1908.] 



The British Freshwater Phytoplankton. 



203 



cylindricum, W. and G. S. West ; Peniwm exiguum, West ; Closterium Vina, 

 Focke ; Tetmemorus minutus, De Bary ; Micrasterias Sol, Ehrenb. 



Euastrum crassum (Breb.) ; Kutz., var. scrobicvlatum, Lund. ; E. crispulum 

 (Nordst.), W. and G-. S. West; E. inerme, Lund.; E. pinnatum, Ralfs ; 

 E. pulchellum, Breb. ; E. svMobatum, Breb. ; E. ventricosum, Lund. 



Cosmarium connatum, Breb. ; G. Debaryi, Arch. ; C. decedens, Beinsch ; 

 C. elegantissimum, Lund. ; C. annvlatum (Nag.), Arch., var. elegans, Nordst. ; 

 C. BTammeri, Beinsch ; C. isthmium, West ; C. Nymannianum, Grun. ; C. ovale, 

 Ralfs ; C. parvulum, Breb. ; C. pseudamcenum, Wille ; C. pseudopyramidatum, 

 Lund.; C. pseudoconnatum, Nordst. ; C. sphairoideiirn, West; C. subvAidvdatum, 

 Wille ; C. variolatum, Lund. 



Staurastrum axuleatum, Ehrenb. ; St. anatinum, Cooke and Wills ; 

 St. Arnellii, Boldt ; St. furcatum, Ehrenb. ; St.. aiconspicuum, Nordst. ; 

 St. arachne, Ralfs ; St. lanceolatum, Arch. ; St,, oxyacanthum, Arch. ; 

 St. pungens, Breb. ; St. scabrum, Breb. ; St. aristiferum, Ralfs ; St. pileolatum, 

 Breb. ; St. pterosporum, Lund. ; St. subscabrum, Nordst. 



Having discussed the most important facts concerning the general distri- 

 bution of Desmids in the British Islands, we can now return to the abundance 

 of the Desmids in the British freshwater phytoplankton. 



We have shown which areas of these Islands possess the rich Desmid- 

 floras, and when one considers that the British lakes are almost all situated 

 in these western areas, it is not very surprising that they possess a plankton 

 containing numerous Desmids. Neither is it surprising that many of these 

 should be the western types, provided that these western types are capable 

 of withstanding the conditions of a limnetic life. 



Therefore we consider that the Desmids of the British freshwater 

 phytoplankton are due largely, and the western types entirely, to 

 the situation of the lakes in the rich desmid-areas of the old 

 formations. 



The antiquity of the geological formations is not a special factor in the 

 occurrence of the numerous plankton Desmids, but in the occurrence of 

 Desmids as a vjhole. The presence of numerous Desmids in the plankton of 

 the lakes follows as a matter of course. 



One does not expect an abundance of Desmids in the plankton of the large 

 Swiss lakes. They are situated in poor Desmid-areas, and in North 

 Switzerland the geological formations are for the most part too recent. 



Most of the Central European lakes are situated in areas relatively poor 

 in Desmidiaceae. In Denmark the formations are Cretaceous and 

 Jurassic, largely overlain by drift, and similarly the lakes of Northern 

 Germany are situated on immense areas of drift, overlying comparatively 



