Dear Paul,

 I am not familiar with mautitiana, but I have seen a (probably) similar
phenomena in annulus.
 As the shells get older, its shell surface became cloudy and whitish that
often made the orange
 ring invisible. This is based on our observation on individually marked
shells in the field.
 I suspect that the older shells excreted surface cover with irregular
crystal layers that
 might scatter light in random directions, so that the nacreous layer was
lost underneath.
 We may be able to verify this hypothesis by cutting shells to examine
microstructures, but
 It may require a SEM.
 Masashi Yamaguchi >>

Masashi,

This may be true, but the annulus is only on the dorsum, Paul's question had
to do with the base and lower margin. We have thousands of hand collected
hand cleaned annulus from Okinawa and it did not seem to be a very common
occurrence. The clouding that Paul asks about seems to happen quite often.

Frank (getting tired) in Massachusetts