FORIMATION OF SHELLS. 



181 



shell, the whole of the two exterior layers of the inner 

 whorls of the shell are removed, leaving only the interior 

 layer, which is consequently very thin when compared with 

 the other whorl, that envelops the whole, and which, re- 

 taining its original thickness, is of suilicicnt strength to give 

 full protection to the animal. That this change has actually 

 been effected is very distinctly seen in the Conus (Fig. 115) 

 by examining a vertical section of that shell, as is represented 

 in Fig. 116. All the inner partitions of the cavity thus laid 



115 



117 



open are found to be extremely 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 de- 

 posite 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 re- 

 moval of the interior whorls. This is found to occur in that 

 of the genus */luricida, which are molluscous animals, res- 



* Fig". 117, which is a transveree section of the same shell, shows the spi- 

 ral convoUitions, and the comparative thinness of the inner portions. It also 

 forms a striking contrast with a similar section of the Cyprxa, Fig 11 -I- 



