180 
HELICID^. 
In my Conchological 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 which closes the aperture of the Clausilicc. 
It consists of a spirally twisted thin shelly plate^ in- 
closed in the last whorl of the shelly 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 lipi. 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. 
