Figure 3. Crystallography of the prismatic units forming the outer layer of Neotrigonia.

A, B. Image quality and phase map, respectively, of a section through the outer prismatic layer of N. margaritacea. The blue color in E indicates that all the mapped material is aragonite, including part of the boss of the left prism. C, D, E. Surface views of bosses of N. gemma. The orientations of the lineations revealed upon natural etching have been indicated with arrows in all cases; the peripheral {110} faces, as well as the approximate boundaries between crystals (broken lines) have been indicated in D and E. F, G. Image quality and cleaned up orientation maps, respectively, of another section through the outer prismatic layer and nacre of N. margaritacea. H. 001 and 100 pole figures of the framed area in G, and color key (bottom) for the orientation map of G. The 001 pole distribution implies that the c-axes of the fibres are mostly parallel to the axes of the prisms, while the 100 pole distribution shows no preferential alignment either within or between prisms. I. Secondary electron image of a polished section through the outer prismatic layer of N. margaritacea. The diffraction contrast reveals the difference in texture between the upper part of the prisms (that enveloped by organic periostracal membranes) and the rest of the prisms. The approximate boundary has been delineated in the left prism. B, boss; Po, periostracum; Pr, prism.