there occurred also Coscinodiscus centralis (probably a variety of C. 

 concinnus). 



This period is particularly characterized by a number of arctic or boreal forms, 

 some of which appeared already in the preceding period, but very rarely. Such northern 

 forms are : 



( 'entropages hamatus. Chcetoceros teres. 



Temora longicornis. Coscinodiscus oculus iridis. 



Pseudocalanus elongatus. C. radiatus. 



Phceocystis Pouchetii. Rhizosolenia setigera. 



Asterionella japonica. Skeletonema costatum. 



Biddulphia aurita. Thalassiosira gelatinosa. 



Chcetoceros boreaiis. T. gravida. 



C. decipiens. Thalassiotlirix Frauenfeldii. 



VIII. Helder 1899. 



Accorcling to the variation in the composition of the plankton I distinguish the 

 following periods. 



Period I. From January 3 th to March 3 rd . The most important species is the 

 boreal Biddulphia aurita. In its company there occurred, although sparingly, some other 

 northern forms, as Chcetoceros debilis, C. diadema and C. teres. The following derive 

 probably from Scotland: Biddulphia mobilensis, Coscinodiscus concinnus and Streptotheca 

 thamesis. 



This period at Helder corresponds to the period I. at Plymouth, but the arctic 

 character of the plankton was more decided at Plymouth than at Helder. It also cor- 

 responds to period V. of St Vaast, where the boreal species were more numerous. 



