LAND SNAILS. 149 
probable explanation_is, that this tree is resorted 
to by the snails for the purpose of feeding upon 
the minute parasitic vegetation which clothes it. 
The supposed scarcity of this snail, and some 
others, in localities where they are in reality most 
abundant, is due to this peculiar habit of ascend- 
ing trees during the summer months; for at 
this season only dead shells will reward a search 
among the herbage at the bases of the trees. 
B. montanus is readily to be distinguished 
from B. obscurus, which accompanies it, by its 
superior size and somewhat polished appearance ; 
B. obscurus, in all stages, 1s covered with a dirty 
incrustation. 
This species is recorded from Wiltshire and 
Hampshire, and in Suffolk near Bury St. Ed- 
mund’s; it is thus local, but plentiful where it 
occurs. 
The specific name, montanus, implies that its 
usual habitat is in elevated situations, and in 
such it is only known in Central Hurope. 
Butimus opscurus — (the Dull Twist Shell) 
(Pl. IX., fig. 82).—This species differs from PB. 
montanus chiefly in its smaller size, for the shell 
is about half an inch in length; and in the white 
reflected margin of the aperture. Albinos. have 
occurred. 
The animal is of a greyish-brown, cr dark 
brown above; the foot is of a lighter colour; 
