Desor (1857) figured and named the type species and, in a footnote to the explanation of plate 22, stated that Cotteau was going to make this the type species of a new genus Macropygus. However, when Cotteau eventually described the species (Cotteau & Triger 1859, p. 176 in Cotteau & Triger 1855-69), he had changed his mind and he placed the species truncatus in the genus of Pygaster. The genus was, however, validly designated by Pomel (1883, p. 76), differentiated from Pygaster by its slightly elliptical peristome. Vadet et al. (2001) also retained the genus, adding the Oxfordian species Pygaster laganoides Agassiz, 1839 and distinguishing it from Pygaster by its coarser and more densely packed tubercles, its wider ambulacra and narrower interambulacral zones.
Cotteau, G. & Triger, J. 1855-1869. Echinides du Departement de la Sarthe. J.B. Balliere et fils, Paris.
Vadet, Remy, N. & Wille, E. (2001). Echinides du Corallien des Ardennes. Memoires de la Societe Academique du Boulonais. Serie Histoire naturelle 21, 52 pp.