  We confirm with high-resolution techniques the existence of mineral bridges
between superposed nacre tablets. In the towered nacre of both gastropods and
the cephalopod Nautilus there are large bridges aligned along the tower axes,
corresponding to gaps (150-200 nm) in the interlamellar membranes. Gaps are
produced by the interaction of the nascent tablets with a surface membrane that
covers the nacre compartment. In the terraced nacre of bivalves bridges
associated with elongated gaps in the interlamellar membrane (> 100 nm) have
mainly been found at or close to the edges of superposed parental tablets. To
explain this placement, we hypothesize that the interlamellar membrane breaks
due to differences in osmotic pressure across it when the interlamellar space
below becomes reduced at an advanced stage of calcification. In no cases are
the minor connections between superimposed tablets (< 60 nm), earlier reported
to be mineral bridges, found to be such.
