6 Caroline Leeraard. 



resting-spores was found at the depth of 5 nietres. At this station the spore-i'ree cells 

 still dominate. Towards the bottom the nuraber of the resting-spores increase in propor- 

 tion to the thin-walled cells, this fact confirming the ruie, that the heavy resting-spores 

 will sink more rapidly than the lighter thin-walled cells. But any considérable sinking 

 could not be noticed yet. The richly laden Upper layers of the water indicate that this 

 species is still in energetic development. 



At the other two stations in the Gulf of Finland, viz: F 50 and F 61, great num- 

 bers of Achnanthes taeniata are to* be found, decreasing from east to west. At these 

 stations the resting-spores prevail, and the development has proceeded further that at 

 station F 41. While the maximum has not sunt anything worth mentioning at station 

 F 50, we find it at the depth of 40 mètres at station F 61. The diatom is here decrea- 

 sing, having begun to sink towards the bottom, the greater number of the cells forming 

 resting-spores. 



At the two stations F 23 and F 30 in the Gulf of Bothnia the behaviour of this 

 species is rather différent. At the most northerly station (F 23) we find it pretty nu- 

 merous with the maximum in the upper layers of the water, but at the same time with 

 resting-spores dominating. The last mentioned fact indicates the retrogression of the 

 species. At station F 30 probably this diatom, occurring in very few cells has almost 

 finished its development and will soon disappear from the plankton. 



At station F 74 in the northern part of the Baltic it is found somewhat mort 

 abundantly than at station F 30, the resting-spores being numerous and the maximum 

 number of cells occurring at the depth of 30 mètres. 



Gliuetoceras debile Clkve. 



(Fig. 3.) 



1894 Cleve, p. 13, pi. 1, f. 2. 



1895 Ostrup, p. 456, pi. 7, f. 89. 



1895 Ch. vermiculus Schutt, p. 39, f. 7 a — c. 



1908 Gran, p. 92. 



1910 Meunier, p. 242, pi. 27, f. 19—23. 



1913 Meunier, p. 43, pi. 7, f. 1—11. 



A few solitary spécimens of some resting spores were to be found. These had two 

 spines on one of the valves and were very probably belonging to Chaetoceras debile. 



The breadth of the chains being very little (8 — 12 tt) 

 the two little wort-like structures of the typical Ch. 

 debile were not distinctly developed ; but Meunier 

 (1913) has given a drawing of this species with 

 resting-spores (pi. 7, f. 11), which are quite similar 

 to those of the Finnish material. 



i'*S^ 



Pig. 3. Chaetoceras debile. 



Resting-spores. 



1000 



Tom. XLVm. 



