The Echinoid Directory

• Test large, circular; ambitus at mid-height.
• Apical disc probably rather large; plating unknown.
• Interambulacral zones with two columns of plates close to the peristome, but four columns of plates from the fifth plate onwards to the apical disc. Each plate with a single large primary tubercle with contiguous areoles.
• Mamelons relatively small, perforate and crenulate; areoles subcircular and incised. No scrobicular ring of tubercles; outside areole tuberculation rather heterogeneous.
• Ambulacra straight and with pore-pairs alternately offset forming a biseries.
• Primary ambulacral tubercle on every other plate at margin; the perradial zone being rather broad and granular.
• Spines relatively slender and cylindrical, with fine ridges according to Cotteau.

Lower Cretaceous (Valanginian), France.

Tetracidaris reynesi Cotteau, 1872, p. 165, by monotypy. Holotype: University of Lyon EM 12131.

•  T. reynesi Cotteau, 1872; Lower Neocomian, France.
• T. kiliani Savin, in Lambert & Thiery, 1910; Valanginian, France.

Cidaroida.

?sister group to Diplocidaridae

Tetracidaris is unique amongst all post-Palaeozoic echinoids in having more than two columns of interambulacral plates in each zone. That this is a derived condition is obvious from the fact that the test is initially constructed as all other cidaroids with just two columns of interambulacral plates in each zone. It is only after the first four plates in each column have formed that the interambulacra become quadriserial. Tetracidaris is identical in its pore-pair and ambulacral structure to Diplocidaris from which it is no doubt derived. Heterocidaris is superficially similar in having four subequal primary interambulacral tubercles abreast in interambulacral zones, but in Heterocidaris there are two primary tubercles to each plate, not four columns of plates. Tetracidaris further differs in having pore-pairs biserially offset.

Cotteau, G. 1872. Echinides nouveaux ou peu connus, 2e article. Revue et Magasin de Zoologie 23, 434-449.