374 Shell Life 



centre of the base of the hist whorl but one. The 

 length of the shell is about one-third of an inch. It 

 affects old mossy trees and lichened rocks, hiding 

 under the loose bark of the one, and in crevices of the 

 other so long as the air is dry; but after rain the 

 whole colony walks out upon the surface. It is 

 widely distributed, but local. 



The Door-shells (Clausilia) belong to a very exten- 

 sive genus, nearly a thousand species being known, 

 but in this country represented by four species only. 

 These are spindle - shaped, twisted to the left like 

 Balea, contracted into a throat behind the pear-shaped 

 mouth. Within the mouth are seen two spiral 

 plates, and on turning the shell over these may 

 be traced through the shell some distance into 

 the body-whorl. In some species there are 

 intermediate ridges on the pillar and the outer 

 lip. Within the throat there is a narrow 

 Plaited twisted plate of shelly matter, attached to the 

 g^eii' pillar by an elastic foot. This plate is the 

 claiisili 117)1, which automatically closes the 

 throat when the mollusk withdraws inside, and which 

 is pushed aside when the animal emerges, and so acts 

 as an operculum. The clausilium is obviously a 

 protection from the inroads of beetles of the family 

 Staphylinidce Sind other small insects, but it is strange 

 that such defence should not be provided until the 

 mollusk has reached maturity. Until the shell is 

 nearly full-grown there is no sign of this device. 

 There is a slight slit-like umbilicus almost hidden 

 by the thickened and expanded lip. They are all 

 herbivorous, and probably at times retain their eggs 

 until hatched. 



