8 P. T. CLEVE, PLANKTON-KESEARCHES IN 1897. 



(30 to 33 p. mille) while the water in the central part of the North Sea had in the 

 summer a salinity of 35 p. mille. It seems as if the Ceratium tripos came round the 

 north of Scotland in the summer and found in the North Sea the most favorable con- 

 ditions for its enormous development. 



C. {tripos var.) macroceros, which is to be considered as a distinct species, is as 

 a rule very rare in the spring, but, in the summer, becomes abundant in the whole of 

 the North Sea and along the Scandinavian coasts, as far as Lofoden. 



Bacteriastrum varians occurs off the Dutch coast and west of Skagen. It is not 

 mentioned by Ostenfeld as occurring in the northern Atlantic. 



Chcetoceros densus follows Bacteriasfruiii varians. 



C. ctirvisetus was met with north- west of Skagerak and west of Bergen, but is not 

 mentioned by Ostenfeld as occurring on the route Shetlands — loeland. It was very 

 common along the Swedish coast from the middle of July and it was seen also at Helder 

 in the same month. It is thus a southern species, which in the summer spreads along 

 the south coast of the North Sea into Skagerak and from there passes with the Baltic 

 current along the Norwegian coast. As it was abundantly found in the autumn of 1896 off 

 the west of Scotland, it also seems to proceed northwards along the west coast of England. 



C. didymus and especially C. Schiittii follow C. curvisetus. 



Guinardia flaccida appeared in May at Plymouth and from the middle of June 

 at Helder. 



Rhizosolenia Shrubsolei was found more or less abundantly from May to October 

 at Plymouth and in June and July at Helder. It was also found, but sparingly, in the 

 styli-plankton off the Shetlands. 



R. Stolterfothii appeared in May at Plymouth, and in July at Helder. It does not, 

 as a rule, reach Skagerak. Ostenfeld mentions it as occurring sparingly at the Shetlands. 



R. styliformis. The occurrence of this species west of the Danish peninsula, and 

 its almost complete absence from other parts of the North Sea lead to the conclusion 

 that it had been carried by a current from the English Channel, since it had been ob- 

 served, although sparingly, in May and June at Plymouth. 



4. Nortli Sea in October— November 1897. 



The State of the plankton prevailing in July and August has not been much altered. 

 Now, as then, the tripos-plankton is the prevailing type, but a number of species, not 

 seen before, appear, the origin of which can be traced partly from Scotland and partly 

 from the English Channel, and which belong partly to the styli-plankton and partly to 

 the neritic plankton of the British Islands. 



The styli-plankton appears both off the coast of Scotland, down to Newcastle, and 

 on the west of the Danish peninsula. 



One species of the northern neritic plankton-type, Ceratium tripos v. longipes, was 

 found along the Dutch coast, but there mixed with southern neritic forms and some 

 species which can be traced from Scotland. As this dinoflagellate appeared also in July 



