The shell layers of the tropical abalone of Haliotis asinina. (A) General view of the shell before (top) and after (lower) the mechanical removal of the periostracal and prismatic layers. (B) SEM micrograph of a cross-section through the abalone shell illustrating details of the periostracum (top) and of the calcified prismatic layer (lower). (C) SEM micrograph illustrating details of the prismatic layer. (D) SEM micrograph of the boundary between the prismatic (top) and the nacreous layer (lower). (E) SEM micrograph of the nacreous layer (oblique view). (F) SEM micrograph of the nacreous layer (cross section) illustrating the columnar superimposition of nacre tablets that is characteristic of gastropod nacre.