I92I. No. 4- THE PHYTOPLANKTON OF SOME XORWKCIA.N LAKES. 33 



in its occurrence than this. I never met with this variety except in the 

 plankton. The Alga is, no doubt, the Sfaiinisfn/ni Daari Hiitf.-Kaas. 



Staurastnmi deject uni var. in fia tu ni W. West. 



W. West : F'reshw. Algæ. W. Ireland. 

 Staiirastntni dcjectnni var. inßatuni is no true plankton tonn. I think 

 it is as often met with in the benthos as in the plankton. 



Staiirastruin forficulatum Lundell. 

 Luxdell: Desm. Suec. 



The Desmid occurred in the lake Sognsvandet in very small quantities. 

 It is no true pelagic form, but it is a great rarity in the benthos, whereas 

 it is not very unfrequent in the plankton. 



Staurastruni jaculiferuni W. West. 



W. West: Freshw. Algæ. W. Ireland. 



This species is one of the commonest plankton Desmids and may occur 

 in great quantities, such as in Jolstervandet. 1 think the biradiate form is 

 a little oftener met with than the triradiate but both may be common in 

 the same sample, and I have observed specimens where the one half was 

 triradiate and the other biradiate. The species was earlier recorded under 

 the name Artlirodesniiis longiconiis Rov & Bissett, under this name it is 

 reported on by Huitfeldt-Kaas. I have not distinguished between its 

 varieties, which, I think, are of no ver}' great importance. 



PL I, Fig's 3 — 5. 



Staiirastnrn /ongispini/ni (Bah..) Arch. 

 Bailey : Micr. Observ. 



The species is very extraordinär}- in the benthos, but is not unfre- 

 quently met with in the plankton. It is, however, only occurring in small 

 quantities, and never obtains great maximums. This Desmid seems to be 

 one of them which are largely distributed in the Occidental Alga-region. 



Staurastruni lu nat uni var. planctonicuin W. & G. S. West. 

 \W. cS: G. S. WV,ST : Scott. Freshw. Plankton. 



This Alga is quite frequent in our plankton, and is an eulimnetic species. 

 It is a ver}' characteristic feature of the Caledonian plankton, and is com- 

 mon in the British lakes. 



Stourastruni Manfcldtli Delp. 

 Delponte : Desm. subalp. 



Staurastruni Maiifeldtii is one of those species which are much more 

 frequent in the pelagic region than in the benthos. It is, however, no very 

 conspicuous constituent of our freshwater plankton. 



Staurastruni niegacanthuni var. scoticuni W. & G. S. West. 



W. & G. S. West: Scott. Freshw. Plankton. 

 The species occurred in two lakes, in one of them viz. Jolstervandet it 

 was common. It is a plankton-variety, exclusively confined to the pelagic zone. 



