Data for: Phylogenetic placement and taxonomic revision of the enigmatic Brazilian genus Monochilus Fisch. & C. A. Mey. (Lamiaceae)

<p><em>Monochilus</em> (Lamiaceae) is an enigmatic genus endemic to Brazil with two rarely collected species currently recognized. Based on morphological features <em>Monochilus</em> is hypothesized to be closely related to <em>Amasonia</em> within subfamily Ajugoideae. However, <em>Monochilus</em> is one of the few genera within Lamiaceae never to be represented in a published molecular phylogenetic study and thus has an uncertain placement with the subfamily. During recent herbarium exploration in association with the Flora of Brasil 2020 project, new specimens of <em>Monochilus</em> <em>gloxinifolius</em> and <em>M</em>. <em>obovatus</em> were uncovered. Based on this material, a new phylogeny of Ajugoideae was constructed based on nuclear ribosomal ITS and the chloroplast markers matK and trnL-trnF. <em>Monochilus</em>, as previously hypothesized, is closely related to <em>Amasonia</em> and placed within Ajugoideae. We discuss the phylogenetic placement of the genus and provide a taxonomic revision with a description, distribution maps, an identification key, preliminary conservation assessments, photographic plates and comments on both the taxonomy and ecology of these two species. Additionally, we provide a lecotypification for <em>M</em>. <em>gloxinifolius</em>.</p>