656 MR. R. GURNEY ON FRESH- AND BRACKISH-WATER CRUSTACEA. 
is a tide of nearly two feet, coming from the Waveney, but 
apparently all the salt comes through the locks from Lake Lo thing; 
at all events, the salinity is highest at the East end of the Broad. 
In many places the shores are covered with Enteromorpha , in 
which lives Splicer oma serratum. 
Rockland and Surlingham Broads I have only twice visited, 
so that my list of their Crustacea is no doubt very incomplete. 
They appear to have few species but immense abundance of 
individuals. In particular, Pleuroxus aduncus was found in 
extraordinary abundance in both Broads, though it is by no 
means a very common species in other parts of the district. 
They are both “ fresh-water ” Broads, though they are tidal. 
Seasonal Distribution of the Cladocera. 
I have given here a table showing the months during which the 
various species have been found to occur, and also the months in 
which sexual forms appeared. Two columns are devoted to each 
month ; in the first column a cross indicates simply that the species 
occurred during that month ; in the second column are given the 
total number of records of males and ephippial females throughout 
the past year. 
Such a table shows at a glance what species are found all the 
year round, and what species only appear in the summer ; but it 
does not show their relative abundance at different times. Some 
idea of relative abundance might be given by presenting all the 
records of each species during each month, but there are great 
objections to this method. The only satisfactory method would be 
to give the total number of records for each month expressed as 
percentages of the whole number of collections taken. Quantitative 
methods, such as are used in the study of Plankton, are not 
available when dealing -with the Cladocera as a whole, since the 
majority of them are littoral species, living among weeds or in mud. 
Observations carried on throughout a whole year, and tabulated 
in this way, are certainly of some value taken by themselves, but 
it is highly probable that, if carried on for a series of years, 
variations in the dates of appearance and disappearance of species 
and in their relative abundance might be found, dependent upon 
meteorological conditions. For instance, the relatively high tem- 
perature of March may have caused several species to appear 
