I have always assumed (poor choice of words?) that a shell listed as "rare"
was one not commonly found or easily acquired.  In this context, would not
the question of how many exist in nature be moot?
 
How about everyone else?  When you look up a mollusk in a book for
identification, did you not assume that "rare" meant rarely found or
acquired?
 
Sylvia S. Edwards
Huntsville, Alabama
[log in to unmask]