420 MR. R. GURNEY ON THE CRUSTACEAN PLANKTON 
post-GIaci;il distribution. In 1907 Apus canniformis, which had not been seen 
in Britain for over 40 years, was found in Scotland by Mr. Balfour-Browne, 
but had again disappeared the following year. Limnicytliere mirahilis is 
claimed by Zschokke as a member of the deep-water fauna of the Swiss lakes, 
which is of northern origin and finds only in deep waters the conditions 
necessary for existence in its southern station. But in 1905 I found this 
species in a shallow rain-pool near Biskra in Algeria, and another species, 
L. incisa, in a stream which irrigates the Oasis of Oumach. 
The Plankton of Ennerdale H 'atev. 
The discovery of Limnocalanus inacrurus in Ennerdale indicated at least a 
possibility that other "relict" Crustacea might be found there, and made it 
most desirable that the lake should be carefully explored. With this 
purpose in view I visited the lake at the end of September 1922, and made 
special efforts to collect the Crustacea of the deep water. The lake is shallow 
at its western end, but the eastern part is a deep narrow trough with a 
maximum depth of 148 feet. The shores aie rocky, and with a very steep 
The weather during my stay was unfavourable, but I was able to search a 
large part of the deep trough with a coarse tow-net and light dredge, and 
also to take plankton samples on three successive nights. Neither Mysis 
relicta nor any Amphipods were taken. This negative result should be tested 
by examination of the stomachs of Char [Salmo alpinus), but I have not 
been able to obtain specimens. 
Table 4 gives the composition of the plankton between September 27th 
and 29th. It is of course impossible to obtain accurate information as to 
the vertical distribution of plankton without the use of a closing net and a 
succession of short vertical hauls, but I do not think that such accuracy is, in 
general, necessary, and a fair approximation to the truth may be got with 
less effort and simpler means. My own collections were made with an 
ordinary small plankton net, which was let down, together with a weight, to 
the required depth and then towed a distance of about 100 yards. Naturally 
the net fished both going down and coming up, but the results of the different 
hauls are so distinct that they do in my opinion give information as to the 
vertical distribution. I have not attempted to compare different hauls by 
absolute numbers, but have counted the individuals of each species in the 
whole or in part of the collection, and expressed their frequency as percentages 
of the whole number of individuals of all species counted. This method gives 
an accurate statement of the comparative frequency, though not of the absolute 
numbers of any species, and it admits of a comparison being made between 
samples from the same or different lakes however they may have been taken. 
Similar figures are given for Wastwater and C^oniston for purposes of 
comparison. 
