178 
HELICID^. 
one above it, and being consequently more tumid 
above the aperture, fixes its distinction from Bulimus^ 
as well as the peculiarity of the appendage. 
The clausium or peculiar elastic valve in the last 
whorl of these shells was first noticed by D’Aubenton, 
in his Distribution Methodique des Coquillages^ in the 
Memoires de V Academie des Sciences de Parisy and 
accurately described by Otho F. Muller, in his ex- 
cellent Historia Vermiuniy &c., in 1773, and by him 
called ossicula and scala. He beautifully and ac- 
curately described its peculiar functions. 
Draparnaud has since described it as a new dis- 
covery, having overlooked Muller’s account, though 
he frequently quotes his work. Cuvier, in his Regne 
Animaly iii. 45., speaks of it, but says he does not 
know its use to the animal. 
In the Annals of Philosophy for 1822, Mr. Miller, 
who also appears not to have seen Muller’s descrip- 
tion — for he specially tells us that he discovered it 
in 1814, and showed it to Dr. Leach in the following 
year, before Draparnaud published his account of it, 
— - gives the following interesting account of its me- 
chanism: — 
Independently of the various contrivances which 
nature has resorted to for the protection of the other- 
wise easily vulnerable Mollusca, it has taken peculiar 
care to guard the apertures of many univalves from 
the intrusion of enemies ; hence the apertures are 
sometimes peculiarly contracted and provided with 
numerous folds and teeth. Other Mollusca have a 
calcareous operculum permanently formed, which 
increases in thickness, and enlarges on a depressed 
