Autor: Backeberg

“Spiny head”, because the genus is completely covered with spines.
A genus of small solitary plants, more or less spherical to elongated, the apex depressed, many low ribs (30-60), divided into spiralled small tubercles. Many spines (25-60), small, but rather stiff and dense, white or yellow.
Flowers diurnal, rather small, appearing in the apex, more or less funnel-shaped to campanulate, with a short tube, self-fertile, greenish, yellowish-green, yellow, orange-coloured or red flame, usually pollinated by insects. Fruits spherical, abundantly setose, usually drying on the plant without opening, floral remains persistent. Seeds rather small, elongated, strongly warty, black. Strophiole absent.
The genus Acanthocephala grows in southern Brazil, from low altitudes up to 1500 m, among grasses, in the cracks of rocky terraces or hilly slopes, in the crevices of rocks, mostly among moss.
Brazil (Rio Grande do Sul).

Currently 2 recognised species:
Acanthocephala graessneri (K.Schum.) Guiggi 2012
Acanthocephala haselbergii* (Haage ex Rümpler) Guiggi 2012

References: “TAXONOMY of the CACTACEAE” – ISBN 978-84-617-3723-9 (Vol. 1)