194 



Mr. W. West and Dr. G. S. West. 



[Oct. 30, 



and Ireland, but is always scarce. We have not observed it in the English* 

 or Welsh lakes. A variety of it — var. contorta — occurs in Loch Euar, 

 Sutherland, which is unique in the curious twisting of its exceedingly short 

 filaments. This variety is not known from elsewhere. 



Throughout all the British lake-areas, but more especially in the west of 

 Scotland and the west of Ireland, species of the genus Surirella form a con- 

 siderable and conspfcuous part of the phytoplankton. The most frequent is 

 Surirella robusta, var. splendida, which sometimes occurs in great abundance, f 

 but S. biseriata and S. linearis are both general. In this respect the British 

 lakes compare with the lakes of Central Africa, in which several plankton- 

 species of Surirella are abundant^ In the Yan Yean Beservoir, Victoria, 

 S. robusta, var. splendida, is also a constituent of the plankton.§ Wesenberg- 

 Lund|| states that various species of the genera Surirella and Cymatopleura 

 occur in the plankton of principally alpine or shallow lakes in level country, 

 and that they have been carried out by rivers and waves into the pelagic 

 region, where they vegetate but for a short period and then perish. We find 

 that in the British lakes, and also in those of Central Africa, the genus 

 Surirella is frequently a true plankton-genus, and the various species which 

 have been recorded both vegetate and multiply in the plankton to an extent 

 we have rarely noticed in other situations. 



Several of the Naviculacefe occur with considerable regularity. 



Centric Diatoms are relatively few and insignificant in the British lakes. 

 Melosira is represented chiefly by M. gramdata and M. varians. The latter 

 is perennial in the plankton of British rivers. Species of Cyclotella are not 

 abundant, and only in Lough Corrib, Galway, have we observed the curious 

 gelatinous colonies which occur so frequently in some of the Central European 

 lakes. 



The genus Rliizosolenia is represented by two (and if the record of 

 R. eriensis be correct, by three) species. R. longiseta is very rare, but 

 R. morsa occurs in some of the lakes of all the British lake-areas. We have 

 observed the resting-spores of this species in the June plankton of Thirlmere 

 in the English Lake District. 



No species of Altheya has yet been observed in any of the British lakes. 



The CHLOROPHYCEiE are well represented in the British lakes, more 



* This species occurs both in the plankton (helioplankton) and the benthos of the large 

 pools in the Midlands of England. 



t Consult W. and G. S. West, loc. cit, Plate 1, figs. 1—4, and Plate 2, fig. 6 (photos). 

 4 G. S. West, in ' Linn. Soc. Bot. Journ.,' vol. 38, 1907, p. 85. 

 § G. S. West, in 'Linn. Soc. Bot. Journ.,' vol. 39, 1909, p. 17. 



|| Wesenberg-Lund, 'Plankton Investigations of the Danish Lakes,' Copenhagen, 1908, 

 p. 42. 



