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Cidaroida (class Echinoidea, subclass Perischoechinoidea) An order of regular echinoids that have a radially symmetrical, subspherical test. The ambulacra are in two columns, with each plate bearing a single pore pair. The interambulacra are conspicuously wider than the ambulacra, each plate bearing one enlarged primary tubercle carrying a primary spine. The teeth are not keeled. There are no gill slits or spheridia. Cidaroida first appeared in the Upper Silurian. There are six subfamilies, within two families, extant.