The box-shaped zooecia of the two living and of five Tertiary species consist of: (1) a smooth primary skeleton including the basal, lateral, and distal walls, and the inner layer of the frontal wall; (2) a finely to coarsely tuberculate superficial layer of the frontal wall. In thin section the primary skeleton is longitudinally lamellar, and the superficial tissue is either wavy-lamellar or fibrous. Bulk X-ray diffraction analysis and differential staining with Feigl's solution suggest that lamellar tissue of both types is calcitic, whereas fibrous material is aragonitic. The living species that has a lamellar superficial layer is wholly calcitic; the living species that has a fibrous superficial layer becomes progressively more aragonitic during zooecial ontogeny.

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