68 
B. Marion Delf. 
In December, 1913, a green scum of Euglena formed over the 
damp mud at the edge of pond II: this has remained for three 
months without apparent change, but Euglenae are rarely found in 
samples of water from the same pond. 
Ascoglena has been found in parts of ponds I, III and VII 
epiphytic on filaments of Tvibonema , Mougeotia and Micvospova : it is 
never found on filaments of Spivogyva, present at the same time. It 
has been found mostly from February to May ; in one case filaments 
of Ti'ibonema were infested almost at their first appearance with 
Ascoglena but later on this was in turn almost displaced by numbers 
of Synedva frustrules. 
Species of Pevidinium occur very frequently but are seldom 
found in any abundance. It seems probable that this may be a 
perennial form, for it has been occasionally found in July and might 
easily have escaped detection when apparently absent in August and 
September. Moreover, Pevidinium can withstand the extreme 
concentration of water, and the exposure to sunlight which follows 
the drying back of a pond even more successfully than such a hardy 
diatom as Navicula. In June, 1913, pond II almost dried up, a small 
shallow pool at one end being the only wet part left. No algal 
vegetation could be detected after careful dredging except the 
empty trustifies of species of Navicula and a few perfectly active 
Pevidinium cells. In the Hampstead ponds Pevidinium is distinctly 
a spring form, becoming most abundant during April and May; the 
Peridineie of the Lake District as observed by West and West 1 were 
of general occurrence, but had a distinct summer phase; those of 
Barton’s Pond were found by Fritsch and Rich 2 to be most frequent 
in warmer weather, but according to these authors, a dilution of the 
water is also favourable. 
On the whole, in the Hampstead ponds it may be said that all 
the Flagellates excepting Euglena are periodically numerous; that 
Syuuva and Dinobvyon are early spring, and species of Pevidinium 
later spring forms; that Ascoglena depends to some extent on the 
presence of suitable host plants, especially Mougeotia ; and that 
Euglena is a summer form apt to appear spasmodically in great 
abundance, but also able to survive winter weather on damp mud in 
a very sluggish amoeboid condition. 
' West, W., and West, G. S. “ Periodicity of the Phytoplankton of some 
British Lakes.” Journ. Linn. Soc. Lond. Bot., XL, 1912. 
3 Fritsch, F. E., and Rich, F. Loc. cit., p. 54. 
