Three species in the accepted classification
Franz Buxbaum established the genus Oehmea in 1951. The current POWO treatment includes O. beneckei, O. sphacelata and O. tonalensis. Alessandro Guiggi transferred the latter two from Mammillaria in 2022. The former names Mammillaria beneckei, M. sphacelata and M. tonalensis remain useful when reading floristic descriptions and labels in older collections. [2-WWW] [Guiggi 2022, POWO, as of 11 October 2026. Guiggi, pp. 2–3]
The type species of the genus is the taxon described as Oehmea nelsonii, now a synonym of O. beneckei. The type identifies the lineage to which the genus name must refer when its boundaries change. The historical epithet nelsonii therefore has nomenclatural significance, although in the atlas it leads to the O. beneckei account. [Breslin and Majure 2026] [2-WWW]
Structural diversity
The historical separation of Oehmea was associated with large, unevenly sculptured seeds and flower structure. Stem and flower dimensions need to be described separately for each species. O. sphacelata forms branched groups of slender stems, and its flowers in the regional flora are most often 1.5–2 cm long. Tubercles, spine number and stem proportions also vary between its two subspecies. [Breslin and Majure 2026, Breslin and Majure, e331. Flora del Valle de Tehuacán-Cuicatlán, pp. 114–119] [Arias et al. 2012, Breslin and Majure, e331. Flora del Valle de Tehuacán-Cuicatlán, pp. 114–119]
For O. sphacelata, the regional diagnosis reports watery sap, glabrous or sparsely bristly tubercle axils, straight central spines, red fruits and black seeds. This combination can be compared with descriptions of species in the historically broad genus Mammillaria. The distribution of these characters across Oehmea requires separate examination of material from each species. [Arias et al. 2012, p. 115]
Mexican habitats
O. beneckei ranges across western and central Mexico. O. sphacelata occurs in Puebla and Oaxaca, and O. tonalensis in Oaxaca. For O. sphacelata, the Tehuacán-Cuicatlán valley flora documents limestone habitats in xerophytic scrub, seasonally deciduous tropical forests and oak forests, at 900–2100 m above sea level. This vegetation mosaic shows how broad a habitat range can occur within one regional set of sites. [2-WWW] [2-WWW] [2-WWW] [Arias et al. 2012, Flora del Valle de Tehuacán-Cuicatlán, p. 117, 119]
Phylogeny and the 2026 proposal
In the 2021 plastid analysis by Breslin and colleagues, O. sphacelata (called Mammillaria sphacelata in the paper) was placed at the base of the clade containing the sampled Coryphantha. The authors identified it as part of a distinct morphological group and noted the need to include other potentially related species. At the time, taxon sampling allowed the interesting position of this one species to be recognised. [Breslin et al. 2021, p. 312–316]
In 2026, Breslin and Majure published the combinations Oehmea napina and O. zephyranthoides, announcing a fuller phylogenomic and biogeographic analysis. On the date checked, POWO retains the accepted names Mammillaria napina and M. zephyranthoides. The taxonomic article presents both proposals together with their basis and the scope of published data. [Breslin and Majure 2026, POWO, as of 11 October 2026] [2-WWW] [2-WWW]