FORMATION OF SHELLS. 251 



tremely thin and transparent, and to consist 

 only of the innermost lamina of the original 

 shell ; as will appear on tracing them up to that 

 outer portion of the section B B, which lies on 

 each side of the proper apex of the shell, and 

 which forms the apparent base. The lines on 

 this part of the section indicate the thickness 

 which each successive whorl had originally, and 

 when it was itself the outermost whorl. The 

 section also shows the vitreous deposit which 

 lines the upper parts of the cavity, and which 

 completely fills up the smaller turns of the spire, 

 near the apex.* 



There are, indeed, instances among shells of 

 the total removal of the interior whorls. This is 

 found to occur in that of 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 other ; but they wholly disappear as the 

 animal approaches to maturity. In other cases, 

 the animal is found to remove exterior portions 

 of shell formerly deposited, when they lie in 

 the way of its farther growth, and when the mouth 

 of the spire is advancing over the irregular sur- 



; 



Fig. 117, which is a transverse section of the same shell, 

 shows the spiral convolutions, and the comparative thinness of 

 the inner portions. It also forms a striking contrast with a si- 

 milar section of the Cyprsea, Fig. 114. 



