The Echinoid Directory

• Periproct opening large and keyhole-shaped; anterior part bounded by plates of the apical disc. Apical disc plating usually absent, but when preserved hemicyclic (no genital plate 5 observed).
• Apical disc opening only weakly pinched near the apex.
• Ambulacra trigeminate throughout with all elements reaching the perradial suture. Two plates bound by a primary tubercle alternate with a single simple element.
• Interambulacral plates considerably wider than tall, with multiple equal-sized tubercles.
• Peristome large, central and with buccal notches.

Early Jurassic (Pliensbachian) to Middle Jurassic (Bajocian, Bathonian), Europe.

 Pygaster macrostoma Wright, 1861, p. 463, by subsequent designation of Hawkins, 1917, p. 167.

The following three species were included in this genus by Hawkins (1917):

• Plesiechinus ornatus (Buckman, 1844); Bajocian, England [includes Hemipedina bonei Wright, 1856]
• P. reynesi Desor, (1857); Late Pliensbachian of France
• P. macrostoma Wright, 1861; Bathonian of England.

Holectypoida, Pygasterina, Pygasteridae.

Subjective junior synonym of Pygaster Agassiz, 1836.

This genus was established on the basis of inaccurate apical disc plating diagrams. Hawkins (1917) discussed in great depth the nomenclature of this group but relied on original drawings by Wright and Cotteau of apical disc plating, which were inaccurate. According to Hawkins the major difference separating Pygaster from Plesiechinus was that the genital plates of Plesiechinus formed an arc around the anterior part of the periproct, while in Pygaster genital plate 2 was supposed to project posteriorly to contact genital plate 4. In fact the type species of the two genera have remarkably similar plate arrangements (illustrated here and under Pygaster). Apart from the slight difference as to whether the periproct opening is strongly (Pygaster semisulcatus) or weakly (Plesiechinus macrostoma) pinched and keyhole shaped, there are no grounds for separating the two taxa.

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

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.