of the Fishery Board for Scotland. 



301 



form figured by Brightwell in M.J. VI., 1858, PI. V. fig. 7, and the form 

 figured in Van Heurck's Synopsis, PI. LXXVIII. fig. 7, and in Peragallo, 

 Monogr. du g. Rhizosol., PI. IV. figs. 12 and 14. The original R. setigera 

 is very delicate, has no markings on the calyptra. There are no rings to 

 be seen on the connecting zone. This form has not been found in the 

 ' Research ' samples, but occurs in the winter and early spring in the 

 Kattegatt and Skagerak, where it forms interior cells, which are identical 

 with Pyxilla baltica, Grun. The figures (PI. fig. 12) show the forma- 

 tion of Pyxilla baltica inside R. setigera. 



The other form, named R. semispina by Hensen was formerly (Bih. t. 

 K. Sv. Vet. Akad. Handl., Bd. XXII. 3, No. 5) named by me R. setigera 

 forma gracilis. It is very rare in the 'Research' samples, some few 

 specimens only having been observed in Nos. 10 and 11. On the other 

 hand, it is of very frequent occurrence in the west Atlantic plankton, 

 which I call tricho-plankton. 



Rhizosolenia Stolterfothii, H. P. (Monogr. d. Rhiz., PI. I. figs. 17, 18). 

 A few specimens only were found in No. 11. 



Rhizosolenia styliformis, Btw. This characteristic Atlantic form was 

 found more or less abundant in most of the ' Research ' samples. It 

 seems to belong especially to the eastern parts of the Gulf-stream. 



Thalassiothrix longissima, CI. and Grun. Of this species, which occurs 

 in the western Atlantic (south west of Iceland) in enormous masses, some 

 few specimens only were found in Nos. 7, 11, 14, 17. 



B. ClLIOFLAGELLATES. 



Ceratium furca, Duj. 

 Ceratium fusus, Duj. 



Ceratium tripos, Ehb. This widely distributed species occurs in 

 different varieties, which it is of importance to distinguish. Ehrenberg 

 named a variety macroceros and another arctica, but did not give any 

 figures of them. In the Infusionsthierschen, PI. XXII. fig. 18, he figures 

 two forms, of which the left is the most common, and may be considered 

 as the type, and is as such figured by Claparede and Lachman. It corre- 

 sponds with the var. baltica, Schlitt (Beschr. d. Plankton-Exp. von 

 Kriimmel, pp. 266 and 302). The right figure corresponds in outline 

 with the variety arctica, figured by Claparede and Lachman, Etudes 

 sur les infusoires, PI. XIX. fig. 3. This is evidently the same as Schtitt's 

 var. labradorica, I.e. The other variety, macroceros, is figured by 

 Claparede and Lachman, fig. 1. This easily recognised form has been 

 named scotica by Schiitt. 



Schiitt adds two other varieties, viz., parvula, the figure of which is 

 insufficient for indentification, and tergestina. The latter has in my 

 paper (in Bih. K. Sv. Vet. Akad. Handl., XXII. 3, No. 5) been named 

 arctica Aur, but was figured by Bailey, already 1854, as Peridinium 

 longipes (Smithsonian contr., Vol. vii. f. 35). To these varieties I 

 will add two, viz., G. tripos var. bucephalus and G. tripos var. horrida. 



I give on Plate figures of all these varieties. 



1. G. tripos type = C. tripos var. baltica, Schiitt, fig. 1, is very common 

 in the North Sea and in the ' Research ' samples. In the Kattegatt and 

 Skagerak it belongs to the summer plankton or the tripos-plankton. 



2. G. tripos var. macroceros, Ehb. = G. tripos var. scotica, Schiitt, fig. 6, 

 very common in the English Channel and the south part of the North 

 Sea. It was found very rarely in the 'Research' samples No. 10. In 

 the Kattegatt and Skagerak it appears in the summer and autumn, 



