Microstructure of the shell of a permian spiriferid brachiopod

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Abstract
The shell of Subansiria sp. is composed of two distinct layers. The primary or outer layer is approximately 0#lb1 mm thick and appears to consist of a granular outer part and an inner portion in which small crystals are oriented normal to the surface of the layer. The secondary or inner layer of the shell comprises elongate fibres inclined obliquely forwards at a low angle from the exterior to the interior of the shell. Transverse sections show the cross‐sectional outlines of the fibres of the secondary layer. The characteristic spatulate outline of the end of each secondary layer fibre on the internal surface of the shell is produced by the overlap of successive fibres. In radial section the fibres of the secondary layer underlying the area of attachment of the muscles in the pedicle valve are similar to those in other parts of the layer except that they are several times larger. The shell is punctate and around each puncta the fibres of the secondary layer are deflected towards the external surface of the shell.

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