The Echinoid Directory

• Test shape circular and depressed?; plating thin and imbricate.
• Apical disc small; with a single enlarged genital plate with hydropores.
• Ambulacra narrow; quadriserial at ambitus and adorally, becoming uniserial adapically. Each plate with a single pore-pair centrally placed, and with marginal tubercle.
• Internally perradial flanges enclose the radial water vessel. Large adradial extensions from adradial plates extend beneath adjacent interambulacral plates.
• Interambulacral zones broad and composed of a large number of small, scale-like plates probably rather irregularly arranged. Individual plates with central primary tubercle that is perforate and non-crenulate; a few smaller secondary tubercles or granules around plate margin.
• Peristome small; plating entirely ambulacral.
• Lantern well developed with large grooved teeth composed of a biseries of elements in a V-shape.
• Spines short, simple; present on ambulacral and interambulacral plates.
• Primitive pedicellariae present consisting of three rods attached to a single tubercle.

Upper Silurian (Ludlow), UK.

Echinocystites pomum Thomson, 1861, p. 106, by subsequent designation of Gregory, 1897, p. 133.

• Only the type species.

Stem group Echinoidea; Echinocystitidae.

Monotypic.

Distinguished from Palaeodiscus by its quadriserial ambulacral plating. Close to Rhenechinus from which it differs only in having slighly more regularly polygonal interambulacral plates and smaller less obvious tuberculation. Gotlandechinus differs in having a uniserial arrangement of pore-pairs close to the perradius

Hawkins, H. L. & Hampton, S. M. 1927. The occurrence, structure , and affinities of Echinocystis and Palaeodiscus. Quarterly Journal of the Geological Society London 83, 574-603, pls 44-46.