88 HELICIDE. 
This shell is local, though abundant where it occurs. 
It is found on the bark of trees; Dr. Jeffreys says 
“chiefly the beech, ash, sycamore, and apple.” TI once 
found it on a willow near Windsor, in company with 
C. rugosa, the young of which it much resembles. 
This resemblance it is necessary to draw attention to, 
or this local species may be passed over. It differs 
from the immature, C. rugosa in being more slender, 
lighter in colour, having a deeper suture, and being 
without any carination along the basal ridge, which is 
a marked characteristic of the young of that species. 
Var. viridula. Greenish white, transparent. 
Genus [X.—CLAUSILIA. 
This genus derives its name from a peculiar charac- 
a little door (Lat. elausélium). 
This interesting provision of nature against the 
teristic 
attacks of such enemies as beetles, &c., differs from an 
operculum thus:—It is not fastened to the animal, but 
to the pillar of the shell by an elastic filament. When 
the animal is within the shell this contractile filament 
draws the clausilium close over the aperture, about 
half a turn from the entrance, and when the animal 
emerges it pushes aside the little “ spring-door,” which 
then hes flat against the columella. 
