PERIODICITY OF THE PHYTOPLANKTON OF SOME BRITISH LAKES. 427 
PERIDINIEE.—Ceratium Hirundinella, which is so ubiquitous in almost 
every part of the world, occurs only in certain lakes of the British lake- 
areas. It occurs in most of the English lakes, but is entirely absent from 
Wastwater, and very probably absent from Buttermere and Crummock 
Water. It is also a notable absentee from the larger lakes of North 
Wales. 
When this Peridinian does occur, it is a summer form with a small 
maximum in August or September. It completely disappears from the 
plankton in the winter months, entering into an encysted state in December 
in the larger lakes, or earlier in the smaller bodies of water *, and re- 
appearing about April or Мау, In the lakes of more southern climates it is 
a perennial constituent of the plankton 1. We have not seen any seasonal 
form-variations of this organism such as those described by Wesenberg-Lund 
from the Danish lakes ў, although several forms frequently occur simul- 
taneously in some of the lakes. 
Ceratium cornutum is а similar summer form. 
Peridinium Willei is the most abundant and widely-distributed species of 
the genus in the English and Welsh lakes, and in some of the Scottish 
lakes. It isa summer form, with a considerable maximum at some period 
from July to September, and, as in the case of Ceratium Hirundinella, it is a 
perennial constituent of the lake-plankton of more southern latitudes. 
Species of Peridinium, like many Diatoms, do not attain a universal 
maximum at one definite period of the year, but the various species reach 
their greatest vegetative development at different times of the year $. In 
some of the pools of the English Midlands there is a summer species (a var. 
of P. cinctum), a spring species (P. anglicum), and a very early spring— 
almost a winter—species (P. aciculiferum), each of which has been shown 
to form resting-cysts at the close of the vegetative period, even though the 
vegetative periods are all at different seasons. 
* * * * ж * * * * 
In studying the phytoplankton of lakes difficulties of comparison are 
every where met with, even when dealing with lakes in the same area. The 
constituents of the phytoplankton are not the same in all, and species 
which oecur abundantly in one lake may not occur in any of the others. 
ж G.S. West, “A Biol. Investigation of the Peridin. of Sutton Park, Warwicksh.,” New 
Phytologist, viii. 1909, p. 193. 
+ Brahm & Zederbauer, in Verhandl, der К. К. zool.-bot. Ges. Wien, 1904, p. 48; G. Entz, 
in Result. der wiss. Erforschung des Balatonsee, Bd. ii. Budapest, 1904; Lemmermann, in 
Archiv für Hydrobiol. и. Planktonkunde, iii. 1908, p. 379. 
t Wesenberg-Lund, ‘Plankton Investigations of Danish Lakes, Copenhagen, 1908, p. 69. 
$ Ц. S. West, in ‘ New Phytologist,’ viii. 1909, p. 194, f. 196, 
