The Echinoid Directory

Gymnocidaris Agassiz, 1838, p. 3

[=Hypodiadema Desor, 1858, p. 61, type species Hemicidaris saleniformis Desor, 1853; =Prodiadema Pomel, 1869, p. 38, type species Cidarites agassizi Roemer, 1839 ]

Diagnostic Features
  • 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 outer edges.
  • 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.
  • Interambulacral plates a little wider than tall, dominated by a large primary tubercle. Narrow band of small secondaries and granules interradially and adradially. Adapical plates with rudimentary tubercle only.
  • 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 open buccal notches giving the peristome edge a crenulate appearance.
  • No sphaeridial pits and no basicoronal interambulacral plate retained in adult.
  • Primary spines much longer than test; simple, cylindrical shaft with smooth cortex, tapering distally.
Middle Jurassic (Callovian) to Lower Cretaceous (Valanginian) Europe.
Name gender feminine
Hemicidaris diademata Agassiz, 1838, p. 49, by subsequent designation of Lambert & Thiery, 1910, p. 168.
Species Included
  • G. diadema (Agassiz, 1838); Oxfordian, Europe.
  • G agassizi (Roemer, 1839); Oxfordian, Europe.
  • G. guerangeri Cotteau, 1856; Callovian, France
  • G. saleniformis Desor, 1853; Valanginian, Europe
  • other named species not checked.
Classification and/or Status

Euechinoidea, Carinacea, Hemicidaridae, Hemicidarinae.


Distinguished from Hemicidaris by the very rudimentary nature of the primary tubercles on adapical interambulacral plates.

Agassiz, L. 1838. Monographies d'Échinodermes vivants et fossiles. Première monographie: Des Salénies. Petitpierre, Neuchâtel, 32 pp., 5 pls.