0. L. Walton 
245 
bank for a considerable distance, but the vegetation became more and 
more peaty. The streamlet follows a bank and hedge which bend 
somewhat, and thus receives the majority of the drainage from the ill 
drained peaty field above; consequently the lower portion receives 
more drainage than the upper and finally the clay is hidden under 
peat. Below the bend, I was unable to find L. truncatula. 
I was for a long time puzzled to account for the absence of Limnaea 
from peat and several factors were studied successively, namely: 
(1) Peat acids, 
(2) Food, 
(3) The relative size of the foot of the mollusc. 
That peat acids play a .part is beyond doubt since Limnaea from the 
vicinity of peat, or within the influence of peat drainage are usually 
stunted and small. Buddicom 1 says of L. truncatula at Church Stretton 
“Lime is necessary for the construction of the shell, and among land 
forms on the Longmynd I have found small fragile forms of Vitrea, 
while specimens of L. truncatula in springs on the hill side are much 
distorted, with thin transparent shells.” 
Taylor 2 also states “ Granitic soils, peaty districts, or other formation 
deficient of the calcic carbonate of which shells are mainly composed, 
are characterised by shells not only of thin texture but dwarf size.” 
I cannot say that I have found any Limnaea from the Aberystwyth 
Area to be either deformed or dwarfed, merely smaller than the normal 
and thin shelled. I now consider the chief factor governing the differ¬ 
ence in distribution of A. truncatula and L. peregra (and their absence 
from peat) to be the relative size and expanse of foot. 
In L. truncatula the foot is small and the shell a narrow spire, while 
in L. peregra it is relatively much broader as also is the shell. L. peregra 
is able to travel upon the surface of soft mud in which truncatula would 
sink, as is readily observable during the progress of field and laboratory 
work. It is significant that the newly hatched young of peregra are 
almost invariably to be found crawling upon the water plants or stones 
on which it is hatched, while truncatula when young is more often to 
be found in similar situations than during later life. 
L. truncatula will leave a soft substratum for a harder if food be there 
available, as for instance fallen leaves that have become coated with a 
1 Buddicom, R. A. (1900), The Land and Fresh Water Mollusca of Church Stretton, 
pp. 182-195. 
2 Taylor, J. W. (1895), Monograph of the Land and Fresh Water Mollusca of the 
British Jsles, Pt. n, p. 85. 
