The Echinoid Directory

• Test concave below and weakly domed above, with rounded ambitus.
• Apical disc large, pentagonal, a little less than half test diameter; plating caducous; presumed monocyclic.
• Ambulacra straight, plating polygeminate throughout; pore-pairs uniform and undifferentiated; at ambitus and above plating quiqegeminate and pore zones more or less uniserial; pore zones weakly arcuate at ambitus and developing into weak phyllodes adorally. Primary ambulacral tubercle to each plate; plate compounding diadematid, with all elements reaching the perradius and all overlapped by the primary tubercle at the ambitus.
• Interambulacral plates a little wider than tall; plates with a single large tubercle, centrally placed, which diminishes in size adapically and adorally. Narrow interradial zone of small secondaries and granules widening a little adapically.
• A naked interradial zone developed adapically in interambulacra.
• Ambulacral and interambulacral tubercles similar in size; perforate and crenulate.
• Peristome about half test diameter; buccal notches small but clearly marked and with lip. No sphaeridial pits.
• Spines and lantern unknown.
• Perignathic girdle with auricles fused perradially and with distinct apophyses.

Cretaceous (Barremian - Cenomanian), western Europe.

• P. bonei (Woodward, 1856); Albian-Cenomanian, western Europe [=Pseudodiadema pseudoornatum Cotteau, 1864, p. 486]
• P. cotteaui Lambert, 1892; Barremian-Aptian, western Europe.

Euechinoidea, Calycina, Phymosomatoida, Emiratiidae, Emiratiinae.

Taxa here have previously been assigned to the genus Polydiadema. However, they are distinguished from true Polydiadema by their uniformly polygeminate plating, with five elements to each compound plate from the subambitus to apex. They are distinguished from Emiratia by having uniserial pore zones throughout and from Allomma by its lack of multiple primary tubercles below the ambitus.

Smith, A. B. & Wright, C. W. 1993. British Cretaceous Echinoids. Part 3, Stirodonta 2 (Hemicidaroida, Arbacioida and Phymosomatoida, part 1). Monograph of the Palaeontographical Society (publication no. 593, part of vol. 147), 199-267, pls 73-92.