28 P. T. CLEVE. THE PLANKTON OF THE NORTH SEA, THE ENGLISH CHANNEL, AND THE SKAQERAK. 



Leptocyliiidrus danicus Cl. — A. In the North Sea in Marek rare S. and SW. 

 of Norway and at Skagen, in June rare at Skagen. — B. In the E. Channel at Plyraouth 

 not rare in August, September and at S:t Vaast in July, August. — C. In the Skagerak 

 more or less scarce from the middle of February to the beginning of June, rare in November. 



Lithodesiuium undulatvim Ehb. — At Helder more or less rare from the end of 

 October to the end of December, east of the E. Channel in November. 



Navicula membranacea Cl. 



Rare in the E. Channel in November. 



Nitzschia seriata Cl. (including ISf. fraudulenta Cl.). — In Mardi more or less rare 

 S. and SW. of Norway, in June W. of Skagen. — In the Skagerak more or less rare 

 from February to the middle of June and very rare in November. 



Kliizosolenia alata Btw. 

 in September and November. 



Not rare in November W. of Hanstholni. At Plymouth 



Tar. corpnlenta Cl. — Rare at Plymouth from August to December. 



Tar. gracillima Cl. — A. In the North Sea. June W. of Skagen, abundant. July, 

 August W. of Skagen and on the Fisher Bank. November rare N. of Skagen. — B. In 

 the E. Channel at Plymouth more or less sparingly from the end of March to July, very 

 common in July and August. — C. In the Skagerak enormously abundant from June 

 to the end of July, then rarer until December. Appears at Måseskär as early as the 

 end of April. 



R. calcar avis Schultze. — From July to November rare W. of the Danish Pen- 

 insula. At Helder from the end of September to the end of November. In the Skagerak 

 rare in October and December. 



y~-. 



r\ 



V 



Fig. n. 



I 



R. delicatula Cl. N. sp. — At S:t Vaast la Hogue rare 

 in April and in November, at Helder in August and September, in 

 the Skagerak common in the first half of June, rare in September 

 and November. 



The annexed iigures (11) represent three specimens in difierent 

 positions, 500 times magnified. Cells cylindrical, straight, 4 to 5 

 times longer than broad, extremely thin--\valled, without distinct 

 transverse rings. Their ends slightly and irregularly convex with a 

 short, excentric spine. Length O, os to 0,07 mm. Breadth 0,ooi4 mm. 



R. oMnsa Hensen. — Rare in February at Måseskär. 



R. robusta Btw. — Rare at Plymouth in March, April and September, more or 

 less common in October, November, December. — In November fi'om the E. Channel to the 

 mouth of Schelde. 



