426 MR. ROBERT GURNEY ON THE CRUSTACEA 
water of high salinity if the locality is suitable in other respects ; 
but the Harpacticidce appear to be quite intolerant of river 
conditions. The scarcity of fresh-water Copepoda in the 
part of the river in question is probably due to scarcity of 
vegetation, owing mainly to the speed of current, and not, in 
the first instance, to the salinity. 
But if there is no established floating fresh-water river 
fauna, so there is no established floating marine fauna. 
Marine Calanoids, when they are found, are almost always 
few in individuals, and nearly all the individuals are mature. 
The actual fauna of the river in this region therefore consists 
of a floating immigrant population, either fresh- water or marine 
in origin, and a sedentary resident community. The residents 
are generally of a marine type. They constitute the “ Brack- 
ish-water ” fauna, but include several species not usually 
considered to fall under that head. 
Whereas the really marine and fresh-water species do not 
mix, except by rare accident, these brackish species have 
a range which leads them to associate almost indifferently 
with either group. 
The associations of the species one with another is made 
clearer by the table given here (table 2) which is constructed 
in the same way as table 1. It gives all the species taken 
in the stretch of river between Six-mile House and Acle 
Bridge in a number of collections made at different times. 
It must be remembered that the collections were always 
qualitative and never quantitative so that the occurrence of 
a single individual of any species has the same value in the 
table as its occurrence in numbers. Still, in spite of this source 
of error, the table does show in a fairly satisfactory manner 
the kind of association one may find in this region. It will 
be seen that the fresh-water Cladocera and the marine Cope- 
poda do not mix, whereas both occur in company with certain 
brackish Copepods and several Malacostraca of a marine 
type. Column No. 7 ( Bosmina longirostris) is a good example 
to take, since it is one of the commonest fresh- water species, 
entirely planktonic, and so a good indication of the limits of 
the fresh-water fauna. The crosses in brackets indicate dead 
