424 MR. ROBERT GURNEY ON THE CRUSTACEA 
in pools on the adjoining marshes. Near Burgh Castle there 
are a number of such pools dug out of clay, each containing 
a brackish fauna somewhat different from the next one. 
For instance, of three pools, one contained only Neomysis 
vulgaris, a second abundance of Gammarus duebeni and a few 
Neomysis, while a third contained a mixed fauna including 
Jaera marina, Idotea viridis and Delavalia palustris besides 
quantities of the little Mollusc Hydrobia ulvce. 
CONCLUSIONS. 
In considering the Crustacean fauna of these rivers one’s 
attention is naturally attracted first to the Tides as the 
factor chiefly affecting the distribution of the species. From 
what has been said it appears that, in the Bure, the tidal 
wave, the influx of salt water, and the distribution of the 
Crustacea are three things having little correspondence. The 
tidal wave is a thing by itself, reaching far up the river and 
accompanied by the tidal stream, but having no influence 
whatever on the fauna. The tidal stream has some influence 
owing to its violence in the lower reaches. But it is the range 
of the salt water which is the really significant element in the 
tidal phenomena. This range I have stated to be practically 
limited by Acle Bridge, but probably accurate chemical 
analysis would be able to trace changes of salinity higher up. 
However, assuming my statement to be correct, the range of 
salt water does not strictly correspond to the range of certain 
Crustacea which are generally supposed to be dependent on 
water of high salinity. Many species extend far beyond this 
point, into completely fresh water. So that one might say 
that, roughly speaking, brackish water reaches to Acle, the 
brackish-water species of Crustacea to Ant Mouth, and the 
tidal wave to Wroxham Bridge. 
The influence of the current is a physical one, largely 
indirect, since a strong stream in a narrow channel scours 
away the soft bottom mud which is the habitat of many 
species, and checks the growth of vegetation upon which 
