Among the gastropods, some muricid shells are composed of an inner aragonitic crossed lamellar layer and a calcitic prismatic outer layer. The analysis of the structure and composition of the two layers of Concholepas shows that the crossed lamellar layer is similar to those of other Mollusc taxa. Sr, Mg and S contents are low in both layers. According to infrared spectrometry, the organic content of the outer calcitic layer is higher than that of the aragonitic crossed lamellar layer. The study of the nanostructure allows for the proposal of a new 3D interpretation of the fine structure of the crossed lamellar layer. The calcitic prismatic layer is compared with the outer calcitic prismatic layer of an archaeogastropod genus: Haliotis.