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 Zrochi. 
“ Ridges, verices, and spines indicate some 
time has elapsed between the deposition of the 
layers. 
“Tf 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 repairmg. 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. 
“Tf the injury is sustained upon the walls 
