326 TVesf : Phytoplankion of English Lake District. 



figs. 13-16; P. alatum Garbini in ' Zool. Anzeig.', 1902, p. 122, 

 fig. A, B. 



This is the only generally distributed species of the genus 

 Peridinium in the English Lake District. It was found in 

 almost all the lakes examined, and often occurred in large 

 quantity, being much more abundant than in any of the other 

 British lake-areas. Like Ceratium hirundinella, it is a summer 

 form, ^vith its maximum in August. Long. 56 /^t ; lat. 60- 

 64 /X ; crass, max. 44-47 fJ-. (Text-fig. 4 a — g) . 



P. Willei is a very distinct species, and maintains its 

 characters very constantly. The group of five small apical 

 plates, forming the extreme anterior margin of the epivalve, 

 is one of its principal features. These plates vary within cer- 

 tain limits, but are very constant in their position (vide fig. 4 

 A-c). Another of its features is the possession of two small 

 wing-like extensions of the ventral margins of the antapical 

 plates. Each of these is furnished with a number of very short 

 and delicate spines, and they are best seen when the cell is 

 very slightly oblique. The plates are rather finely areolated, 

 the areolations gradually becoming more pronounced as the 

 age of the cell increases. 



v.— -THE PECULIARITIES OF THE ENGLISH 

 LAKE-PLANKTON. 

 The phytoplankton of the English Lakes contains a varied 

 assortment of Algae, 64 per cent, of w^hich belong to the Chloro- 

 phycese, 21 per cent, to the Bacillarie^, and only 9.5 per cent, 

 to the Myxophycese. Thus, the phytoplankton is essentially 

 Chlorophyceous, with a plentiful admixture of Diatoms, and 

 but a few Blue-green Algae. 



Species. 



Varieties. 



Chlorophyceae | 



96 



Desm.) 



24 



4 



6 



2 



Bacillarieae 

 Myxophyceae 

 Flagellata . . 

 Peridinieae 



3 



o 



2 



o 



Total 



188 species 20 vars. 



Of 120 species of Chlorophyceae, 96 are Desmids, so that 51 

 per cent, of all the species recorded for the plankton belong to 



Naturalist, 



