Limnoplankton 439 



plankton of many of the western European lakes (W. & G. S. W., '02 ; '05 ; 

 '06; '09 A and B; Lemmermann, '04; Wesenberg-Lund, '04; '08; Bachmann, 

 '11 ; etc.). Star-dispositions of T. floccnlosa are very uncommon, but they 

 have been observed by Holmboe ('99) in Norway and by Wesenberg-Lund 

 ('04) in Denmark. 



Fragilaria crotonensis is a very characteristic plankton-diatom of the pools 

 and lakes of Europe and North America, but does not often occur in large 

 quantity. In Loch Ruar, in Sutherland, there is a unique variety var. 

 contorta with curiously twisted short filaments (W. & G. S W., '05). 



The phytoplankton of the large African lakes is peculiar in the almost 

 entire absence of many genera of diatoms which are a dominant feature of 

 the lakes of north temperate areas. Asterionella, Tabellaria and Rhizosolenia 

 are wanting or of small importance. Four species of the last-named genus 

 have recently been recorded by Woloszynska ('14) from Victoria Nyanza, but 

 star-dispositions of diatoms are practically confined to several species of 

 Synedra of the section Belonastrum, 



Attheya is a characteristic plankton-genus closely allied to Rhizosolenia 

 and although frequent in the lakes of continental Europe has not been found 

 in the British lakes. 



In all the British lakes and in the large African lakes the genus Surirella 

 plays an important part in the phytoplankton. In the British lakes Surirella 

 robusta var. splendida is the most frequent form, sometimes occurring in great 

 abundance, but S. biseriata and S. linearis are both general (W. & G. S. W., 

 '05 ; '09 B). In the great lakes of Africa a number of beautiful species of 

 Surirella occur in abundance, many of them, such as S. Nyassee, S. Malombm 

 and S. Fullebornii, being apparently restricted to that part of the world 

 (O. Miiller, '05 ; G. S. W., '07 ; Ostenfeld, '09 ; etc.). These African species 

 show many transition stages from one to the other. Certain species of 

 Cymatopleura have also become true constituents of the plankton ; they 

 are common in the Irish lakes (W. & G. S. W., '06) and in Victoria Nyanza 

 (G. S. W., '07), and Wesenberg-Lund ('04) records Cymatopleura elliptica as 

 being a typical plankton-organism in one of the Danish lakes. 



The Centric diatoms of the plankton are only conspicuous in certain 

 lakes. Apart from Rhizosolenia, various species of Melosira are often well 

 represented. In the European area M. granulata may occur in such quantity 

 as to give a Melosira-phase to the plankton and the same is true of the Yan 

 Yean Reservoir in Australia. The records of this species show such erratic 

 vegetative periods 1 in relation to water-temperatures that it is not improbable 

 that various species have been recorded under the name of ' Melosira granu- 

 lata ' or that there are different biologic forms of this diatom. In Windermere, 



1 Rhizosolenia morsa is apparently as erratic as Melosira yranulata in its occurrence. It 

 frequently has a double maximum, the autumn maximum being tlie larger (W. & G. S. W., '12). 



