156 THE HALL OF SHELLS. 



her mysteries to seek diligently for them she 

 rewards us generously. You have learned that 

 some shells secrete layers of nacre, with which 

 many of our most beautiful ones are lined and 

 which composes a great part of others. You 

 remember, too, how injuries inflicted by borers 

 are repaired. These facts open our eyes to a 

 part of the mystery. Shells were once a por- 

 tion of the mantle or delicate films which were 

 separated from it. These gradually harden, 

 becoming united to tissues previously thrown 

 off by the mantle. So layer is joined to layer 

 in forming shells so delicate a wave may break 

 them or solid like these Trochi. 



"Eidges, verices, and spines indicate some 

 time has elapsed between the deposition of the 

 layers. 



" If shells are broken we can sometimes see 

 the process of shell growth, if patient enough 

 to watch the work and to wait the completion 

 of the delicate repairing. First a moist exuda- 

 tion — a secretion from the injured part of the 

 little animal — covers the spot. Gradually this 

 exudation becomes filmy upon exposure to the 

 air, and, as the process continues, grows calcare- 

 ous or horny according to the character of the 

 shell undergoing repairs. 



" If the injury is sustained upon the walls 



