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The protegulum of acrothelid inarticulate brachiopods is characterised by slender. elongate, hilaterally arranged spines which are interpreted as adaptations to facilitate settlement on a soft substrate. They provide a mechanism for positioning the gape clear of the substrate and a means of shell eversion to permit recovery from an overturned position. Holoperipheral growth and attainment of a low subconical form by the pedicle valve during later growth stages provide different strategies for solving these problems, and the protegular spines become non-functional and largely worn away. The structures arc illustrated in a species of Orbithele from the Upper Cambrian of Australia.