The shell ultrastructure of six auloporid coral species was investigated and shown to have been originally constructed of laminar calcite. The mineral succession begins with a subcuticular granular layer and continues with a secondary laminar layer, which makes up the bulk of the skeleton. The laminar layer is constructed of lath-like crystals 2–4 μm thick and 10–20 μm wide, and is penetrated by granular rods, which originate from the primary granular layer and protrude into the calicular cavity as spinose septa. The laminae deflect around the granular rods in an inwardly convex pattern.