The Echinoid Directory

• Test tall, hemispherical, flattened beneath.
• Apical disc small, less than one-third test diameter; plating dicyclic. Genital plates large, hexagonal, G2 generally the largest. Periproct subovate with smooth edges; plating unknown.
• Ambulacra narrow, straight. Plating trigeminate; compounded in acrosaleniid style above ambitus, with all three elements extending to perradius. Pore-pairs uniserial above and at ambitus, expanded adorally to form phyllodes. Primary tubercles large on oral surface, much smaller but still discernible (uniting two of the three elements in each triad) above but becoming entirely simple adapically.
• Interambulacral plates a little wider than tall, dominated by a large primary tubercle. Narrow band of small secondaries and granules interradially and adradially. Tubercles decrease in size adapically.
• Primary tubercles perforate and crenulate. Those of the ambulacra only a little smaller than those of the interambulacra subambitally.
• Peristome large, more or less flush, with deep buccal notches.
• No sphaeridial pits and no basicoronal plate.
• Spines and lantern unknown.

Jurassic (Bathonian-Oxfordian), Europe, North America, Arabian Peninsula.

Diadema florescens Agassiz, 1840, p. 17 by monotypy.

• H. florescens (Agassiz, 1840); Oxfordian, western Europe.
• H. legayi (Cotteau, 1884); Bathonian, France.
• H. brightoni (Philip, 1963); Middle Jurassic, USA.
• H. nanituberculata (Kier, 1972); Callovian, Saudi Arabia.

Acroechinoidea, Carinacea, Pseudodiadematidae.

Most specimens labeled as Hessotiara in museum collections are simply the juvenile of a Gymnocidaris. Hessotiara differs from Pseudodiadema principally by having a rather sharp decline in the size of primary ambulacral tubercles a little above the ambitus. In Pseudodiadema, the tubercles decrease in size gradually towards the apex. The difference is slight and there may be no clear distinction possible between these two genera. Hypodiadema is also very similar but has smaller, less well developed ambulacral tubercles above the ambitus.

Pomel, A. 1883. Classification méthodique et Genera des Échinides vivante et fossiles. Thèses présentées a la Faculté des Sciences de Paris pour obtenir le Grade de Docteur ès Sciences Naturelles 503, Aldolphe Jourdan, Alger, 131 pp.