HELICID.E. 211 



" Of all the wonderful contrivances employed by 

 nature for the protection of the Mollusca, there is 

 none which is more calculated to excite the admira- 

 tion of the conchologist than the clausium, an elastic 

 appendage which closes the aperture of the Claiisilice. 

 It consists of a spirally twisted thin shelly plate, in- 

 closed in the last whorl of the shell, and attached to 

 the columella by an elastic pedicle. When the ani- 

 mal is retracted within its shell, this shelly plate nearly 

 covers the aperture at a little distance within the 

 mouth, and coming in contact with a transverse plait 

 on the outer lip, leaves only a small canal, formed 

 between the outer plait and the posterior angle of the 

 mouth, and sometimes an elongated longitudinal plait 

 on the inner lip. When the animal wishes to protrude 

 itself, it pushes the plate on one side into a groove 

 situated between the inner plait and the columella, 

 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. 

 This curious structure, and also the plaits of the 

 mouth, which are intimately connected with it, are 

 not formed until the animal has nearly reached matu- 

 rity. It is best exhibited by breaking off the outer 

 part of the aperture to the distance of about half a 

 whorl, when it will generally be found free ; but in 

 order to exhibit it behind the columella in its natural 

 position, when the animal is exserted, it is necessary 

 to kill the animal in that situation (by drowning it), 

 and then suffer it to dry before the outer lip is broken 

 off, and the pedicle will thus become fastened to the 

 side by means of the dried mucus (of the body) ; it 



