Article: Shell structure of the Devonian retziid brachiopod Plectospira ferita

Electron microscope studies of Plectospira ferita (von Buch) show the presence of two inorganic layers, primary and secondary, as well as a third, outermost one interpreted here as a periostracum. This is the first time a periostracum has been recognized in the fossil state. The structure of the periostracum of P. ferita is described and a comparison is made with periostraca of living brachiopods. The micro-ornamentation of the shell of P. ferita is described; pits and tubercles occurring on the surface of the primary layer are considered in detail.