MR. U. GURNEY ON FRESH- AND RRACKISH- WATER CRUSTACEA. 639 
such a destruction can be shown to occur ; but I believe that many 
of the more sedentary species have acquired the power of resisting 
such ranges of salinity that they are enabled to live permanently in 
the intermediate zones, though properly belonging either to fresh or 
to salt water. It is quite probable that differences in adaptibility 
could be shown to exist between groups of individuals of the same 
species taken from different localities. 
In consequence of the tidal nature of the rivers, the true fresh- 
water fauna is invaded by a number of species which belong 
properly to brackish-water or are even typically marine. Some 
of these species have established themselves beyond the limits of 
even abnormal tidal influence. It has therefore been found 
necessary to include these brackish-water forms in my lust, and 
in fact it is very difficult to know how or where to draw the line. 
I have thought it best to include in my list all those species which 
I have found associating with true fresh water forms, since obviously 
any geographical boundary is out of the question. For instance, 
at the Yacht Quay in the North River at Yarmouth a tow net 
collection may at times be made up entirely of quite characteristic 
fresh water forms; at other times the species may be entirely 
marine; and at others again partly marine and partly fresh-water. 
So much depends upon the state of the tides and liver currents. 
I have, however, not always followed even the above rule, for had 
I done so, I should have been forced to include many chance marine 
immigrants such as Acadia di.<raudata or Carcinu >• nuicnas. Those 
that have been included are generally such species as I consider 
may habitually occur in the conditions in which they have been 
found. 
But, apart from the tidal nature of the rivers, and the consequent 
introduction into the Norfolk fauna of semi-marine species, the 
physical conditions obtaining in the Broads in general are some- 
what peculiar owing to their proximity to the sea. The salinity of 
the water is always a little above the normal of pure fresh water, 
probably owing to salt spray blown in from the sea. This relatively 
high salinity can hardly be without influence upon the presence 
and distribution of certain species. 
In the list that follows, where no mention of localities is made, 
it must be understood that the species in question is widely dis- 
tributed, and further information must be sought in the table of 
distribution given at the end. 
