Microplankton from the Finnish Waters during the Montli of May 1912. 



15 



As mentioned above it was difficult to distinguish this species froin the Coscino- 

 discus lacustris. When using high magnifying powers on glowed matei'ial it was possible 

 to see the structure of the valve distinctly. The valve is tlat with delicately rounded 

 edge. The pores are lying in radial rows being almost parallel within rather large 

 sectors. On the spécimen drawn, the diameter of wich was 67 <<, were found about 

 14 sectors with 12 — 18 rows within each sector nearest the edge. The central pores 

 are a little more scattered than the peripheral ones. Along the margin of the valve 

 three series of spines could be seen. The row nearest the centre consisted of scattered 

 large spines of varying size, while the two outer rows were formed of délicate spines. 

 The spines of the row nearest the edge were placed just outside the small ones of the 

 inner row. Generally two spines of each of the outer rows were found between two of 

 the large spines of the innermost row. The margin of the valve had fine radial Unes. 



The size was inconstant, the diameter varying from 20 to 100 n. 



As to the distribution it is once more to be noticed, that the numbers of the tables 

 are a little too high. Some of the cells ought to have been noted as Coscinodiscus 

 lacustris; hut when the material was strongly heated it appeared that Thalassiosira bal- 

 tica was generally more numerous than Coscinodiscus lacustris. 



This diatom was found at all the stations and in every depth in quantifies, which 

 with the reservations mentioned varied between 120 cells pr. litre (station F 41, 50 

 mètres) and 35400 cells pr. litre (station F 30, 80 mètres). At the stations of the Gulf 

 of Bothnia (F 23 and F 30) this species was more numerous than at the other stations. 



Pig. 14. Ihalassiosira battica with parasite. 



a. Cell in front-view. 



b. Cell in side-view. 



1200 

 1 ' 



N:o 5. 



