no eet OLE 
VOL. XIV. (3) HOLOCENE DEPOSITS 201 
in this country and in Denmark. It is probable that 
its presence in the upper bed at Clifton Hampden, and in 
the modern deposits at Walthamstow, is due to the des- 
truction of an older deposit and subsequent redeposition : 
an occurrence which must often happen with fluviatile 
deposits bordering on the main stream. Certain it is that 
this species offers a most perplexing problem, the solution 
of which we must leave to the future, having at present 
no clue. 
Dreissensia polymorpha is represented by two valves 
from bed B. The occurrence of this species 7 situ, in 
what is obviously an old deposit, is of great importance, 
since it has generally been considered to have been intro- 
duced into this country in the early part of last century. 
The species were first noted in these islands in 1824, by 
Mr J. de C. Sowerby, the examples having been obtained 
‘n the Commercial Docks; and he suggested that it had 
been imported into this country from the Danube and the 
Russian rivers, on timber. This view has received the 
adhesion of practically all writers on British Mollusca, 
with the exception of Dr J. Gwyn Jeffreys.’ He expressed 
the opinion that it was really a native of these islands and 
of northern Europe, gave various reasons for this conclu- 
sion, and stated his belief that it would in the future be 
proved to be indigenous; and then “the ingenious 
theories which have been put forward to account for the 
mode of its transfer across the sea will not require further 
discussion.” In 1890, a single valve was recorded” as 
having been found in a fresh water deposit at Whitefriars, 
London, which was probably of Roman age. Unfortu- 
nately, this example was not found zz stu, so that the 
evidence was not so conclusive as might have been 
1 British Conchology, 1862, Vol. 1, pp. 47-50 
2B. B. Woodward “On _ the Pleistocene (non-marine) Mollusca of the London 
district.” Proc. Geol. Assoc., Vol. xi., p. 342- 
