E. Jørgensen. 



C. socialis Lai u. 



In large quantities dm-ing the diatom intiow in spring. Endo- 

 cysts very frequent. 



Distribution: Arctic coast form. Occurs in quantities also on 

 the west coast of Norway during the diatom inflow in spring. 

 Only occasionally round the Faeroe Islands. 



I b. Peniiatæ. 



7. Synedreæ- 



Tlialassiothrix Cl. et Grin. 

 T. lonøssinia ('l. et Grun. 



In 1899 rare and scarce, in 1900 not observed. 



DistrUmtion : Oceanic species, distributed over the northei'n 

 part of the Atlantic from the American to the European side, right 

 up to Spitzbergen. On the west coast of Norway in 1898 found 

 during most months of the year, but always scarce. Very rare 

 round the Faeroe Islands. 



T. nitzscbioides Grun. 

 (PI. VI, tig-. 11). 



Gkun. in VH. Syn. (L. 88), pi. 43, f. 7—10. 



(In the tables T. Frauenfeldii and T. Fr. var. nitzschioides). 



This species is certainly the same as the one wliich, in nearly 

 all plankton tables, is called T. Frauenfeldii. Strangely enough, 

 Cleve refers (L. 40, p. 356 & 357) to VH. Synopsis pi. 37, f. 11— 

 12 and, for „the variety" javanica Grun., f. 13. The species which 

 corresponds to figures 11 and 12, and which I know from the 

 Indian Ocean, is, however, very different from the one which is so 

 frequently seen in the coast plankton in our latitudes. The „variety" 

 javanica Grun. does corre.spond better with regard to the closeness 

 of the puncta, but, in other respects, differs so much that it can 

 hardly be considered to be the same as our common plankton spe- 

 cies. On the other hand, it is also in structure so different from 

 what is taken to be the main species that it cannot be united with 

 it unless, (as perhaps is the case, although I have never seen any 

 mention of such) there are a number of intermediate forms. 



But our plankton species, as far as I can see, corresponds in 

 every respect to 'Thalamothrix? nitzschioides Gnvs. It is true that 

 this species does not answer well to the characteristics of the genus 

 Thalassiothrix, but is yet so different in important points from the 

 genus Synedra that I think it should better be referred as a sub- 

 genus 7 haJassionema (Grun.) to Thulussiothri.r rather than be re- 

 tained in the genus Synedra. 



1 have earlier thought, from the description given by De 'Joni 

 of the structure of these forms (L. 50, p. 672 — 673), that all three 

 {T. Frauenfeldii, T. javanica and 7'. nitzschioides) should be con- 

 sidered as one species (cfr. E. Jørgensen L. 91, p. 22), but 

 have since not found, after my own experience, any definite reason 

 for so doing. 



More or less frequent, especially during the diatom inflow in 

 the spring of 1900, but never noticed in such large quantities as in 

 more southerly samples. 



Distribution: This species is ceilainly neritic, but the genuine 

 T. Frauenfeldii is probably oceanic. Is widely distributed along 

 the coasts of the northern Atlantic and its arms, northwards as far 



as Greenland. On the west 

 quantities in the spring. 



coast of Norway common, in large 



Fragilaria Lyngb. 

 F. oceanica Cl. 



Cl. L. 22, p. 22, pi. -1, f. 25 a, b. Gran L. 65, p. 8, pi. 1, 

 f. 6-9. 



In large quantities during the diatom inflow, at other times 

 absent. Strange to say it appears all at once in large quantities. 



Distribution: Arctic coast form. According to Ciæve also 

 found in Hudson's Bay. Is not found so far south as Bergen, on 

 the west coast of Norway. At Ona frequent in March — April, 

 according to Gran (L. 70, p. 180). 



JF. cylindrus Grdn. 

 (Plate VI, fig. 9). 



Gbun. L. 83, p. 55, pi. 2, f. 13. Gran L. 65, p. 8, pi. 1, 

 f. 4—5. 



Occurs together with the preceding species in large quantities 

 in spring. 



Distribution: On the coast of North Siberia, Frantz Joseph's 

 Land and Greenland; in quantities in May in Davis Strait (Cleve 

 L. 26). Probably a neritic arctic species. Not mentioned before 

 from Norway. In the Barents Sea, 71" 48' n. lat., 49" 38' e. long. 

 'Vs 1900, numerous together with the preceding species. 



F. islandica Grun. 

 (PI. VI, fig. 10). 



Grunow in VH. L. 88, pi. 45, f. 37. 



In some few samples from 1900 during the diatom inflow in 

 spring a species was observed, which I think is identical with 

 the one above mentioned. It occurred sparingly, but in rather long 

 chains. I have not, however, succeeded in seing it in valvar ^iew. 



Distribution: Jan Mayen (1. c.) By Cleve also once observed 

 numerous in the North Sea (L. 27, p. 3), together with northern 

 neritic species. 



8. Plagiograinineæ. 



<•!> |ilio(l<'siiiis Williams 



(W. Sm.) Grun. 



Hardly a genuine plancton form. Here, as on the west coast 

 of Norway, probably brought in by (or torn ott' from) species of 

 fixed algæ (sea — weed). 



g. Tahellarieæ. 



€iraiiiiiiato|thora Ehrb. 

 G. islandica Ehrb. and G. oceanica Ehrb. 



Not genuine plankton forms, only torn off from algae, or car- 

 ried on with them. 



