HELICID^E. 209 



having the last volution slenderer than the one above 

 it, and being consequently more tumid above the 

 aperture, fixes its distinction from Bulimus, as well as 

 the peculiarity in 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 VAcademie des Sciences de Paris, and accu- 

 rately described by Otho F. Miiller, in his excellent 

 Historium Vermium, &c, in 1773, and by him called 

 ossiada and scala. He beautifully and accurately 

 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 Eigne 

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

 know its use to the animal. 



In the Annals of Philosophy for 1 822, 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 mecha- 

 nism. 



" 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 froir 

 the intrusion of enemies; hence the apertures are 

 sometimes peculiarly contracted and provided witl 

 numerous folds and teeth. Other Mollusca have a 

 calcareous operculum permanently formed, which in- 

 creases in thickness, and enlarges on a depressed 



