Hello, Everyone!

I ran into an unusual situation with a shell I got at
auction the other night.  It is an Elephant Snout type
volute, not positive of the species yet.  But it had what
looked like a periostracum on it, partially gone.  When I
put it in weak bleach, it started coming off; but I
discovered that it seemed to be a very thin colored glaze
that looks like mica when it flakes off.  I think this is
not a true periostracum, but don't know what to call this
layer.

Do other volutes in this genus, Cymbium,I think, also have
this glaze?  I know very little about this group of
volutes.

Thanks,
Linda