7 8 E. Marion Delf. 
of the larger microscopic worms digesting a plant of Closterium 
nearly half the length of its own body, as well as various diatoms 
and unicellular green bodies which must have been either Chlaniy- 
domonas , or zoospores of about the same size^ these worms are 
very numerous in the Hampstead ponds. 
The Bacillary. 
These form one of the most characteristic features of the 
vegetation of the ponds. They are probably present all the year 
round, though much diminished in the summer months, and they 
have a marked vernal maximum from February to May, when they 
form delicate brownish strands around any submerged plants, but 
particularly around Elodea and the old decaying leaves of Poa 
fluitans. 
The genus which is most widely distributed is undoubtedly 
Navicula, especially a small species, which abounds in the mud of 
all the ponds. Gyrosigma has been found in ponds II and VIII, 
and is also a mud-loving diatom. Tabellaria fenestrata is very 
abundant as well as of frequent occurrence, but is principally 
epiphytic in habit; Synedra is also very common and is either free- 
swimming or epiphytic in habit. 
There is a certain amount of difference in the diatomic flora 
of the ponds. Tabellaria is characteristic of ponds I, III, IV and V, 
and infrequent or absent in the rest; it is associated with Elodea 
in the first three and with Poa jlzutans in the last of these ponds. 
Synedra is particularly common in ponds III and IV, although found 
more or less frequently in all the ponds. Gyrosigma and Gompho- 
nema are mostly found in ponds II and VIII, although also seen 
occasionally in other ponds. Cocconeis has been observed epiphytic 
on Microspora, Tribonema and Vaucheria, principally in pond V. 
Most of the diatoms of the Hampstead ponds are spring forms, 
dominant from February to May (1912, 1913, 1914), with a slight 
secondary maximum from October to December (1912), or in 
December (1913). They seem to disappear wholly in summer, 
although some may prove to be perennial on closer investigation. 
Tabellaria fenestrata (and perhaps also Synedra) seems to be 
associated with phanerogamic rather than filamentous hosts. Both 
of these diatoms reach their maximum during the colder months 
of winter or early spring, when the decay of the previous year’s 
vegetation is in an advanced state ; at the same time, these are 
usually rainy months, so that a supply of nitrogen from decay, and 
