(Subfamily Byttnerioideae), Tribe Lasiopetalae.
Habit and leaf form. Shrubs; non-laticiferous and without coloured juice. ‘Normal’ plants. Leaves well developed. Plants with roots; non-succulent. Leaves cauline. To 0.5–2 m high. Self supporting. Mesophytic. Not heterophyllous. Leaves small, or medium-sized; alternate; with blades; petiolate. Petioles wingless. Leaves with ‘normal’ orientation; simple; not peltate. Leaf blades neither inverted nor twisted through 90 degrees; dorsiventral; entire, or dissected; flat; ovate, or obovate, or oblong, or elliptic; pinnately veined; cross-venulate; cordate (‘usually with 2 prominent lobes’ (Wheeler 1987)). Mature leaf blades adaxially pubescent (stellate hairs scattered, or only along the veins and margins), or glabrous; abaxially pubescent (stellate hairs scattered on blade, or only on veins). Leaves with stipules (usually 2 when present, or tiny), or without stipules. Stipules intrapetiolar; free of the petiole; free of one another; leafy (usually), or scaly; caducous, or persistent. Leaf blade margins entire. Leaf anatomy. Hairs absent; complex hairs present. Complex hairs stellate.
Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite. Plants homostylous.
Inflorescence and flower features. Flowers aggregated in ‘inflorescences’; not crowded at the stem bases. Inflorescence few-flowered. Flowers in racemes, or in cymes (rarely). Inflorescences simple, or compound (rarely); leaf-opposed. Flowers pedicellate; bracteate. Bracts deciduous. Flowers bracteolate. Bracteoles 3, often slightly connate at the base. Flowers small; regular; not resupinate; neither papilionaceous or pseudo-papilionaceous; 5 merous; cyclic; tetracyclic, or tricyclic (corolla rarely absent). Perianth with distinct calyx and corolla; 10, or 5 (rarely); 2 -whorled, or 1 -whorled (rarely); isomerous. Calyx present; 5; 1 -whorled; gamosepalous; lobed; lobulate to toothed (‘ovate to triangular’). Calyx lobes about the same length as the tube. Calyx prominently 1 veined; hairy (densely stellate hairy, but the hairs on the outside usually longer and larger), or glabrous (on the outside and silky stellate-hairy inside); exceeding the corolla; regular; neither appendaged nor spurred; pink, or purple, or green (rarely); persistent (much enlarged, scarious or coloured at fruiting). Calyx lobes ovate to triangular. Corolla vestigial (‘usually small and scale-like’), or absent; 5; 1 -whorled; not appendiculate; polypetalous; regular. Petals ‘small and scale-like’. Androecium present. Fertile stamens present. Androecial members definite in number. Androecium 5, or 10. Androecial members free of the perianth; free of the gynoecium; markedly unequal (but only when staminodes present), or all equal (rarely when staminodes present, commonly when staminodes absent); free of one another, or coherent (shortly united at base to adjacent stamens or staminodes); 1 -whorled. Androecium exclusively of fertile stamens, or including staminodes (rarely). Stamens 5; all more or less similar in shape; isomerous with the perianth; filantherous. Filaments filiform. Anthers separate from one another; all alike; dehiscing via short slits (toward the inside); bilocular. Fertile gynoecium present. Gynoecium 3(–5) carpelled. The pistil 3(–5) celled. Carpels reduced in number relative to the perianth, or isomerous with the perianth (rarely). Gynoecium syncarpous; eu-syncarpous; superior. Ovary plurilocular; 3(–5) locular. Gynoecium stylate. Styles 1; simple; apical; hairless, or hairy (at the base). Placentation axile.
Fruit and seed features. Fruit non-fleshy (‘usually crustaceous’); dehiscent; a capsule. Capsules valvular. Dispersal unit the seed.
Geography, cytology, number of species. Native of Australia. Endemic to Australia. Australian states and territories: Western Australia, South Australia, and Victoria. Northern Botanical Province, or Eremaean Botanical Province, or South-West Botanical Province. A genus of ca 32 species; ca 32 species in Western Australia; 31 endemic to Western Australia.
Additional comments. Named after the Thomases a Swiss family several of whose members were plant collectors.