4 Messrs. G. S. Brady and D. Robertson on the 
these rivers are numerous Bekwati lakes or meres, locally 
called “ broads,” occupying’ doubtless, areas whic were for- 
merly depressions of the sea-bed. These have at the present 
day all the external Urine ST of freshwater lakes; they 
d dense growths of sedge and rushes, and their shallow 
waters supporting rast Greis of aquatic weeds, Pota- 
i iai 2n dicc illum, Chara, water-lilies, and the like. Yet 
they are to a greater or less degree su ject to the influence of 
the idm rising and falling to some inconsiderable extent ; 
and though the water which thus ebbs and flows must usually 
e fresh, we are informed that in some broads sea-water has 
been known to penetrate in sufficient quantity to kill the fish. 
There can be no doubt that the changes which produced the 
present aspect of the district are still in progress, that the 
broads are yearly becoming shallower, and that, owing partl 
to the débris brought down by the rivers, partly to the choking 
arising from constantly decaying vegetation, they will at no 
distant date cease to exist. In 1827, Mr. Taylor stated their 
depth to range from 15 to 30 feet; at the present time, 3 to 
15 feet would be a tolerably correct estimate. Mr. Stevenson 
tells us that “ Mr. Gunn estimates the growing-up process, 
from subsidence id vegetable matter, aided by drainage, at a 
ceeded scere e adds, * to my knowledge, where, 
some fifteen years bk. g could pull a boat through, is now a 
pathway almost firm e nough for a marsh-man in boots.” The 
rise and fall of the tides along the Norfolk coast is debrüniali 
small, averaging at Yarmouth only three or four feet; yet, 
owing to the low level of the district, they affect the rivers for 
a very great distance inland. e Rev. Canon ingsley, i in an 
interesting paper on “ the Fens,” in ‘Good Words’ for 1867, 
states that, were it not for the great sea-sluice of Denver, on 
the Ouse, the tides would be felt to within ten miles of Cam 
bridge. There can be no difficulty, then, in understanding 
how a fauna introduced when the whole East-Anglian district 
was overspread by the sea, should hold its ground for a 
lengthened period, while its habitat was year by year becoming 
less subject to marine influences, and that the more hardy or 
more plastic species should remain even after fresh water 
entirely usurped the place of salt, while at the same time a 
new fauna derived from the landward side was also gradually 
establishing itself, as the conditions of existence became more 
favourable. It is, indeed, impossible to account in any other 
way for the existence in the more remote broads of Norfolk, 
in the river Cam at Ely, and in the dykes about Whittlesea, 
of species purely marine (or, at least, decidedly estuarine) in 
