| Term | Definition |
| HYPOSTRACUM | (1) Inner layer of shell wall, secreted by entire epithelium of MANTLE (original usage of term); (2) part of shell wall secreted at attachments of ADDUCTOR MUSCLES. | |
| MESOSTRACUM | [need definition]; (refers to some aspect of CHITONS) | |
| MYOSTRACUM | Part of shell wall secreted at attachments of ADDUCTOR MUSCLES (of bivalves); see also: HYPOSTRACUM. | |
| OSTRACUM | Outer part of CALCAREOUS wall of shell, secreted at mantle edge [original usage]; entire calcareous wall of shell [usage by some later authors]; see also: PERIOSTRACUM. | |
| PERIOSTRACUM |
The outermost molluscan shell layer, composed of CONCHIOLIN; = DRAP MARIN (by French naturalists); erroneously called "epidermis" by authors; see also: OSTRACUM. Photo: Cymatium pileare (Linne, 1758) |
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| PROTOSTRACUM | Same as PRODISSOCONCH I. | |