PiKtistplanktuii. 



!»5 



dirtcrent to liotli ( '. rniluiltis ami ('. cixtnilis may lu' rescued from 

 cliaos. 



The two ijioiuineiit author.s Ckiniiw and Ivatthay, who liave 

 given extensive and tlioi'oui;h monoirraphs on the difticnlt genus 

 Co!>ci7todiiicug, have in their exceedingly exact dcscrii)tion of the 

 differences in the structure of the valve omitted other iiniioitant 

 distinctive features, especially the shape of the valve in sidi' view. 

 Both of them, especially Rattray, indeed often particularly refer 

 to the convexity, but not by any means in every instance, and they 

 often include forms which correspond in structure, but differ cou- 

 sidcralily in convexity, in the same species. This circumstance, 

 unfortunately, makes Grunow's work, which in other respects is 

 so exceedingly thorough, some^\■hat incomplete and \\aiiting in 

 clearness. 



As far as my experience goes, the convexity of the valve is 

 precisely a very certain distinctive feature, and comparatively easy 

 to apply to living species in the plankton. It is even, as far as 

 I can see, the only guiding thread which will sei-ve to lead us out 

 of an otherwise hopeless and interminable maze. Only it must 

 always be remembered that this — just as is the case with regard 

 to structure — is only one distinctive feature, and may lead to the 

 same unnatural piecemealing which the structure has caused; but 

 that both distinctive features in conjunction can give good results. 



What I have, in one instance, in the tables called C. ocuhcs 

 iridis, is the above mentioned coarser forms of C. centralis. 



DMiilndion: Appears to be an arctic and boreal oceanic 

 species, which is rare with us. It is found scattered at several 

 places on our northern coast, especially in deep water. 



Appears to extend along the northern, especially the arctic 

 coasts of the Atlantic (Jan Mayen and Spitzbergen, in E. Jokgex- 

 SEx L. 92 named C. or»/»s iridis) and in the districts where the 

 arctic and Atlantic waters mix. 



t', nitidas Gkeg. 



Greg. L. 74, p. 499. pi. 10, f. 45. A. Schm. L. 127, p. 94, 

 pi. .3, f. 32, L. 128, pi. 58, f. 18. 



Only found singly in two of the samples, from the \'est Fiord 

 I. 0—50 m., '/'i 1899. and from Senjeu '-'/i, — 130 m. 



Probably only come in by chance and really a bottom form» 

 as it is frequent in bottom samples. (Cfr. under that heading.) 



F.iioflia 



{Hcmidisciis Wallich. 



E. ^ihha Haii 



Haii.. in Pkit(!1. L. 123, p. 852. pi. 8, f. 22. Hemiiiscus 

 aineiformis Wallich L. 137, p. 42, pi. 2, f. 3—4. 



Very rare with us, in deep water, probably come in with At- 

 lantic water. Hitherto hardly found so far north. 



Distribution: Chiefly a subtropical and southern temperate 

 Atlantic form, according to Cleve (L. 40, p. 330) rare north of 

 50" northern latitude. 



Known both from the European and American side of the At- 

 lantic. 



Aotinocyrlii» Khbb. 

 A. Khrenhergi Ralks. 

 Fre(iuent in the plankton. 



Tiiis species is difticult to distinguish from those closely re- 

 lated to it, and seems to vary 8onsideral)ly. 



Distrihidion : .Seems to occur all the year round on the west 

 coast of Norway, and probably also on the north coast, here at 

 any rate in the months January— May. Otherwise widely distri- 

 liuted around the European coa.sts of the North Atlantic and its 

 arms. Occurs exceptionally right up to Greenland, but is not an 

 arctic form. iScems to have a much widei- distiibution, judt-'ing from 

 the statements in De Toni. (L. 50). 



A. Ralfsi (W. S.M.^ Hai.1'8. 



This .species seems to be much rarer on our coasts, both on 

 the west and north, than the preceding one. 



Reference should be made to the chapters on bottom samples, 

 where it occurs somewhat more frequently. It is, however, cer- 

 tainly a genuine plankton form, and not a bottom form. 



Distrihditn : .Seems to have a more southerly distribution than 

 the foregoing species. Like the latter, it has also been found at 

 Greenland. 



.1. siihtilis (Greg.) Ralks. 



Ralfs in Pritchard L. 123, p. 835. Van Heueck SjTiopsis 

 (L. 88), p. 216, pi. 124, f. 7. Eupodisnts suhtilis Greg. L. 74, 

 p. 501, pi. 11. f. 50. 



Very rare. Occuired very scarce in the plankton from Kva-n- 

 angen -*/. 1899, 0— Ho m. 



Distrilndion : Known from the coasts of England, Spain, The 

 Mediterranean, The Azores and The Pacilic Ocean. 



Note. A. sparsus (Greg.) Rattk. seems to occur in the plank- 

 ton from the northern and western coasts of Norway; but as it is 

 difticult to discern between this species and A. Ehrcnbeiyi, I have 

 not included it in my tables. Besides, another form occurs, which 

 certainly is specitically different from A. Ekrenbergi, and is perhaps the 

 same as the genuine Eupodiscus crossus W. Sm. Earlier, I took 

 it to be A. crassm VH. but have later become somewhat uncertain 

 respecting this species, whose description (by De Toni and Rattray) 

 does not agree well with Van Heurck's drawing. In spite of 

 considerable labour, I have not yet been able to come to any de- 

 tinite conclusion, so that I have not tabulated this form either. 



For further particulars, reference should be made to the chapter 

 on bottom samples. 



A. subocclliitus (Gri-n.) Rattr. 



Rattr. L. 125. p. 145. Cosci)>odiscHs curvatulus var. siiboccl- 

 lata Grcn. L. 83, p. 83, pi. 4, f. 15. Actinoeijchis ciirvatuhis 

 Jan. in. A. Schmidt L. 128, pi. 57, f. 31. 



This beautiful diatom is verj^ like Ccscinodiscus citrratulus and 

 is probably often mistaken for it. Possibly, therefore, it is not 

 quite so rare as it seems to be. 



Hitherto only found in a few plankton samples of 1900 (The 

 Skjerstad Fiord, V*, B', V and XII; The Salten Fiord "A). 



Distribution : Certainly not sufficiently known. Judging from 

 the available accounts, only found fossil and at various places in 

 the Ant-arctic regions. 



