Petalidium Nees (Acanthaceae) is a genus of about 40 species. All species have 2 large bracteoles, small 4 or 5-fid calyx, 4 stamens in 2 pairs, ovary with 2 ovules in each cell, mature placentas elastic and seeds covered with hygroscopic hairs. The genus Pseudobarleria T.Anderson which only contained species with a 4-fid calyx was placed in synonymy. The centre of diversity is the north western part of Namibia and south western Angola, with a subcentre in the Huns-Orange area of southern Namibia. A single species occurs on the Indo-Pakistan subcontinent. Such a disjunct distribution is not uncommon in taxa from arid south western Africa and known in other Acanthaceae genera like Blepharis, Monechma, Phaulopsis and Rhinacanthus. The Petalidium genus was described by Nees in Plantae Asiaticae rariores in 1832 with Petalidium barlerioides the type of the genus. The first specimens of what are today known as Petalidium species, were identified as Ruellia barlerioides Roth described in 1821 in India. The earliest collection in this study is from Angola from 1838. Many type specimens were collected by Welwitsch in Angola in the 1860s at the same time as the first specimens were recorded for Namibia by Marloth and Schinz.