The Echinoid Directory

Hemidiadema Agassiz, in Agassiz & Desor 1846, p. 351

Diagnostic Features
  • Test depressed, flattened above and below.
  • Apical disc small (about one-quarter test diameter); plates usually preserved bound to corona; monocyclic, with large subcircular periproct.
  • Ambulacra straight, relatively wide. Pore-pairs uniserial throughout; no phyllodes. Plate compounding irregular, all elements extending to perradius and a single column of primary tubercles in each zone.
  • Interambulacral plates wider than tall, with large, centrally placed primary tubercle. Narrow adradial and interradial zones of dense uniform secondaries and granules.
  • Primary tubercles perforate and crenulate.
  • At ambitus and aborally elongate sutural depressions developed beneath the tubercles giving the test a sculptured appearance.
  • Peristome moderately large, weakly sunken, with feeble buccal notches.
  • No sphaeridial pits; no basicoronal plate.
  • Lantern and spines unknown.
Distribution
Cretaceous (Hauterivian - Cenomanian), Europe.
Name gender neuter
Type
Hemidiadema rugosum Agassiz, in Agassiz & Desor, 1846, p. 351, by monotypy.
Species Included
  • H. rugosum Agassiz, 1846; Upper Aptian - Cenomanian, Europe.
  • H. neocomiense (Cotteau); Hauterivian, France
Classification and/or Status

Euechinoidea, Camarodonta, Glyphocyphidae.

Monotypic.

Remarks
Very close to Glyphocyphus but with irregular ambulacral tuberculation, primary tubercles being uiserial rather than biserial.  The genus has been thoroughly revised by Clement (2013).

Agassiz, L. & Desor, P. J. E. 1846-1847. Catalogue raisonné des familles, des genres, et des espèces de la classe des échinodermes. Annales des Sciences Naturelles, Troisième Série, Zoologie: 6 (1846), 305-374, pls 15-16; 7 (1847), 129-168; 8 (1847), 5-35, 355-380.


Clement, A. 2013. Revision des especes Hemidiadema rugosum Agassiz, 1846, et H. neocomiense (Cotteau, 1869) (Euechinoidea, Camarodonta, Glyphocyphidae) du Cretace inferieur du Bassin parisien (France). Carnets de Geologie Memoire 2013/01 (CG2013_M01)