Taxonomic revision of holocystitid genera (Echinodermata: Blastozoa: Diploporita)

Brightonicystis is a valid genus characterised by nine or more oral plates, with two in the B and D ambulacra not reaching the inner mouth edge; five ambulacral facets shared by oral and facetal plates; two generations of smooth thecal plates with buried humatipores.Brightonicystis contains two named species, Brightonicystis gregaria and Brightonicystis salmoensis, with other unnamed taxa known from Canada and Sweden, all of which are Hirnantian.All other holocystitid genera are Silurian.A review of the Holocystitidae confirms the established genera Holocystites and Brightonicystis both characterised by five ambulacra with shared facets, as well as Trematocystis, Pustulocystis, and Paulicystis all characterised by four ambulacra.The hypothesis that the differences between Pentacystis (without orals) and Osgoodicystis (with two or six orals) are taphonomic is reviewed and rejected.The peristome border and food grooves should be absent in Pentacystis if the oral plates on which they occur were lost during preservation.Pentacystis species show both features developed on facetal plates.Osgoodicystis is reinstated as a valid genus with six narrow orals.Moyacystis gen.nov., type species Osgoodicystis cooperi, bears only two orals (O1 and O6) and facetal plates with oral pores in the peristome border.Pentacystis, Osgoodicystis, and Moyacystis gen.nov., all possess five ambulacral facets restricted to single facetal plates and a smooth thecal surface.The Australian genus Austrocystites is a junior synonym of Trematocystis.Holocystitid subfamilies and subgenera introduced by Frest and Strimple (in Frest et al. 2011) are not discussed herein.