Diagnostic Features
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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
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Apical system tetrabasal, with four genital pores
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Petals typically unequal, with petals V and I longer than others, interporiferous zones wide, poriferous zones narrow, closing distally, all ambulacral plates double-pored
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Periproct supramarginal, longitudinal, in groove extending from opening to posterior margin
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Peristome oblique, anterior
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Bourrelets absent
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Phyllodes slightly widened, double pored, pore-pairs arranged in two series in each half ambulacrum
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No buccal pores
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Tubercles only slightly larger on the adoral surface than on adapical
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No naked sternal area
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Distribution
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Early Cretaceous (Valanginian) to Late Paleocene of Europe, Kazakhstan, North America, Trinidad and North Africa
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Name gender |
feminine |
Type |
Nucleolites olfersii L. Agassiz, 1836, p. 133; by subsequent designation of Melville, 1952, p. 1.
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Species Included |
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P. olfersii (L. Agassiz, 1836); Hauterivian, Early Cretaceous of France and Switzerland
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P. faringdonense (Wright, 1875); Lower Greensand, Upper Aptian, Early Cretaceous of UK.
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P. cruciferum (Morton, 1830); Paleocene of Denmark, Belgium, The Netherlands, Trinidad and the United States.
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P. texanum Smith & Rader, 2009; Lower Albian, Texas, USA
Lambert & Thiery (1909-1925) list 11 species within this genus.
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Classification and/or Status |
Irregularia; Cassiduloida; Pygaulidae
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Remarks |
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.
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