In a series of postings in the summer of 2019, Bruce Bayer discussed the taxonomy of Haworthia retusa and related H. turgida, H. pygmaea, H. mirabilis, H. magnifica, H. emelyae, H. mutica, H. groenwaldii, H. floribunda. Here are supplemental references.
H. retusa (L.) Duval in Pl. Succ. Hort. Alenc.: 7 (1809): Aloeretusa L. (1753). Lectotype, designated by Scott (1985): Illustration in Commelin, Horti Med. Amstelod. 2: t. 6 (1701). Epitype, designated by Breuer & Metzing (1997): Blikbonnie, E Riversdale, Dekenah s.n. NBG144772 (NBG).
retusa: with leaf-tips bent back thumb-like.
Rosette stemless, slowly or seldom proliferous, to 12cm φ. Leaves 10-15, turgid, rigid, with pronounced retused end-area, pointed tips, variously lined and windowed. Without surface spination and usually without spination on margins and keel. Colours brownish or green and seldom purpling. Inflorescence simple, robust, to 30cm. Flowers compacted on inflorescence, white with greenish-brown veins.
H. retusa var. turgida (Haw) Bayer (2012)H. turgida Haw. in Suppl. Pl. Succ.: 52 (1819). Neotype, designated by Breuer & Metzing (1997): Swellendam, Breede River Bridge, Bayer 2420 (NBG).
Rosette partially stemless, proliferous, 5-10cm φ. Leaves 20-40, turgid often as thick as broad, recurved or slightly retused, generally mottled, yellow-green to pink in sun, margins and keel lightly spined. Inflorescence simple, 15-20cm. Flowers slender, brownish-white with darker venation.
H. pygmaea V.Poelln. in Repert. Spec. Nov. Regni Veg. 27: 132 (1930). Neotype, designated by Breuer & Metzing (1997): hills E Great Brak, Fourcade 4759 (BOL).
Rosette stemless, slowly proliferous, 6-10cm φ. Leaves 12-15, retused, round-tipped, surface pellucid with obscure raised tubercles, sometimes intensely papillose. Inflorescence simple, robust, to 30cm. Flowers white with greenish veins.
H. mirabilis (Haw.) Haw. in Syn. Pl. Succ.: 95 (1812): Aloemirabilis Haw. (1804). Neotype, designated by Bayer (1977): Illustration in Curtis’ Bot. Mag.: t. 1354 (1811). Epitype, designated by Breuer & Metzing (1997): Skuitsberg, between Caledon and Greyton, Bayer 2453 (NBG).
Rosette stemless, proliferous, to 7cm φ. Leaves 10-15, retused, 3-4cm X 1,5cm, markedly retused, acute above, face translucent and lined, dark green, with marginal spines turning reddish in the sun. Inflorescence slender. Flowers narrow, elongate, biarcuate bud, upper lobes pinched at tips.
H. mirabilis var. magnifica (V.Poelln.) Bayer (2012): Haworthiamagnifica V.Poelln. in Repert. Spec. Nov. Regni Veg. 33: 240 (1933): H. maraisii var. magnifica (V.Poelln.) Bayer : 131 (1976). Lectotype, designated by Breuer & Metzing (1997): Riversdale, Ferguson.
Rosette stemless, slowly proliferous, to 8 cm φ. Leaves spreading, retused to ground level, dark green to purplish, scabrid to finely spined margins, end-area slightly translucent between the veins, surfaces with small slightly raised tubercles. Inflorescence slender, to 40cm. Flowers brownish veined, few open, upper lobes pinched at tips.
H. emelyae V.Poelln. in Repert. Spec. Nov. Regni. Veg. 42: 271 (1937). Lectotype, designated by Breuer & Metzing (1997): [unpublished image] (B).
emelyae: for Emely Ferguson.
Rosette to 10cm φ, seldom proliferous. Leaves 15-20, distinctly retused, pointed, barely translucent, with scattered elongate small flecks, with obscure raised tubercles, lined, reddish-brown hued. Inflorescence simple. Flowers 15-20, white.
H. mutica Haw. in Revis.:55 (1821). Lectotype, designated by Bayer (1978): [image] (K; later published in Excelsa 8: 50 (1978). Epitype, designated by Breuer & Metzing (1997): NE Soetrivier Bridge, Bayer KG623/69 (NBG).
mutica: without a point.
Rosette stemless. non-proliferous, 6-8cm φ. Leaves 12-15, retused, blunt-tipped, brownish-green, in habitat developing purplish cloudiness, barely pellucid with several longitudinal lines. Inflorescence simple, to 20cm. Flowers white with brownish veins.
Description: Always growing singly, but in a couple of cases growing in groups of plants of different ages. Rosette: acaulescent, 40-50 mm Ø (exceptionally up to 100 mm). Leaves: 5-7(8), fleshy, spreading, end-area in most cases very rounded (resembling H. springbokvlakensis), 20-25 mm long, 13-18 mm broad, 11-13 mm thick. Leaf surface of back and leaf-base, smooth, opaque; colour dull dark green. Margins: opaque, smooth. Back without keel. End-areas: 13-18 mm long, face translucent, with 3-5 whitish parallel lines, the middle one the longest (sometimes parallel lines are ± partly of greenish colour). Surface of the end area, rough with small papillae. End area of some specimens are with ± whitish, ± raised dots, in a few cases the dots are blended into cloudy flecks. Leaf-tip: very rounded, without teeth. Inflorescence: 300-600 mm long, 1.2-2 mm diam., Peduncle: 200-300 mm long. Sterile Bracts: 15-20, 5-7 mm long. Raceme: 100-300 mm long. 8-18 flowers, 2-5 flowers open at one time. Fertile Bracts: 3-4 mm long. Pedicels: 2-4 mm long, 1-1.2 mm diam. Flowers: 14-16 mm long. Perianth: 3 -4 mm diam. Flower-face: 9-12 mm long. Upper part: 6-7 mm long, 3-5 mm broad. Lower part: 5-7 mm long, 5-9 mm broad. Fruit: 11-18 mm long, 2.5-4 mm thick. Flowering time February–March. Type Information Collector: J. Groenewald s.n. (= MBB7801) Locality: 3420 (-BA), Buffeljagsrivier, Western Cape, SA.
H. floribunda V.Poelln. in Repert. Spec. Nov. Regni Veg. 40: 149 (1936). Neotype, designated by Bayer (1982): [unpublished image] (B). Epitype, designated by Breuer & Metzing (1997): Blackdown, NE Heidelberg, Bayer 158 (NBG).
floribunda: many flowered.
Rosette stemless, up to 3cm φ, slowly proliferous. Leaves 20-30 dark green opaque, upto ovate-lanceolate, spreading, twisted with flattened, rounded tip, margins scabrid to dentate. Inflorescence simple, to 250mm. Flowers 10-15, greenish-white, few open together.