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



Streptotheca thamesis Shrubs. — From January to March rare at Plymouth, in 

 March W. of Scotland and midway between Scotland and Norway, in JSfovember common 

 in the E. Channel and sparingly along the Dutch coast to about 56° N. 6° E. 



Thalassiosira gelatinosa Hensen. — In the Skagei'ak rare from January to the 

 beginning of April and in November. Rare in December at Helder. 



T. gravida Cl. — In the Skagerak rare from February to the end of April and 

 in December. At S:t Vaast la Hogue in December. — In the North Sea rare in 

 March on the Fisher Bank. 



T. Nordenskiöldii Cl. — From January to March rare (at Måseskär common from 

 9. II. to 21. III.) in the Skagerak. In March rare S. and SW. of Norway and at Skagen. 

 In November rare in the Skagerak. 



Thalassiothrix Frauenfeldii Grun. — A. In the North Sea in March abundant 

 S. and SW. of Norway and at Skagen, in June not rare at Hanstholm. — B. In the 

 Skagerak from January to June, very abundant in March, and very common in November, 

 December. 



T. longissima Cl. & Grun. — A. In the North Sea in March rare S. and SW. of 

 Norway and at Skagen, in November rare at 58°12'N. 5°30'E. — B. In the Skagerak 

 rare from January to April (common at Måseskär from the 9th to the löth of February), 



An Attempt to classify the Plankton-organisms according- 

 to their Geographical and Seasonal Distribution. 



Although the exploration of the North Sea in 1898 has been carried out four times 

 only and the data are by no means complete, I venture to classify the plankton-organisms 

 according to their distribution at different seasons. This classification may be in some 

 cases erroneous, for want of sufficient data, but as a rule I consider it correct and in all 

 cases useful as a starting point and basis for future researches. The organisms may be 

 classified as follows. 



A. Forms that, as a rule, are confined to the space above the 50 metre plateau on the bottom. 



1. The whole year: Corycceus anglicus. 



2. From March to June: Cerataulina Bergonii, Ditylum Brightwellii (also in No- 

 vember and December). 



