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.

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

Cidarites mamillanus Roemer, 1836, by subsequent designation of Savin, 1903, p. 136,

Lambert & Thiery (1911, p. 189) also took the type species as P. mammillanum Roemer (Cidaris), refering specifically to the illustration in Desor & de Loriol [1868-72. Echinologie Helvetique. Terrain jurassique, p. 154, pl. 25, figs 2-3]. However, they (Lambert & Thiery 1925, p. 565) subsequently claimed that the specimen illustrated and described by Desor & de Loriol was different from Roemer\'s Cidaris mammillanum. They recognized that the specimen of Desor & de Loriol that they referred to in setting up this genus was synonymous with Wright\'s Diadema davidsonii and later corrected the type species designation for Polydiadema. It seems highly likely, nevertheless, that Wright\'s species is synonymous with Romer's, as Wright (1856, p. 132) originally believed.

• P. mammillanum (Roemer, 1836); Oxfordian, Europe.
• P. priscum (Agassiz, 1840); Oxfordian, Europe.
• P. bourgueti (Agassiz, in Agassiz & Desor, 1846); Valanginian-Berriasian, ?Aptian, Europe, North Africa and the Middle East.
• P. bigranulatum (Gregory); Cenomanian-Turonian, Egypt [includes T. humei Fourtau and T. balli Fourtau].
• P. depressa (Agassiz, in Agassiz & Desor, 1846); Aalenian-Bathonian, western Europe [?=T. homostigma (Agassiz, in Agassiz & Desor, 1846); Bajocian, France; =T. subcomplanata (d'Orbigny, 1850); Bathonian, France].
• P. delgadoi (de Loriol); Hauterivian, Portugal.
• P. deslongchampsi (Cotteau, 1869); Upper Toarcian, France.
• P. fittoni (Wright, 1868); Aptian, England.
• ?P. gauthieri (Cotteau, 1885); Upper Pliensbachian, France [no phyllodes adorally, pore-pairs uniserial and tubercles very small!]
• P. guerangeri (Cotteau, 1859); Cenomanian, France, Portugal.
• P. inaequale (Agassiz, in Agassiz & Desor, 1846); Callovian, France.
• P. neglecta (Thurmann, in Desor, 1856); Upper Oxfordian, Kimmeridgian, France
• P. langi (Desor, 1856); Oxfordian, France.
• P. princeps (Thurmann, in Desor, 1856); Oxfordian, France
• P. randenensis (de Loriol, 1871); Oxfordian, Switzerland
• P. rotularis (Agassiz, 1836); Neocomian- Barremian, France
• P. superba (Agassiz, in Agassiz & Desor, 1846); Oxfordian, Europe
• P. travisensis Smith & Rader, 2009; Albian, Texas, USA
• Trochotiara priscum (Agassiz, 1840); Oxfordian, western Europe
• T. inaequale Agassiz, in Agassiz & Desor, 1846); Bathonian-Callovian, France
• T. prisciniacense (Cotteau, 1856); Lower Jurassic, France.

Lambert & Thiery (1910) included a number of other species in this genus, all with true polygeminate plating. These differ from Polydiadema in having 5 elements to each compound plate, and represent an unnamed taxon within the Emiratiidae. They include:

• P. bonei (Woodward, 1856); Albian-Cenomanian, western Europe.
• P. cotteaui Lambert, 1892; Barremian-Aptian, western Europe.

Euechinoidea, Calycina, Phymosomatoida, Emiratiidae.

Not distinguished from Trochotiara in any significant way. Diplopodia differs in having biserial pore zones adapically. Tiaromma is similar but can be distinguished by having polygeminate plating above the ambitus.

Lambert, J. 1888. Note sur un nouveaux genre d'Echinide de la Craie de Yonne. Bulletin de la Societe des Sciences historiques et naturelles de l\'Yonne 42, (2), 1-14, 1 fig.