OF THE EAST NORFOLK RIVERS. 
425 
others are dependent. The influence of the salt is mainly 
a direct physiological one, and must be felt, in a greater or 
less degree, by all species exposed to it. It is also a factor 
which is capable of accurate estimation, so that one is tempted 
to lay greater stress upon its importance than is, perhaps, 
really justifiable. 
Speaking now solely of the stretch of the Bure from Thurne 
Mouth downwards, I believe that there is no indigenous 
floating fauna, whether fresh or salt. As a general rule 
townet collections, though sometimes producing a fair 
diversity of species, are remarkable for the paucity of 
individuals. The number of individuals taken diminishes 
rapidly below Acle, and at the Yacht Quay, where the water 
is thick with mud and larger debris, townet collections may 
be almost blank. 
It seems most probable that any fresh- water species taken 
in the townet are derived from the Broads or from the upper, 
relatively still, reaches of the river. If their numbers were 
not constantly recruited from above they would speedily die 
out, not solely by reason of a higher salinity or speed of 
current, but because the conditions of life are unstable and 
continually varying. Certain species, such as Daphnia 
cucullata, are probably killed by a very slight trace of salt, 
but others, such as Bosmina longirostris, are certainly capable 
of maintaining themselves in water of a high salinity. The 
latter is found in Hickling Broad where the salinity is high 
(about 70 grains of Chlorine per gallon) ; but it maintains itself 
with difficulty, and is found only in small numbers. Hickling 
Broad is rather a useful comparison in cases of this kind ; 
for here, and in Horsey Mere, where the conditions of life 
appear to be favourable except for the high salinity, certain 
species of fresh-water animals are found which are absent 
from the river in water of equal salinity ; so that their absence 
from the river must be due to other causes, such as current 
or fluctuations of salinity. 
The same remarks apply in a lesser degree to the more 
sedentary fresh-water species, such as the Cyclopidce and 
Harpacticidce. Of the former some are often found living in 
VOL. VIII. 
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