16 Caroline Leewaard. 



At all the stations except station F 41 sorae of the Thalassiosira-cells had enclosed 

 in themselves a globnlar body, no doubt a parasite. (Fig. 14.) The cell-wall of the 

 latter appeared to be of other material than the silicious cover of the diatom, the wall 

 of the parasite tnrning violet when treated with chloroïodide of zinc. The contents of 

 the parasite consistée! of black-tinged grains larger than the chromatophores of the host- 

 cell. Several little bubbles were found surrounding the parasite-carrying Thalassiosira- 

 cells. These bubbles turned pale violet with chloroïodide of zinc, and they were not 

 noticed in the healthy Thalassiosira-cells. 



The parasite being soine preliininary undetermined Chytridiaceae is probably not the 

 species Olpidium mentioned by Gran, Meunier and Ostenfeld. 



1900b Gran, p. 123, pl. 9, f. 8—9. 



1910 Olpidium phycophagmn Meunier, p. 222, pl. 25 etc. 



1913 Kolderup Rosenvinge, p. 14"). 



1910 Ostenfeld, p. 158. 



B. FOKMS THAT LIVE IN BRACKISH OR FRESH WATER. 

 MARINE LITTORAL FORMS. 



Besides the oceanic diatoms mentioned several species were found belonging to 

 brackish and fresh water or to the littoral flora. 



For the most part only solitary spécimens were found of the species. Only Diatoma 

 tenue oecurred in rather large numbers, brackish water being its natural habitat; so it 

 was thriving well in the Gult of Finland, where the water had a low salinity. 



As it was very difticult to détermine the species when counting, they were all in- 

 cluded in the tables under the same heading as given to tins chapter with the exception 

 of Diatoma tenue. , 



The occurrance ot ail thèse diatoms was mainly limited to the upper layers of sta- 

 tion F 41, the most easterly station in the Gulf of Finland At stations F 50, F 61 

 and F 23 a few spécimens were found, while they were totally missing at stations F 74 

 and F 30. 



The species noted hâve for the most part been determined on heated material 

 according to the „Synopsis' - of Van Heurck. The following species were found: 



Amphiprora decussafa Grun. (?) 



(Fig. 1 ô a. l 



V. H. S. pl. 22, f. 13. 

 1894—95 Cleve I, p. 18. 



Marine. One single fragment was found (station F 41, 10 mètres). 



Tom. XLVlir. 



