BEMOVAL OF WHORLS. 



119 



that envelopes the whole, and which, retaining its ori- 

 ginal thickness, is of sufficient strength to give full 

 protection to the animal. That this change has actually 

 been effected, may be seen distinctly in the cone fig. 3. 

 Fig. 4, is a transverse section of a shell of this kind, 



Fig. 3. 



Fist. 4. 



showing the spiral convolutions and the comparative 

 thinness of the inner portions. 



Instances indeed occur among shells of the total re- 

 moval of the interior whorls. This is the case with the 

 genus auricula, which are molluscous animals, respiring 

 by means of pulmonary organs. In the young shell 

 of this tribe, the partitions which separate the cavities 

 of the whorls are incomplete, and twine parallel to each 



