Different layers of the shell will have different proportions of calcite, aragonite, and the organic material: the hypostracum
, for example, has a much higher proportion of aragonite than the subnacreous layer, which has multiple sheets of calcite separated by conchiolin proteins; the prismatic layer has crystals of both calcite and aragonite; the periostracum is composed almost entirely of organic material (Galtsoff 1964).
The plates are composed of calcium carbonate and are composed of three layers, outer or dorsal tegmentum, articulamentum, and ventral hypostracum. In Euleptochiton spatulatus the articulamentum layer is restricted to narrow triangular areas extending from the apex on the posterior margin of the intermediate plates and the mucro on the tail plate to the anterior margin forming prong-like extensions called sutural laminae (Fig.
Some of the horizontal canals curve over the edge of the articulamentum layer and extend vertically to the ventral surface of the hypostracum layer (Fig.
2.10 arrow at contact of hypostracum and articulamentum layers).
No distinction could be seen between the thin tegmentum and thicker hypostracum layers based upon the preserved shell microstructure.