The Echinoid Directory

• Test small (less than 20 mm); flattened below, gently domed above.
• Apical disc small; dicyclic; generally flat and raised in smaller individuals; plates firmly bound to corona. Ocular plates small and expanding outwards, projecting beyond genital plates. Genital plates projecting interradially; gonopore opens towards end of projection. Single large primary tubercle occupies the centre of each genital plate.
• Ambulacra straight and rather broad; plating trigeminate throughout; pore-pairs uniserial aborally, expanded into short phyllodes adorally. Aboral pore-pairs small and oblique, not conjugate.
• Plate compounding goniopygid-style with two elements extending to the perradius and housing the primary tubercle, the upper being distinctly smaller, alternating with a single small demiplate. A single primary tubercle to each compound plate.
• Interambulacral plates a little wider than tall; dominated by a primary tubercle.
• Primary tubercles imperforate and non-crenulate, with massive mamelon and no platform. Tubercles largest at ambitus and decrease in size adapically and adorally. Only a few small granules around the edges.
• Peristomial very large (at least half test diameter); circular, with shall, rather broad buccal notches with shallow tags.
• Primary spines over upper surface short and peg-like with flattened ends, interlocking to form a protective pavement; at ambitus spines short and weakly fusiform, shorter adorally.

Acropeltis aequituberculatus Agassiz, in Agassiz & Desor, 1846, p. 340, by monotypy.

Echinacea, Arbacioida, Acropeltidae

Presumed monophyletic.

The spine canopy of Acropeltis was described by Roman et al. (1991) and is very similar to that seen in the modern Colobocentrotus. As argued by those authors, this taxon was presumably adapted for life in highly turbulent rocky coastal environments. Acrocidaris has a similar tuberculation, but its ambulacra are polygeminate, its tubercles perforate and its plating diadematid. Distinguished from all other acropeltidae by its single large primary tubercle on each genital plate.

Agassiz, L. 1838. Monographies d'Échinodermes vivants et fossiles. Première monographie: Des Salénies. Petitpierre, Neuchâtel, 32 pp., 5 pls.

Mortensen, T. 1935. A monograph of the Echinoidea II. Bothriocidaroida, Melonechinoida, Lepidocentroida and Stirodonta. C. A. Reitzel, Copenhagen.

Roman, J., Vadet, A. & Boullier, 1991. Echinoides et brachiopodes de la limite Jurassique-Cretace a Canjuers (Var, France). Revue de Paleobiologie 10, 21-27.