Hi Vivian!
Welcome to Conch-L, the one-stop conchology information station for this end of the universe!  There is a condition known as "Byne's Disease" which can cause deterioration of shells.  It has been discussed in depth on Conch-L, so instead of launching into the same discussion again, I'll tell you how to find the information.  First, go to:
http://listserv.uga.edu/archives/conch-l.html
Then, click on "search the archives".
Then, type "Byne Disease" into the search box, and hit GO.  That will bring up all the past Conch-L postings on that topic.  Type it the way I typed it above because some folks wrote Byne, some wrote Bynes and some wrote Byne's.  By typing "Byne Disease" you will bring up all the postings, regardless of which spelling was used.
Regards,
Paul Monfils