Discoidea

Discoidea is a well-known genus of fossil Echinoidea which includes two species which are typical of the Lower Chalk and Upper Greensand; the most conspicuous feature of this genus is the presence of a series of ten raised ridges along the inner lower side of the test (or shell); these radiate from the mouth. Around the mouth is a raised girdle for the support of the jaws; the exact relations of this structure have been much discussed.