The Echinoid Directory

• Test flattened below and above; ambitus rounded.
• Apical disc large, pentagonal, a little less than half test diameter; plating caducous; presumed monocyclic.
• Ambulacra straight, plating trigeminate; pore-pairs uniform and undifferentiated; at ambitus more or less uniserial; developing into short phyllodes adorally. Primary ambulacral tubercle to each plate.
• Plate compounding diadematid in style, with all three 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 distinct interradial naked zone in each interambulacrum adapically.
• Ambulacral and interambulacral tubercles similar in size; perforate and crenulate.
• Peristome about half test diameter; buccal notches small but clearly marked and with raised lip. No sphaeridial pits.
• Spines simple and solid; lantern unknown.

Lower Jurassic (Toarcian) to Upper Cretaceous (Cenomanian) western Europe.

Diadema priscum Agassiz, 1840, p. 21, by original designation.

• Trochotiara priscum (Agassiz, 1840); Oxfordian, western Europe
  
• T. inaequale Agassiz, in Agassiz & Desor, 1846); Bathonian-Callovian, France
• T. prisciniacense (Cotteau, 1856); Lower Jurassic, France.

Euechinoidea, Calycina, Phymosomatoida, Emiratiidae.

Subjective junior synonym of Polydiadema Lambert, 1888. Presumed paraphyletic.

Distinguished from Tetragramma and Diplopodia by having uniserial zones of pore-pairs adapically, and trigeminate plating more or less throughout. Tiaromma differs in having quadrigeminate plating adapically. Colpotiara has simple ambulacral plating adorally without primary tubercles but is otherwise similar. Loriola has an elongate pentagonal apical disc that projects into the posterior interambulacrum and its apical disc is larger than its peristome, whereas Trochotiara has a circular or pentagonal apical disc which is smaller than the peristome.

N.B. T. elegantula (Cotteau, 1859); Cenomanian, Sarthe = Tiaromma michelini.

T. noemiae (Cotteau & Gauthier, 1895); Upper Cretaceous, Iran = young Heterodiadema libycum.

Lambert, J. 1901. Note sur quelques oursins bajociens de Comberjon (Haute-Saone), communiques par M. P. Petitclerc. In Petitclec, P. Supplement a la faune du Bajocien inferieur dans le Nord de la Franche-Comte.  Vesoul, L. Bon, p. 233-241, 2 pls.