The vast majority of species of Holectypidae belong to this genus, including all those from the Jurassic. There are 62 nominal species, but many should probably be synonimized.
This taxon is distinguished from Philolectypus by the position of its periproct. In Holectypus the periproct is separated from the peristome by just one or two interambulacral plates, whereas in Philolectypus it is separated by between four and six plates in each column.
Holectypus boschmai Engel 1964 from the Maastrichtian of Limburg, the Netherlands is 20 mm in diameter with 4 gonopores However, it is based on a single specimen found in museum collections and its provenance is almost certainly wrong.
Desor, E. 1842. Des Galérites: Monographies d’Echinodermes vivans et fossiles, par Louis Agassiz, Monograph 3, iv + 94 pp., 13 pls. Petitpierre, Neuchâtel.
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.
Engel. 1964. Zool. Mededel. 39, p. 236, pl. 1, figs 1-2.