135 



which furnishes the adult \vitli its lašt coating of enamel can be ex- 

 erted to the formation of as many superincumbent layers as may be 

 necessary to replace all that has been decomposed. That a disso- 

 lution takes place there can be no doubt : "the shell gradually 

 swells," says Lieut. Hankey, " and cracks, becomes thinner, and 

 duUer in colour, and finally disappears ;" a circumstance -n-hich may 

 be easily credited -vvhen it is remembeied that the Mttrex possesses 

 the faculty of reraoving spines or any similar obstacles to its advance- 

 ment of growth, and that the Pholades and other terebrating mol- 

 lusks exercise a power of absorbing which enables them to penetrate 

 the hardest limestone rock. The microscopical structure of the 

 Cowry shell is, moreover, of a nature peculiarly tenacious of absorp- 

 tion ; it is composed of a large ąuantity of carbonate of lime in pro- 

 portion to the amount of membranous substance ; and this accounts 

 for its surface becoming vitrified, as it \vere, to such a highly polished 

 statė of enamel when in contact with the acidity of the soft parts. 



There is another circumstance in Lieut. Hankey's naiTative to 

 which attention should be given, respecting tlie formation of the 

 new sheU. The glutinous matter, which has the appeai-ance of shell- 

 lac, and is so fragile that it yields to the touch, does not assume the 

 narrow cylindrical Bulla form ; it does uot follovv the original plan of 

 revolving round a columellar axis, but is of the wide ventricose 

 shape of a Cymba, and rapidly consoHdates into the adult shell. 



With these generalizations I think it may be assumed that the 

 Co\vry possesses the faculty of decomposing, during one or more 

 periods of its existence, any portion of the shell that is liable to re- 

 sist its advancement of growth ; that the rene'vval of the shell is ac- 

 compHshed within a comparatively short space of time ; and that the 

 columella with its internal spirai partitions remains undisturbed. It 

 may, however, be inferred that it is an operation of extremely rare 

 occurrence, and one W'hich only happens under peculiar couditions. 



