  Nacre is a layered, iridescent lining found inside many mollusk shells, with
a unique brick-and-mortar periodic structure at the sub-micron scale, and
remarkable resistance to fracture. Despite extensive studies, it remains
unclear how nacre forms. Here we present 20-nm, 2{\deg}-resolution
Polarization-dependent Imaging Contrast (PIC) images of shells from 15 mollusk
shell species, mapping nacre tablets and their orientation patterns, showing
where new crystal orientations appear and how they propagate across organic
sheets as nacre grows. In all shells we found stacks of co-oriented aragonite
(CaCO3) tablets arranged into vertical columns or staggered diagonally. Only
near the nacre-prismatic boundary are disordered crystals nucleated, as
spherulitic aragonite. Overgrowing nacre tablet crystals are most frequently
co-oriented with the underlying spherulitic aragonite or with another tablet,
connected by mineral bridges. Therefore aragonite crystal growth in nacre is
epitaxial or near-epitaxial, with abrupt or gradual changes in orientation,
with c-axes within 20{\deg}. Based on these data, we propose that there is one
mineral bridge per tablet, and that "bridge-tilting" is a possible mechanism to
introduce small, gradual or abrupt changes in the orientation of crystals
within a stack of tablets as nacre grows.
