OF THE EAST NORFOLK RIVERS. 429 
to fresh, marine forms would find ideal conditions in which 
to accomplish a migration into fresh water. 
Also we know that the rivers and Broads of East Norfolk 
are relics of what were once arms of the sea, and that, within 
historically recent times, these bays and estuaries supported 
a marine fauna. Close to Ludham Bridge some mud cleared 
out of the river bed this year was thick with shells of 
Scrobicularia piper ata and Cardium edule, mixed with present- 
day Unios, and looking as fresh as if they had contained 
living animals. Why is it that the marine fauna has been 
driven out ? Why have not some species stood their ground, 
gradually accustoming themselves to the gradually changing 
conditions ? Actually the fauna of the Broads, as at present 
constituted, presents no features indicating a past marine 
occupation. It has been shown in this paper that salinity is 
of importance as a factor of distribution, but a good deal of 
evidence has also been adduced to minimise its importance, 
and indeed there are yet other factors which must exercise 
much influence on any marine species immigrating into fresh 
water, or attempting to maintain themselves in a freshening 
and shallowing arm of the sea. One of these- factors is 
Temperature. 
All our rivers and Broads are very shallow, and consequently 
the temperature of the water must be exceedingly variable, 
since the heat of the sun can readily penetrate through the 
6 feet or so of our deepest Broads. The temperature may 
vary from day to day or even from hour to hour, whereas the 
depth and volume of the sea causes a temperature varying 
nothing from day to day and little from month to month. 
The difference of temperature conditions must be a very 
serious barrier to the immigration of open-sea species, such 
as Mysis oculata , the parent form of My sis relict a, and the 
fact that this and other “ Relict ” species only occur in large 
deep lakes is some evidence of the importance of uniform 
temperature to them. It has been said* that the differences 
between the fresh-water and marine fauna decreases from the 
Poles to the Equator, so that in certain tropical regions one 
finds a large number of species of marine type inhabiting 
* P. & F. Sarasin Materialen zur Naturgeschichte der Insel Celebes, 1S9S. 
