SHEPPARD : INTER-GLACIAL GRAVELS OF HOLDERNESS. 
7 
apposition, and I cannot find that any such specimen has been taken 
in this neighbourhood. 
The occurrence of this fresh-water shell, mixed witli the marine 
shells in these gravels has been the subject of much controversy, 
and has frequently been the cause of rather heated discussions 
amongst local geologists. A careful search through the literature on 
the subject showed that the presence of this shell had several times 
been recorded, but in not one single instance have I been able to find 
a satisfactory explanation of the same. In order to get this I was 
requested to write to Mr. P. F. Kendall, and his reply to the com- 
munication was gratifying to us all. As the purport of this letter 
has not previously been made public, the present seems a very fitting 
opportunity for quoting it, more especially as it is so relevant to the 
immediate purpose of this paper. 
" Cyrena fluminalis is in two ways a puzzling shell: it is odd that 
its occurrence should be so limited, and it is still more strange that a 
shell at present so strictly southern in range should occur in such 
profusion in a glacial deposit." 
" First, as to its limited glacial range in Yorkshire. I think 
that, as Clement Reid suggests, there ma^ be some connection 
with the gap in the Wolds. At the present day the shell is of 
fluviatile habitat, and probably was mainly so in glacial times, but 
6/valve shells are not carried out to sea in large numbers by modern 
rivers, and therefore Reid's explanation, taken by itself, will not 
account for the profusion of these shells intermingled with true 
marine species. No more will it account for the form of the mounds 
in Holderness. These are not marine but glacial features." 
" Now for a suggestion. We have every reason to believe that 
Holderness was invaded again and again by the oscillating ice-front. 
Between a retreat and an advance there would be left a tract of land 
with many lakelets. May not Cyrena have inhabited some of these, 
and been involved at the next invasion with the marine shells 
dragged up by the ice ? That's one suggestion. Another, and to 
me more probable hypothesis is that these shells belong to a period 
before the first advance of the Norse Ice. At this time the old 
Bumber would debouch somewhere on Holderness (there would be a 
