CLOSE-SHELLS. 87 



its inhabitant has retired within it. But the next 

 genus we have to notice, Clausilia, is characterized 

 by the possession of an apparatus for partially closing 

 the last whorl of the shell within the mouth, which 

 is neither epiphragm nor operculum. It consists of a 

 valve (clausilium) attached to the pillar of the shell 

 by an elastic hinge, and when the animal wishes to 

 protrude itself, according to Dr. Gray (cf. " Zool. 

 Journ." i., p. 212), it pushes the plate on one side 

 into a groove situated between the inner plate and 

 the columella, or pillar, where it is detained by the 

 pressure of the body of the animal, leaving the 

 aperture free ; and when the animal withdraws itself, 

 the plate springs forward by the elasticity of its 

 pedicle and closes the aperture. 



Of this singular genus we have already noticed 

 (p. 66) three species as being found upon the London 

 clay. A fourth, Clausilia rolphii (PL VIII., fig. 10), 

 may now be named as peculiar to chalky and calca- 

 reous soils. It is larger than C. rugosa (or nigricans), 

 but smaller than laminata (bidens of Gray) and bipli- 

 cata. It differs from the first named in being more 

 ventricose, and in having coarser striae and a larger 

 and broader mouth ; it is also lighter in colour. It 



