1. Cup-shaped, plated body of pelmatozoan echinoderms. The calyx is made up of several rows of plates. If there is no stem, a centrodorsal plate is succeeded by a ring of five basal plates and then by a further ring of radial plates above them. Above these two rows of plates come the plates making up the arms. Where these two rows of plates are present the calyx is said to be ‘monocyclic’. In some species a third row of plates, the infrabasals, is present beneath the basals and the calyx is then said to be ‘dicyclic’. In some cases radial plates may be compounded (i.e. split transversely into two plates) and then, depending on their position in the calyx, they may be known as ‘infraradial’ or ‘supraradial plates’.

2. Bowl-shaped depression at the top of a calcareous coral skeleton, usually formed by the upper edges of the septa.