The aganippine shield-backed trapdoor spiders of the monophyleticnigrum-group ofIdiosomaAusserers. l.are revised, and 15 new species are described from Western Australia and the Eyre Peninsula of South Australia:I.arenaceumRix & Harvey,sp. n.,I.corrugatumRix & Harvey,sp. n.,I.clypeatumRix & Harvey,sp. n.,I.dandaraganRix & Harvey,sp. n.,I.formosumRix & Harvey,sp. n.,I.gardneriRix & Harvey,sp. n.,I.gutharukaRix & Harvey,sp. n.,I.incomptumRix & Harvey,sp. n.,I.intermediumRix & Harvey,sp. n.,I.jarrahRix & Harvey,sp. n.,I.kopejtkaorumRix & Harvey,sp. n.,I.kwonganRix & Harvey,sp. n.,I.mcclementsorumRix & Harvey,sp. n.,I.mcnamaraiRix & Harvey,sp. n., andI.schoknechtorumRix & Harvey,sp. n.Two previously described species from south-western Western Australia,I.nigrumMain, 1952 andI.sigillatum(O. P.-Cambridge, 1870), are re-illustrated and re-diagnosed, and complementary molecular data for 14 species and seven genes are analysed with Bayesian methods. Members of thenigrum-group are of long-standing conservation significance, andI.nigrumis the only spider in Australia to be afforded threatened species status under both State and Commonwealth legislation. Two other species,I.formosumRix & Harvey,sp. n.andI.kopejtkaorumRix & Harvey,sp. n., are also formally listed as Endangered under Western Australian State legislation. Here we significantly relimitI.nigrumto include only those populations from the central and central-western Wheatbelt bioregion, and further document the known diversity and conservation status of all known species.