438 MR. GURNEY ON THE CRUSTACEA OF EAST NORFOLK RIVERS. 
occurs at Acle, growing on stones on the river bed, but not 
on the reeds by the bank. Quantities of it drifted down the 
river on up-rooted reeds between July 4th and 10th. Below 
this point I have not found it except once in the Muckfleet. 
Above, it has been found growing at various points as far as 
Ant Mouth ; growing thickly under the arch of Ludham Bridge, 
and even in Barton Broad. It was decidedly rare in Barton, 
occurring only on one or two posts at the south end. In the 
Thurne River it has long been known to occur abundantly, 
reaching into Hickling Broad. I have not found it myself in 
the river, though I cannot profess to have hunted there very 
diligently, but in Heigham Sounds and Hickling Broad it 
was abundant on the reeds. It was not seen in Horsey Mere. 
It is worth noting that it did not appear to grow upon reeds 
in a reed bed or where there was much shelter ; it was only 
on the reeds which grew scattered in the open water or in the 
Deep Dike that colonies were found. 
This interesting species is a recent immigrant into rivers. 
It was first discovered by Agardh in 1816 in salt water at 
Stockholm ; its name was given to it by Allman (in 1843) 
who found it in fresh water (?) at Dublin. After that a 
number of observers recorded it from various places, generally 
in brackish water. It seems to have rapidly adapted itself 
to fresh water and has migrated up the River Seine and the 
Elbe. It found its way from the Elbe into the great water- 
pipes of Hamburg and there established itself, to the great 
obstruction of the pipes. Something of the same sort seems 
to be happening here, for it is not found in the salter reaches 
of the rivers, but in those parts in which the water is nearly 
always practically fresh. In Barton it is, of course, in as fresh 
water as can be found in these parts. Probably it will, 
before long, be found up the Bure as far as Wroxham. 
