180 HELICIDiE. 



In my " Conchologlcal Observations" {Zoological 

 Journal, i. 212.) I gave the following more par- 

 ticular account of this appendage: — 



" 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 Avhich closes the aperture of the Claiisilioe. 

 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 animal 

 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 maturity. 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. 



