The Echinoid Directory

Pygaster L. Agassiz, 1836, p. 185

[=Plesiechinus Pomel, 1883, type species Pygaster macrostoma Wright, 1861; =Megapygus Hawkins, 1912, type species Pygaster umbrella Agassiz, 1839 [=Pygaster semisulcatus (Phillips, 1829)], objective; =Macropygus Pomel, 1883, p. 70, type species Pygaster truncatus Agassiz, 1840. ]

Diagnostic Features
  • Test subpentagonal to subcircular in outline; weakly conical in profile.
  • Periproct opening large and keyhole-shaped; bounded anteriorly by plates of the apical disc. Apical disc plating usually absent, but when preserved apparently hemicyclic (no genital plate 5 observed).
  • Ambulacra trigeminate throughout with all elements reaching the perradial suture. Two plates bound by a primary tubercle alternate with a single simple element.
  • Pore-pairs uniserially arranged throughout.
  • Interambulacral plates considerably wider than tall, with multiple equal-sized tubercles.
  • Peristome large, central, with buccal notches.
Middle Jurassic (Bajocian) to Late Cretaceous (Cenomanian) of Europe.
Name gender masculine
Clypeus semisulcatus Phillips, 1829 by subsequent designation of Savin, 1905, p. 187.
Species Included
  • Pygaster semisulcatus (Phillips, 1829); Middle (Bathonian-Callovian) and Upper Jurassic (Oxfordian-Kimmeridgian), western Europe [=Pygaster tenuis Agassiz, 1839 and P. umbrella Agassiz, 1839].
  • P. ornatus Buckman, 1844; Middle Jurassic (Aalenian, Bajocian), England.
  • P. reynesi Desor, 1857; Early Jurassic (Pliensbachian), France.
  • P. macrostoma Wright, 1861; Middle Jurassic (Bathonian), England.
  • P. laganoides Agassiz, 1839; Bajocian-Bathonian, Kimmeridgian, Europe [=P. morrisi Wright, 1851].
  • P. trigeri Cotteau, 1857; Callovian, France.
  • P. gressleyi Desor, 1842; Kimmeridgian, Europe.
  • P. truncatus (Agassiz, 1840); Cenomanian of France.
Classification and/or Status

Holectypoida, Pygasterina, Pygasteridae.

Probably paraphyletic, by exclusion of Pileus and Anorthopygus.


This genus has a very primitive morphology for irregular echinoids, with the periproct still apical in position and in broad contact anteriorly with the plates of the apical disc. Smith (1979) provided an analysis of its mode of life based on functional morphology. For reasons why the genus Plesiechinus Pomel, 1883 is synonymized see Plesiechinus.

The following taxa were described as pedinoids or orthopsids but are actually juvenile Pygaster.

Agassiz, L. 1836. Prodrome d'une monographie des Radiaires ou Échinodermes. Mémoires de la Société des Sciences naturelles de Neuchâtel, 1, 168-199.

Barras, C. G. 2006. British Jurassic irregular echinoids. Monograph of the Palaeontographical Society no. 625 (part of volume 159 for 2005), 1-168, pls 1-14.

Hawkins, H. L. 1917. Morphological studies of the Echinoidea Holectypoida and their allies. Geological Magazine Dec VI, 4, 160-168.

Smith, A. B. 1979. A comparative study of the life style of two Jurassic irregular echinoids. Lethaia 11, 57-66.

Radwanska, U. 1999. Lower Kimmeridgian echinoids of Poland. Acta Geologica Polonica 49(4), 287-364, pls 1-49.