Nora,
 
Another candidate for your large mystery shell is Hemifusus colosseus.
It looks a lot like Volema tuba or Hemifusus tuba and has a gorgeous
iridescent pink interior.  They grow rather large, 250mm.  There is a
lot of confusion with this group and I do not know of a single, really
good source.  Apparently the compendium may have some labels reversed
(from an earlier thread on Conch-L).  Get Joe to pick up a nice, cheap
flatbed scanner and you can tap into some great expertise to help with
those IDs.
 
Tom Eichhorst in New mexico, USA