Acanthopsis adamanticola differs from A. disperma in being a biennial/perennial (vs. an annual herb), with a glaucous appearance usually due to short, flat appressed hairs on the leaves (vs. green leaves with appressed, strigose or spreading hairs), usually with prominent tufts of long, silky hairs at the base of the petiole (vs. no tufts of hairs at the base of petiole), thinner inflorescences (8–)10–12(–15) mm in diameter (vs. (12–)13–15(–20) mm in diameter), with slender primary bract spines usually covered with a short, velvety indumentum (vs. broader primary spines almost glabrous to villose). Acanthopsis adamanticola is distinguished from A. ludoviciana by the white corolla throat (vs. a lemon-yellow throat) and the more northwesterly distribution in the Sperrgebiet, Namibia (vs. the Richtersveld, South Africa).

Type: — NAMIBIA. Karas: Klinghardt Mountains, in basin. Sperrgebiet, (2715 BD), 23 September 1996, Mannheimer & Mannheimer 278 (holotype WIND!; isotype PRE0838901 -0!).

A. disperma sensu Mannheimer et al. (2008: 219 fig.), misapplied name, non Nees.

Biennial or perennial, compact subshrub up to 100 mm high, with reduced branching (internodes 3–5(–7) mm long). Leaves oblanceolate (30–)40–60(–70) × (7–) 10–15 mm; margin flat to undulate, spinose, spines fine, up to 2(3) mm long, yellow; appears glaucous, usually due to appressed, densely packed short flat hairs; often with scattered longer hairs on main vein; base attenuate; petiole 3–8 mm long, usually with tufts of long, silky hairs at base. Inflorescences 25–45(–65) mm long, (8–)10–12(–15) mm in diameter. Bracts cup-, wedge- or fan-shaped, 15–17 mm long, base 5–7 mm long, middle and upper bracts ending in 5 primary spines; central primary spine often much shorter than basal secondary spine from lateral primary spines and very often compound (with 2(3) pairs of marginal secondary spines); lateral primary spines usually with 1 or 2 basal and 0 or 1–4 marginal secondary spines; primary spines usually recurved and secondary spines spreading in fruit; bract base pubescent with deflexed to spreading short hairs, ring of long, silky hairs adaxially at base of spines; spines usually velvety with silky short hairs, rarely almost glabrous. Bracteoles straight to slightly curved with the broadest part near the base, 5–6 mm long, silky hairy especially at tip. Calyx with dorsal sepal ovate, apiculate to cuspidate, 10–11 mm long, densely silky hairy, 7–9-veined from base; ventral sepal ovate, (8–) 9–10 mm long, densely silky hairy, 5–7-veined from base; lateral sepals 7–8 mm long, densely silky hairy. Flowers blue-purple with white throat; corolla 25 × 9–11 mm, tube 9–10 mm long; central lobe wider than long or equal, slightly constricted at base, truncate to emarginate. Stamens with dark brown anthers, 2 mm long; filaments 4–6 mm long, glandular, hairy towards base. Style with patch of glandular hairs at base. Capsules ovate in face view, flattened, glabrous, shiny, 6–7 × 3–4 mm, 2-seeded. Seeds 4 × 3 mm, covered with long white hygroscopic hairs.

Etymology: —The specific epithet, adamanticola (from the Latin adamas = diamond and cola = dweller), is in reference to the fact that this species is near-endemic to the Sperrgebiet (restricted diamond mining area) in southwestern Namibia.

Distribution, ecology and phenology: — Acanthopsis adamanticola is restricted to the inselbergs and rocky outcrops of the coastal plains of the Sperrgebiet and the western foothills of the escarpment northeast of Rosh Pinah, Namibia (Fig. 1) at elevations between 500–750 m (up to 1200 m in the southeast). This area falls within the Gariep Centre of Endemism (Van Wyk & Smith 2001). It is found in sandy crevices associated with rocky outcrops and inselbergs in the Succulent Karoo of Rutherford & Westfall (1994) or Desert and Succulent Steppe of Giess (1971). The plants receive winter rainfall of less than 100 mm per annum, with an average of more than 50 days of fog per year (Mendelsohn et al. 2002). Flowering time: mainly August–October.

On the label of Ward 13875 it is noted that the plants are “much eaten by game”.

Notes: — Meyer (1961) noted that contrary to the species concept of Acanthopsis disperma, the collection of De Winter & Giess 6183 is a perennial with “ 4 cm long branches”; this specimen is here included in A. adamanticola.

According to Vollesen (2000) plants of Blepharis obmitrata C.B. Clarke (1901: 29) from the coastal desert (Namib) in Angola have a dense appressed silvery indumentum which is considered an adaptation to the special climate. Specimens from the higher lying areas around Rosh Pinah, Namibia, with prominent tufts of hairs at the base of the petiole and which appear glaucous but lack the typical short, flat appressed hairs on the leaves are preliminarily included in A. adamanticola: De Winter & Giess 6330 (PRE!, WIND!), Giess 13800 (M!, PRE!, WIND!), Giess ex Wendt 14712 (WIND!), Giess 12941 (WIND!).

The Sperrgebiet and adjacent areas are still botanically under-collected and further fieldwork is necessary to clarify the relationships between A. adamanticola and A. disperma (hairy bract form), especially from the transitional zones between the ranges of these two species.

Conservation status: — Acanthopsis adamanticola is a near-endemic of the Sperrgebiet National Park and restricted to inselbergs and rocky outcrops. Although the park has been fairly comprehensively collected during the past 15 years, this species is known from relatively few localities, with probably less than 10000 plants existing in the wild. As the park is being opened for tourism, including 4x4 routes and a lodge in the Aurus Mountains, a preliminary status of Rare (Raimondo et al. 2009) is assigned to this species (S. Loots, pers. comm. 2014).

Additional specimens examined: — NAMIBIA. Karas: 57 km from Rotkop on power-line track. SW slope of larger koppie, (2615 DD), 23 October 1987, Kolberg & Maggs 200 (PRE!); Diamond Area 1, foothills of Tsabiams Inselberg, 576 m, (2715 BA), 4 September 2002, Bartsch, Loots & Mannheimer SB1010 (WIND!); Klinghardtberge, südlicher Teil, Umgebung des [Klinghardt Mountains, southern part, vicinity of] ‘ Sargdeckel’, (2715 BC), 16 September 1977, Merxmüller & Giess 32073 (M!, WIND!); Klinghardtberge, nördlicher Teil, Quartzithügel (NW), (2715 BC), 18 September 1977, Merxmüller & Giess 32146 (M!); Diamond Area 1, Klinghardt Mountains area: Kaiser’s Camp, 500 m, (2715 BC), 5 December 1996, Ward 13875 (PRE!); Diamond Area 1: Klinghardt Mountains, 700 m, (2715 BD), 21 September 1996, Burke 96111 (WIND!); Klinghardt Mts., (2715 BD), 20 September 1922, Dinter 3973 (BOL!, PRE!); Diamond Area No.1. Inselberg of Klinghardt Mountain Range, 728 m, (2715 BD), 5 August 2001, Germishuizen 10074 (PRE!); Sperrgebiet; slopes of Tsaus Mountains. Hill slope, 27°8’20”S 16°13’3”E (2716 AA), 26 September 1996, Mannheimer 318 (PRE!, WIND!); Diamantgebied 1. Tsaus Spinnenberg, in Gestein [rocks], (2716 AA), October 1977, Wendt 13/2 (WIND!); Sperrgebiet; eastern side of Klinghardt Mountains, 726 m, 27°26.02’S 16°04.14’E (2716 AC), 6 August 2001, Smook 11204 (PRE!, WIND!); Diamond Area No.1. Aurusberg Mountains. West side, 507 m, (2716 CA), 8 August 2001, Germishuizen 10185 (PRE!); Sperrgebiet; northern end of Aurus Mountains on western side, 1988 ft, 27°36.55’S 16°14.23’E (2716 CA), 8 August 2001, Smook 11264 (PRE!, WIND!); Southern Namib: Diamond Area No. 1; Aurus Mountains (North), 630 m, 27°31’S 16°11’E (2716 CA), 22 April 1988, Ward & Seely 10243 (PRE!, WIND!); Sperrgebiet; eastern side of Aurus Mountains at highest peak of range, 27°38.95’S 16°18.98’E (2716 CB), 10 August 2001, Smook 11314 (PRE!, WIND!); Diamond area no. 1; Aurusberge; hills, (2716 CB), 23 October 1974, Watmough 875 (PRE!); Sperrgebiet: Bushmanberg, 661 m, (2716 CD), 6 September 2003, Klaassen, Bartsch & Loots EK1133 (WIND!); Diamond Area no 1. Gravelly flats at foot of stony mountain slopes. Obib Fountain, (2816 BA), 4 September 1958, De Winter & Giess 6183 (M!, PRE!, WIND!).