264 THE ENTOMOSTRACA OF EPPING FOREST.
very well indeed. In five instances, however—namely, Pteu-
roxus trigonellus, Bosmina longirostris, Cypria ophthalmica, Cyclops
strenuus, and C. bicuspidatus, the supposed annual period of extinc-
tion probably does not exist, for the first-mentioned species has
been found at all times of the year, and each of the others in
eleven months out of the twelve. But it is only fair to remember
that the method of taking the earliest and latest records obtained
during a period of several years is a very severe test of periodicity,
as it is one which probably more than compensates for the imper-
fections and accidents of collection.
Besides the species given in the table it is very pro-
bable that several others are periodic in their appearance (e.g.
Eurytemora lacinulata), but the small number of times they have
been seen renders a definite statement on this subject impossible
at present.
Seasonal distribution of species.
Owing to the want of system in collecting, it is quite out of
the question to attempt to obtain the cycle of seasonal distribu-
tion of species for the whole district by taking the averages for
each month during the whole period, as was done with the
Cladocera of Wanstead Park. The best that can be made out
of the data at command is to show the months in which each
species has been found, and to deduce from this the approximate
cycle of seasonal distribution. This has been carried out in the
twelve columns following the names of the species in Table I.
Granted that the collections are fairly numerous, that they
are made without any special object except that of getting
an exhaustive list of species, and further, that they are not
spread over too long a period (so as to avoid the inclusion of
a number of exceptional seasons), then this method should give
approximate, results of real value, although, of course, not for a
moment to be put before those obtained by more systematic work.
In the present case the results yielded by the foregoing method,
from the data supplied by the whole Epping Forest area, corres-
pond remarkably well with those obtained by the "average"
method from Wanstead Park. This will be clearly seen from
the following summary which gives the number of species of each
of the three main groups of fresh-water Entomostraca taken in
each month :—