178 HELICIDiE. 



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 Methodiqiie des Coquillages, in the 

 Memoires de V Academic des Sciences de Paris^ and 

 accurately described by Otho F. Miiller, in his ex- 

 cellent Historia Vermium, &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 Miiller's account, though 

 he frequently quotes his work. Cuvier, in his Regne 

 Animal, iii. 45., speaks of it, but says he does not 

 know its use to the animal. 



In the Annals of Philosophj for 1822, Mr. Miller, 

 who also appears not to have seen Miiller'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 JMollusca have a 

 calcareous operculum permanently formed, which 

 increases in thickness, and enlarges on a depressed 



