210 PNEUMONOBRANCHIATA. 



spiral plane, as the opening of the shell extends with 

 the growth of the animal, thus continually assimilating 

 to its size, and when the animal retreats, excluding it 

 completely from all external intrusion. In the Clau- 

 sula, nature has continued the protection afforded by 

 means of contractions and folds, and also added an 

 opercular appendage. The inhabitant of the Clau- 

 silia, when nearly full grown, secretes a thread-like 

 elastic calcareous filament, one of whose ends is af- 

 fixed to the columella. This filament makes half a 

 spiral turn round the columella, insinuating between 

 its folds. When the animal finishes its shell and 

 completes the aperture, it secretes, at the unattached 

 end of the filament, a spoon-shaped calcareous lamina 

 conforming at its margin to the contour of the aper- 

 ture. The lamina is somewhat smaller than this, 

 and its margin is rounded. 



" Its adhesion to an elastic filament enables the ani- 

 mal to push it, when it comes out of its shell, against 

 the columella ; and the same elasticity closes it, on the 

 inhabitant retreating, thus securing it from intruding 

 enemies. Thus, then, this valve may be compared 

 to a door provided with an elastic spring. The elas- 

 ticity of the filament may be restored to its full power 

 (in the empty shell) by sometimes immersing it in 

 water, as I have ascertained in a section, made with a 

 view to this inquiry." (Ann. Phil. hi. 378.) 



Draparnaud has named this valve-like appendage 

 the clausium, and Cuvier (Regne Animal, ii. 409.) 

 states, " de cette lame on ignore l'usage dans l'animal 

 vivant." 



In my conchological observations (ZoologicalJour- 

 ?ial, i. 212.) I gave the following more particular 

 account of this appendage. 



