MR. R. GURNEY ON FRESH- AND BRACKISH-WATER CRUSTACEA. 655 
being separated from any direct connection with the Bure by the 
Muckfleet sluices. They have a rich pelagic fauna, and probably 
the apparent scantiness of the littoral fauna is only due to 
insufficient search. 
The records under the head of the Muckfleet only refer to the 
lower part of it, close to the sluices. At this point the dyke widens 
out into a large, shallow pool thickly grown up with weed, which 
is for the most part Myriopliyllum , and it is here that most of 
my collections have been made. The Muckfleet is peculiarly inte- 
resting, owing to the great fluctuations of salinity to which the 
water is subject. On the three occasions on which the water 
has been analysed, the salinity was 19 59, 235 09,* and 24‘48 
grains of Chlorine per gallon, the salinity in the river outside 
the sluices being at the same times 1 2*24, 39 18, and 20 81 
respectively. It seems probable that the salinity in the Muckfleet 
changes slowly, so that if it is raised very high by slow infiltration 
of water during exceptional tides in the river, it will remain at 
this high level after the river salinity has sunk to the normal. 
The changes in the fauna correspond largely to the changes in 
the composition of the water. There is a constant fauna repre- 
sented by a few common Copepods and by Gammcinia duebeni, but 
at times there may be found species evidently derived from the 
upper fresh- water reaches, such as Daphnia cucullata, and at others 
those derived from the river, belonging to the brackish-water fauna. 
Among the latter are Neoinysis, Pahenionetes, and Cyclops cequoreus. 
Tachidins littoralis, though a marine type, can adapt itself to fresh 
water, and is therefore a constant inhabitant. 
The two columns devoted to Fritton and Sprowston districts 
contain records from ponds and ditches only, most of the ponds 
being ordinary farm ponds. The “Fritton” district lies South 
of Fritton Lake, and includes Ilerringfleet, Lound, and Blundeston. 
The “ Sprowston ” district includes Rackheath, Thorpe, and 
Sprowston. 
Oulton Broad is one of the most interesting Broads of the 
district. The salinity of its water, though subject to great 
fluctuations, is always high, so that its fauna is not rich, and 
is made up for the most part of brackish-water species. There 
* A month previous to the day on which this analysis was made there was 
an exceptionally high salt tide in the river. 
