The Echinoid Directory

Plagiochasma Pomel, 1883, p. 59

[=Dochmostoma Duncan, 1891, p. 176; type species Nucleolites olfersii L. Agassiz, 1836, p. 133; by subsequent designation of Kier, 1962, p. 93 (objective);=Trematopygus d'Orbigny, 1857, p. 374; type species Nucleolites olfersii L. Agassiz, 1836, p. 133; by subsequent designation of Lambert & Thiery, 1921, p. 338 (objective); =Rhopostoma Cooke, 1959, p. 26; type species Anachytes cruciferum Morton, 1830, p. 245; by monotypy]

Diagnostic Features
  • Test of small to medium size, elongate, anterior outline rounded, posterior slightly indented by anal groove, greatest width posterior to centre, adoral surface rounded marginally, depressed at peristome
  • Apical system tetrabasal, with four genital pores
  • Petals typically unequal, with petals V and I longer than others, interporiferous zones wide, poriferous zones narrow, closing distally, all ambulacral plates double-pored
  • Periproct supramarginal, longitudinal, in groove extending from opening to posterior margin
  • Peristome oblique, anterior
  • Bourrelets absent
  • Phyllodes slightly widened, double pored, pore-pairs arranged in two series in each half ambulacrum
  • No buccal pores
  • Tubercles only slightly larger on the adoral surface than on adapical
  • No naked sternal area
Early Cretaceous (Valanginian) to Late Paleocene of Europe, Kazakhstan, North America, Trinidad and North Africa
Name gender feminine
Nucleolites olfersii L. Agassiz, 1836, p. 133; by subsequent designation of Melville, 1952, p. 1.
Species Included
  • P. olfersii (L. Agassiz, 1836); Hauterivian, Early Cretaceous of France and Switzerland
  • P. faringdonense (Wright, 1875); Lower Greensand, Upper Aptian, Early Cretaceous of UK.
  • P. cruciferum (Morton, 1830); Paleocene of Denmark, Belgium, The Netherlands, Trinidad and the United States.
  • P. texanum Smith & Rader, 2009; Lower Albian, Texas, USA

Lambert & Thiery (1909-1925) list 11 species within this genus.

Classification and/or Status
Irregularia; Cassiduloida; Pygaulidae

Kier (1962, p. 89) reports that Plagiochasma is very similar to Pygaulus and Pygopistes. However, Plagiochasma differs in having a supramarginal periproct, where it is inframarginal in Pygaulus, and marginal in Pygopistes.

P. M. Kier. 1962. Revision of the cassiduloid echinoids. Smithsonian Miscellaneous Collections, 144 (3) 262 pp.

J. Lambert & P. Thiery. 1909-1925. Essai de nomenclature raisonnee des echinides. Libraire Septime Ferriere, Chaumont, 607 pp., 15 pls.

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, 131 pp. Aldolphe Jourdan, Alger.