The Echinoid Directory

Rhopostoma Cooke, 1959, p. 26

Diagnostic Features
  • Test of small to medium size, oval, 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, posterior oculars not in contact
  • Petals poorly developed, columns of weakly-conjugate pores sub-parallel and merging into small more oblique pore-pairs adambitally, with no clear distal termination. All ambulacral plating simple, except towards the peristome where a triad pattern becomes evident and some elements may even become occluded.
  • Periproct supramarginal, longitudinal, in groove extending from opening to posterior margin
  • Peristome moderately large, trigonal; strongly sunken with sloping walls and no surrounding zones of small dense milliaries
  • Bourrelets absent
  • Phyllodes slightly widened, double pored, pore-pairs arranged in a single series in each half ambulacrum, becoming offset towards the peristome
  • No buccal pores
  • Tubercles large, scrobiculate adorally, slightly smaller and less scrobiculate adapically
  • No naked sternal area
Palaeocene (Middle Danian-Late Thanetian) of Denmark, Belgium, The Netherlands, Trinidad and the United States.
Name gender neuter
Anachytes cruciferum Morton, 1830, p. 245; by monotypy.
Species Included
  • Only the type species
Classification and/or Status

Irregularia; Cassiduloida; Pygaulidae

Subjective junior synonym of Plagiochasma Pomel, 1883.


Cooke (1959) erected this genus on account of its greatly reduced phyllodes. All other characters present in the type species of this genus are typical of Plagiochasma.

C. W. Cooke. 1959. Cenozoic echinoids of eastern United States. United States Geological Survey, Professional Paper, 321, 106 pp., 43 pls.

J. W. M. Jagt. 2000. Late Cretaceous-Early Paleogene echinoderms and the K/T boundary in the southeast Netherlands and northeast Belgium - Part 4: Echinoids, Scripta Geologica, 121, 352-353.

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

A. B. Smith & C. H. Jeffery. 2000. Maastrichtian and Paleocene echinoids: a key to world faunas. Special Papers in Palaeontology, 63, 406 pp.