Hi
As far as I know M. . (now in the Genus Balinese) occurs on the sand.  The
shell Patella is also used in making Royal purple.  I don't think M.
trunculus (Hexaplex) was used to make Royal purple but some snails could be
used.  All snails have two "feeler" tentacles.  All shells have red blood
because of hemoglobin and not cyan based blood as in a Horse Shoe Crab.  The
shell grows all the time for repair and body growth.  There have been shell
mounds found in the area of the Mediterranean Sea.  It takes MANY thousands
of shells to make a small amount of purple.  The clear liquid the mollusk
produces is collected and boiled? or exposed to the sun to make it turn
purple.  Now I will run around the house to see if my SMALL library of books
has more accurate information.
    Shells of the world (small paperback), Shells of the Northern Hemisphere,
Shells (by K.R.Wye), Shells and shell collecting (a small white hardcover
book), and Kingdom of the Seashell might be a good sources of information (I
didn't really look in those books because time is against me and I might be
thrown off line again).
    If this info. helps, glad to help.  I will include my email address so
you can respond to me privately or post to the list.  Did I help?
Jordan *
P.S. Only senators or important officials wore Royal purple.