Fcn-ms, Ccmiposition, and Structure of Shells* 343 



sary ; and in passing it repeatedly over the hole, leaves a layer 

 of calcareous matter each time, until the new piece has ac- 

 quired the requisite thickness. I will not deny that other 

 parts may occasionally secrete shell. The operculum, in the 

 tribes furnished with it, is certainly secreted by the part to 

 which it is attached ; and the skin investing the body appears 

 to furnish the material required for the formation of the 

 septa of multilocular shells, and the tubes of the Tubicolae. 



The shell is to be considered as a model of the body of its 

 architect ; so that, when we observe two shells to differ re- 

 markably in their figure, we may infer, with perfect safety, 

 that the animals differed no less ; and it is this agreement 

 between the living snail and its dead envelope which gives to 

 the study of the latter an importance it could not otherwise 

 possess. Even the various processes, foliations, and spines, 

 which vary and ornament the outer surface of shells, are the 

 result of similar variations in the collar. " The form of the 

 shell," I translate the words of Blainville, " and even the pre- 

 dominance of the animal matter over the earthy, bear a rela- 

 tion to the form of the skin or of the cloak, and to the age of 

 the snail. Thus, the tubular, spinous, and lamellated elonga- 

 tions, which occur so frequently on the outer surface, are the 

 consequences merely of prolongations, lobes, and fringes of 

 the cloak ; and the notches and channels of the aperture are 

 also produced by the habitual though intermittent protrusion 

 of some organ, as of the respiratory tube, or of the head, or 

 of the oviduct, &c." * 



When, however, we come to apply the above general doc- 

 trine of the formation of shells to particular cases, we meet 

 with some apparent exceptions. The cowries (Cypr^^^a), for 

 example, by an addition of matter to the edges of their lips, 

 would soon have the aperture entirely closed, as you will per- 

 ceive on examining any species of that genus. To get rid of 

 this difficulty, Bruguiere and others have imagined that the 

 animal threw off the shell when it had become too small for his 

 necessities, and then formed another more capacious, and bet- 

 ter fitted for his ease. This theory labours under insurmount- 

 able difficulty, nor does it seem required by the circumstances 

 of the case. The Cypr<^V, in their immature state, have a 

 very different form from that they have when full grown. 

 When young, they are very thin and brittle, with an evident 

 spire, and a wide aperture, the margins of which are not 



* Manuel de Malacologie, p. 94. — I may here, once for all, acknowledge 

 the great assistance I have received from this work, in the compilation of 

 this Letter. 



