FORMATION OF SHELLS. 223 



which is thus abandoned, being very thin and brittle, 

 and having no support internally, soon breaks off, 

 leaving what is termed a decollated shell ; examples 

 of this occur in the Cerithium decollating, the I^n- 

 limns decollatus, &c. The young of the genus 

 Magilns has a very thin shell of a crystalline tex- 

 ture ; but when it has attained its full size, and has 

 formed for itself a lodgment in a coral, it fills up 

 the cavity of the shell with a glassy deposit, leav- 

 ing only a small conical space for its body ; and it 

 continues to accumulate layers of this material, so 

 as to maintain its body at a level with the top of 

 the coral to which it is attached, until the original 

 shell is quite buried in this vitreous substance. 



The forms of the Cone and Olive shells are such 

 as to allow but a small space for the convolutions 

 of the body of the animal, which accordingly be- 

 comes, in the progress of its enlargement, exces- 

 sively cramped. In order to obtain more space, 

 and at the same time to lighten the 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 

 M ith the other whorl, that envelopes the whole, and 

 which, retaining its original thickness, is of suffi- 

 cient 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 exa- 

 mining a vertical section of that shell, as is repre- 

 sented in Fig. 110. All the inner partitions of the 

 cavity thus laid 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 



