10 P. T. CLEVE, PLANKTON-RESEARCHES IN 1897. 



Steplianopyxis turgida has about the same distribution. 



I consicler that the following species arrive from the north of Scotland in the 

 North Sea: 



Ceratium furca. Coscinodiscus ocidus iridis. 



C. fusus. Rhizosolenia styliforinis (also through the Eng- 



C. tripos v. bucephala. lish Channel). 



Biddulphia mohilensis. Steplianopyxis turgida. 



Coscinodiscus concinnus. Halosphcera inridis. 



' Araong species, which occur in the southern North Sea and arrived through the 

 English Channel, I note the following: 



Cerataulina Bergonii. This species appeared at Helder in April and May, and was 

 found at Plymouth in July and October. It has not been mentioned by Ostenfeld as 

 found in the northern Atlantic. 



Ditylum Brightwellii occurred at Plymouth from January to May, at Helder in 

 April, and is not mentioned from the northern Atlantic. 



Eucampia zodiacus was found abundantly at Helder in September and October, 

 also at Plymouth in October, but not in the northern Atlantic. 



Lifhodesmiiim undulatum is very characteristic of the neritic plankton along the 

 Continental coast. 



Rhizosolenia alata foUows R. styliformis as a rule, and it may be assumed that it 

 enters the North Sea both from the north and from the south. 



R. calcar avis occurs usually in the company of R. styliformis. 



R. robusta is an inhabitant of the eastern tropical Atlantic, west of Africa, and has 

 never before been seen in the northern Atlantic. This remarkable species was found in 

 September and October both at Plymouth and at Helder, in October between the Färöes 

 and Iceland (Ostenfeld). It reached Hanstholtn (Jutland) in the beginning of November 

 and arrived oif the Swedish coast at the end of the same month. 



I give the following list of the species, which enter the North Sea through the 

 English Channel. Species, marked *, may also arrive round Scotland. 



Bacteriastrum varians. Guinardia flaccida. 



Cerataulina Bergonii. Lithodesmium undulatvm. 



*Gliaitoceros curvisetus. * Rhizosolenia cdata. 



C. densus. R. calcar avis. 



C. didymus. R. robusta. 



*C. Schuttii. R. Shrubsolei. 



Ditylum Brightwellii. R. Stolt herfothii. 



Eucampia zodiacus. *R. styliforinis. 



It is worthy of remark, tliat the chceto-pAanhon already begins to appear in the autumn. 

 Chcetoceros decipiens was then found by Ostenfeld on the south side of Iceland and 

 round Scotland. Is was not rare at Plymouth in the end of November and in the be- 

 ginning of December. 



