Ken,
 
I saw John Bernard's (Shelloak) answer to your query and thought I would add.  You
should probably check out the Jacksonville Shell Club web page as they have almost
all if not all of the Ranellidae (Cymatium) pictured on the site with the most
recent taxonomy.  I didn't see your particular subspecies of Cymatium parthenopum
but there are three subspecies shown.  Worth a look.  The site is:
 
http://home.sprynet.com/sprynet/wfrank/jacksonv.htm
 
Tom Eichhorst