| Summary: | In the shells of the Pelecypods belonging to the Pinnidae family, the calcareous prismatic units of the outer layer are long-standing references for biomineralization studies. To elucidate how the mechanism of prism formation enables both shell elongation and thickness increase, a top-down structural analysis of these classical “simple prisms” has been carried out, taking advantage of shell sampling on actively mineralizing animals. Particular attention was paid to the morphological and structural patterns of the calcareous units sequentially produced at the margins of the growth lamellae. This pre-prismatic part of the shell allows for studying the mineralizing stages not taken into account in prism reconstructions based on samples taken from older areas of the shell. Examination of the microstructural sequence shows that within the actively mineralizing area of the shell, a step-by-step structuring process is continuously running, providing a renewed view of prism formation as it makes obvious the progressive occurrence of their specific patterns. Given the critically endangered status of the species, a better knowledge of the mineralization process associated to shell growth may become handy for future studies aimed at understanding the health status of individuals based on their shell records.
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