CHANGES OP SHELL WITH AGE. 365 



sight to belong to the same tribe. But in all these cases, the 

 form of the young shell may be traced in that of the adult. The 

 accompanying figures of the Pteroceras show this change in a 

 moderate degree ; in other genera it is much more remarkable. 

 In another group of shells, of which the common Cowry is an 

 example, a still more curious alteration takes place. In the 

 young shell the edge is sharp, and the mouth an opening of con- 

 siderable breadth. This state continues as long as the shell is 

 increasing in size ; but when it has arrived at adult age, the outer 



FIG. 658. CYPKAA EXANTHKMA : a, young shell ; b and c, back and front views 

 of adult shell. 



lip is thickened, and brought so near the other as to leave but a 

 narrow chink between them. At the same time, a prolongation 

 of the mantle on each side deposits a new layer of shelly matter 

 on the outside of the previous one ; and as the two' prolonga- 

 tions meet along the back, (the line of their junction beino- 

 usually evident on the shell), this additional coat, which is very 

 hard and porcellanous in its texture, incloses the whole previous 

 shell. The operculum is principally confined to the aquatic 

 Gasteropoda. It is sometimes of the same texture as the shell 

 itself, and sometimes horny. It does not always close the entire 



