244 
Liver Rot of Sheep 
origin; rock, gravel, sand and peat are, in general, devoid of these 
mollusca. 
Since L. truncatula is very widely distributed (even occurring on the 
islands of the w. coast of Ireland) it may be taken that its antiquity 
is considerable and it may have followed the retreating glaciers of the 
ice sheet; its habitat would be governed by whatever modified the 
distribution of clay and it would readily extend its upward range by 
crawling slowly upstream, while floods would, in some cases, renew its 
downward distribution. Given suitable conditions both species occur 
throughout the Area, from sea level to 1250 feet, and there is no doubt 
they would range higher were conditions right. Speaking broadly, 
L. truncatula inhabits shallow streams and ditches with a clayey floor, 
or layer of diatomaceous mud; L. peregra prefers a deeper mud and 
a less rapid flow. Where these conditions merge the two species may 
overlap to a limited extent. 
In one or two deep ditches L. truncatula was found to inhabit the 
margins and sides (sometimes crawling on the damp zone just above 
water level) while L. peregra occupied the softer and deeper mud about 
the centre. This ditch ran east and west, and there were decidedly 
more specimens of L. truncatula on the eastern side, than on the more 
sunny western, and more crawling above the water line. Peat—one 
of the most important physical factors of the Area—is inimical; at 
any rate I have failed to find L. truncatula among peat, while in water 
draining from it the size is small and the shell thin. 
It is probable that peat acids, lack of lime and the presence of much 
soft flocculent vegetable matter are the limiting factors, and not any 
lack of food. It sometimes happens that clay is exposed around the 
margins of peaty tracts, and there L. truncatula, will often be found, 
thus causing dangerous spots in an otherwise free area. The following 
may be cited as an example: in November, 1913, I examined a small 
holding situated in the s.e. region, and about 800 feet above sea level. 
This place is just on the boundary of a large expanse of boulder clay, 
and so many sheep had died from rot from time to time that the 
occupier despaired of keeping any. After examining several boggy 
ditches without result, L. truncatula, was at last discovered in a muddy 
patch just where drainage from a little lane entered the fields. On 
tracing this drainage to its source it was seen to ooze out of a wet patch 
on the margin of an otherwise dry grass field, this spring marking the 
margin of the boulder clay. 
The streamlet was next followed downward, and clay showed in its 
