The Echinoid Directory

• Test flattened below and above; ambitus rounded.
• Apical disc large, subpentagonal, a little less than half test diameter; plating caducous; when preserved monocyclic, with angular interior indicating pavement of polygonal periproctal plates.
• Ambulacra straight, plating trigeminate at ambitus, becoming irregularly quadrigemminate adapically; pore-pairs uniform and undifferentiated; at ambitus and above more or less uniserial; no phyllodes adorally. Pore-pairs very widely separated on ambital plates. Primary ambulacral tubercle to each plate; plate compounding diadematid, 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. Row of small adradial secondaries developed adorally. A narrow naked interradial zone developed adapically.
• Ambulacral and interambulacral tubercles similar in size; perforate and crenulate, the mamelon particularly small.
• Peristome about half test diameter, distinctly invaginated; buccal notches small but clearly markewd and with lip. No sphaeridial pits.
• Spines long, fine and cylindrical, solid without cortex. Lantern unknown.

Cretaceous (Albian-Cenomanian), western Europe.

Diadema michelini Agassiz & Desor, 1846, p. 347, by subsequent designation of Smith & Wright, 1993, p. 237.

• T. blancheti (Desor, 1856); Upper Albian-Lower Cenomanian, western Europe.
• T. michelini (Agassiz & Desor, 1846); Albian-Cenomanian, western Europe.

Phymosomatoida, Emiratiidae, Diploporiinae.

In having trigeminate plating up to the ambitus and then quadrigeminate plating aborally Tiaromma resembles Tetragramma. However, Tetragramma is immediately distinguished by its biserial pore zones and by having enlarged secondary tubercles on interambulacral plates that approach in size the primaries. Tiaromma is ike Loriola in form, but that taxon has a more pentagonal disc that is elongate to the posterior and has only trigeminate plating adapically.

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

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

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.

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.