252 
Liver Rot of Sheep 
hollows, some proportion of the snail population of a ditch may survive, 
while the remainder perish, or only a few may die and the rest be saved 
by rain. Much depends not only upon situation, but also upon the 
water supply, type of soil, vegetation, duration of drought, amount of 
sunlight, cloud, fog, dew, etc., etc., rendering the whole matter complex. 
Some snails will not recover when replaced in water after two days 
drying should they have been in a very exposed position and subject 
to continuous sun and dry wind; in other cases where more favourably 
situated from five to eight days pray elapse. Should any of the snails 
however, become buried in the mud their chances of survival are greatly 
increased, and experiments conducted in May, 1916, in the case of 
snails buried beneath 2| inches of damp soil, about half recovered when 
replaced in water at the end of 14 days, but after three weeks (artificial) 
drought all were dead. It has several times happened that one or two 
large snails will be found among the fresh stock of riewly hatched young, 
following a drought and it is thought that these may owe their survival 
to burial. Perhaps the most frequent and probable cause of such 
burial is the trampling of certain ditches by cattle, this while crushing 
many, at times doubtless preserves a few. 
Local Rainfall Data. 
At the commencement of the Survey (Oct. 1913) L. truncatula was 
widely distributed in the Area and of considerable size. During the 
summer droughts of 1914-15 it was almost impossible to obtain a suffi¬ 
cient number of specimens to carry on experiments and observations. 
The spring and early summer of 1916 saw a great increase owing to wet 
conditions, but in July a drought again killed large numbers. It is 
well known that wet years are a necessary prelude to outbreaks of rot, 
hence examination of the local rainfall for a number of years should 
be of interest. The rainfall data obtainable for the Area are by no 
means as complete as could be desired. Full details are available for 
Aberystwyth (which however is not typical of those portions of the 
Ai*ea more especially troubled by Rot) and I have been able to obtain 
valuable particulars for Gogerddan (83 ft.) some three miles from 
Aberystwyth; Hat'od (580 ft.) near Devils Bridge, and Machynlleth 
(41 feet) three miles to the n. of the Area (see Map, Fig. 1). 
The Chart, Fig. 2, shows the relative amount of rainfall at these stations 
and the amount in inches between 1905 and 1915. Thus, within the Area 
surveyed the rainfall shows a difference of 15 to 25 inches between the 
coast and an elevation of 600 feet, 12 miles inland (Aberystwyth and 
