I92I. No. 4- THE PHYTOPLANKTOX OF SOME NORWEGIAN LAKES. 29 



in Western Europe, but I think that unexperienced authors ha\e contused 

 it with sterile specimens of Mougcotia. There is, however, no excuse for 

 doing so, as the diiTerence is striking enough if carefully looked for. 



Genus Clostcriiiin Nitzsch 1817. 



Clostcriitiii sctaccitm I'or. cloiigatitiii W. & G. S. West. 



W. & G. S. West : A further Contribution to the Freshw. PI. Scott. Lochs. 



The cell-wall was colourless and the striolations very delicate. It occurred 

 in one lake in Jotunheimen viz. Heimdalsvandet. 



Size: long. 860 \l, lat. 13 ;x. This variety is, as far as we know, an 

 exclusive plankton-varietv, being originally described from Loch an Sgath, 

 Lewis, Outer Hebrides. 



Genus Micrastcrias Ac. 1827. 



Micrastcrias radiata Hass. 



Hassall: Brit. Freshw. Algæ. 



This Alga occurred in small quantities in Orrevandet. It seems to be 

 a semilimnetic form, but it is oftener met with in the plankton than else- 

 where. It seems principally to have a western distribution in our Country. 



Micrastcrias Sol (Ehrbg.I Kuetz. 



Ehrenberg : Verbr. u. Einfluss mikr. Leben in Süd- u. Nord-Amerika. 

 The species was very common in the Figgenelven river in Jæderen. 



Micrastcrias Sol var. ornata Nordst. 



Nordstedt : Desm. Brasil. 

 I have only observed this variet}- from a single lake, and it seems to 

 be no frequent constituent of the Norwegian freshwater plankton. 



Genus Cosmariiiiii Corda 1834. 

 Cosinariiiiii abhrcviatiim var. plaiictouiciiiii W. & G. S. West. 



W. & G. S. West : Fürth. Contrib. Plankt. Scott. Lochs. 

 This variety is a true plankton form, and it is generally distributed in 

 our countrv. It occurred in great quantities in the plankton of Bes- 

 vandet. I have drawn a typical specimen. 

 PI. I, Fi^. 18. 



Cosniariii/ii coiitractiiiii Kirchn. 



Kirchner: Alg. Schlesien. 

 The tvpical Cosiiiariiiiii contractum is a very frequent Desmid in the 

 Norwegian lake-plankton, being distributed throughout the whole country. 

 It seldom occurs in very large quantities, but it is without doubt a true 

 plankton organism, being far more frequent in the plankton than in the ben- 

 thos. The var. cllipsoidcwn is ver}' common in bogs, indeed it is one of 

 the commoner Desmids in Norway. 

 PI. I, Fig. 15. 



Vid.-Selsk. Skrifter. I. M.-N. Kl. 1921. No. 4. 3 



