Abstract

A new species of primitive cystoid, Cardiocystites pleuricostatus, is described from the Caradoc of Shropshire. Its superb preservation clarifies the morphology of this problematical taxon and shows it to belong to the Rhipidocystidae. Its highly flattened theca cannot be indicative of a recumbent mode of life, contrary to previous interpretations, since its arms and brachioles formed a conical filter. The flat, thin-plated theca may have been a specialization either for aiding gaseous exchange, or for providing hydrodynamic lift to the crown.