C. L. Walton 
251 
the snails can live on moist ground, quite away from any quantity of 
water for considerable periods, is sufficiently proved by the fact that 
1 have kept them alive for eleven weeks on moist grass and moss, even 
when infested with the larval forms of the liver fluke.’’...“The snails 
thus left” (by floods) “continue to wander and feed, so long as the 
bottom of the grass remains moist. Even on land which is not liable 
to flood this snail may exist in large quantities, having crawled from the 
ditches, ponds, streams and marshy spots through the moist grass. 
A drought may render the snail dormant but, unless continued too 
long,- it revives at the first shower of rain.” In a footnote to another 
paper 1 , Thomas quotes Sir C. Lvell who states that in Maderia, L. trun- 
catula was accidentally introduced by the Portuguese, and spread so 
widely as to be found even in pools and ruts of roads. I have noted 
the snail in ruts on roads, etc., in the Aberystwyth Area, but have 
never observed any such phenomena of abundance after floods as in 
that noted by Thomas in Oxfordshire where conditions probably differ 
considerably from those found in Mid Wales. A considerable series 
of experiments were carried out to test the effects of drought upon 
L. truncatula. My experience during the last three years may be stated 
as follows: Direct observation in the field, and Laboratory experiments 
have both amply proved that the snail can resist drought for a relatively 
short time, though the time necessary to cause death depends upon 
both physical and meteorological factors which are very variable. 
Should the spot inhabited be a shallow hard bottomed ditch lying under 
a bank in full and continuous sun glare, the water evaporates rapidly 
and the death of the mollusc soon follows. Should the situation be 
a comparatively shady one, or overgrown, or floored with moisture- 
retaining mud, evaporation is correspondingly delayed and the molluscs 
have a greater chance of surviving a drought period. Many streams 
and wet spots are fed by springs and are not affected by a drought until 
it has lasted long enough to stop such sources of supply. Some spots 
are but rarely touched by drought, and there are a few in the Survey 
Area that have not dried since the work commenced; but these are 
very exceptional cases. L. truncatula never attempts to follow retreating 
water and when overtaken by drought remains with the mouth of the 
shell pressed closely to the ground. Thus even after a few days a com¬ 
paratively damp spot may still remain under the shell, and should 
drought continue the snail withdraws further into the shell and so 
remains until either water returns or death supervenes. In damp 
1 Quart. Journ. Microsc. Sci., 1883. 
