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



Hexasterias problematica Cl. — By this name I denote provisionally a unicellular 

 alga, which I have found in several collections. It is a flat disc, diameter 0,04 mm., 

 with six, at the ends truncate and denticidate empty processes, twice as long as the radius 

 of the disc. Having seen only specimens preserved in spirit I am unable to decide 

 whether the chromatophores are yellow or green. 



Fig. 6. Hexasterias problematica Cl. 500 t. m. 



Habitat: Helder, rare in December. Outside the North Sea at the following points: 

 Iceland (Westmannaö 26. X. 98), the Färöes (23. VII. 98), at 51° N. 2° 5' W. (14.'XI. 98), 

 at 66°30'N. 22°30'W. (29. V. 99) and at 61°37'N. 6°40'W. (20. V. 99). 



Diatomaceae. 



Asteriouella japonica Cl. (in Cl. & Moll. Diat. N:o 307, 1882. A. glacialis Ostenf. 

 probably Castrac. 1886 A. spatliulifera Cl.). — At Plymouth in March and April, rather 

 rare in September. Very rare in the Skagerak on the 31st of January. 



Asteromplialus heptactis Ralfs. • — Very rare in March at 63° 1' N. 1°36'E. 



IJacteriastrxim delicatulniii Cl. — Very rare in June W. of Stadt in Norway. 



B. varians Lauder. — In March at Plymouth, in July, August NW. of Heligoland. 

 At Helder coinmon in August, September, rare in October. At S:t Vaast la Hogue more 

 or less sparingly in November, December. 



Bellerochea mallens Btw. — In March very rare W. of Jutland (55° 43' N. 7°29'E.). 

 In November at S:t Vaast la Hoguc, from the Channel to the coasts of Holland, at Helder 

 and in the Skagerak. 



Biddulphia aurlta Lyngb. — In Mardi on the whole sparingly round the south 

 coast of Norway, at Skagen, on the Fisher Bank and above the depression S. of the Dogger 

 Bank. — At Helder rare in February. In the Skagerak from January to March. In 

 November common at Helder, rarer in the Skagerak and at S:t Vaast la Hogue. 



