1	"Leaves with margins recurved to revolute, discolorous with upper surface darker than undersurface. Stamens 10–50. Seeds smooth or minutely colliculate, 0.8–1.7 mm long. Occurring south of Perth"	2
1	"Leaves with margins usually level or gently incurved to inrolled, concolorous or with upper surface paler than undersurface, if with upper surface darker then occurring north of Perth. Stamens 12–200. Seeds shallowly to deeply reticulate-pitted, 1.3–2.65 mm long"	8
2	"Young stems 3-angled, glabrous, each angle with a rounded ridge that is directly opposite the petiole above. Leaves in whorls of three, with oil glands in 2 or 3 main irregular rows. Flowers solitary in the axils. Androecium of 15–26 filaments, united for at least half of their length"	3
2	"Young stems all or mostly + terete, 4-angled or 4-winged. Leaves all or mostly opposite, dotted with numerous minute oil glands. Flowers up to 4 per axil but all solitary in H. minus. Androecium of 10–50 filaments, united only at base or free"	4
3	"Leaf c. 7 mm long, 1–1.5 mm wide; apical point c. 0.5 mm long. Petals 3.5–4 mm long, persistent. Anthers all of a similar size. Ovules 8–10 per loculus"	"Hypocalymma connatum"
3	"Leaf blades 3–6 mm long, 1.8–2.3 mm wide; apical point scarcely developed. Petals 2–2.5 mm long, deciduous. Anthers very variable in size. Ovules 1 per loculus"	"Hypocalymma verticillare"
4	"Leaves cordate or very broadly ovate to almost circular, 4–19 mm long. Ovules 2 per loculus, erect. Seeds with a hilum near centre of inner surface"	5
4	"Leaves ovate to obovate or broadly elliptic, 15–40 mm long. Ovules 6–12 per loculus, horizontal. Seeds with a hilum near base of inner surface"	7
5	"Petals deep pink-purple, 6–7 mm long. Ovary almost fully superior in flower. Seeds 1.6–1.7 mm long, minutely colliculate"	"Hypocalymma speciosum"
5	"Petals white or pale pink, 1.2–4 mm long. Ovary c. 1/2 inferior in flower. Seeds 0.8–1.2 mm long, smooth"	6
6	"Leaves cordate or depressed cordate, 6–19 mm long. Petals 2.5–4 mm long. Stamens 25–40, opposite petals as well as sepals. Mature style 2.5–5 mm long"	"Hypocalymma cordifolium"
6	"Leaves ovate to cordate, 4–8(–10) mm long. Petals 1.2–2 mm long. Stamens 10–20, opposite sepals but not petals. Mature style 0.6–1.2 mm long"	"Hypocalymma minus"
7	"Young stems and peduncles glabrous; petals 4.5–6.5 mm long"	"Hypocalymma myrtifolium"
7	"Young stems and peduncles with minute hairs. Petals 6–9 mm long"	"Hypocalymma phillipsii"
8	"Flowers solitary in the axils. Petals 6–14 mm long, bright pink, deciduous. Stamens much shorter than the petals"	9
8	"Flowers paired in all or many of the axils. Petals 1.5–5.5 mm long, white, yellow or pink, persistent or deciduous in fruit. Stamens about as long as or exceeding the petals"	12
9	"Leaves linear in outline, thick. Ovary 2-locular in all or most flowers"	"Hypocalymma puniceum"
9	"Leaves narrowly ovate to cordate or elliptic. Ovary 3(4)-locular in all or most flowers"	10
10	"Leaves sessile, broadly ovate to cordate, 4–6 mm wide. Petals c. 14 mm long. Ovules 8–10 per loculus"	"Hypocalymma sp. Cascade (R. Bruhn 20896)"
10	"Leaves with a petiole 0.5–0.9 mm long, blade narrowly ovate or obovate to almost elliptic, 1.5–3 mm wide. Petals 6–10 mm long. Ovules 3–5 per loculus"	11
11	"Young stems smooth. Leaf margins gently incurved. Petals 8–10 mm long"	"Hypocalymma sp. Lake King (R.W. Purdie 3936)"
11	"Young stems minutely tuberculate. Leaf margins strongly incurved. Petals 6–8 mm long"	"Hypocalymma melaleucoides"
12	"Ovary 2-locular in all or most flowers. Bracteoles usually deciduous, the flowers usually ± sessile except in H. syvestre"	13
12	"Ovary 3-locular in all or most flowers. Bracteoles usually persistent, the flowers usually ± sessile"	19
13	"Leaves 50–70 mm long. Petals white or cream. Ovules 8–11 per loculus"	"Hypocalymma longifolium"
13	"Leaves 8–30 mm long. Petals pale to bright pink. Ovules 1–4 per loculus"	14
14	"Ovules 1 per loculus"	15
14	"Ovules 2–4 (usually 3) per loculus"	16
15	"Shrub 0.3–1 m high. Branchlets with densely clustered leaves, the largest usually with a petiole 0.4–1.3 mm long and blade 7–15 mm long"	"Hypocalymma jessicae"
15	"Shrub up to 1.6 m high. Branchlets with widely spaced, ± sessile leaves, the largest 20–32 mm long"	"Hypocalymma elongatum"
16	"Leaves 15–30 × 1.2–3 mm, distinctly wider than thick. Petals bright to deep pink, 4–5.5 mm long. Seeds usually 1.9–2.1 mm long"	"Hypocalymma robustum"
16	"Leaves 3–16(–25) × 0.6–1 mm, very thick. Petals pale to bright pink, 2.7–4 mm long. Seeds usually 1.4–1.8 mm long"	17
17	"Young stems with with star-like protruding oil glands. Leaves 3–9(–13) mm long, with stellate protrusions like those on the stems"	"Hypocalymma asperum"
17	"Young stems ± smooth. Leaves 6–25 mm long, ± smooth"	18
18	"Leaves usually densely clustered on the branchlets, usually with a well defined petiole 0.4–1.3 mm long"	"Hypocalymma jessicae"
18	"Leaves well spaced on the branchlets, ± sessile or with a poorly defined petiole up to 0.5 mm long"	"Hypocalymma asperum"
19	"Ovules 2–5 per loculus. Flowers yellow and with style base continuous with the ridges on summit of ovary. Stamens 45/50–200"	20
19	"Ovules 1 in all or most loculi. Flowers white, pink or yellow, if yellow then with style base inset into ovary. Stamens 12–55"	25
20	"Leaves with long hairs. Stamen filaments united for 1/3 to almost half their length"	"Hypocalymma hirsutum"
20	"Leaves glabrous or with short hairs. Stamen filaments united for up to 1/3 of their length"	21
21	"Young stems glabrous, ± terete. Leaves densely arranged, ± as broad as long, ciliate. Ovules 3–5 in most loculi. Stigma entire. Seeds c. 1.6 mm long"	"Hypocalymma sylvestre"
21	"Young stems prominently tuberculate to hairy or, if glabrous, then 4-angled. Leaves  not very densely arranged, usually much narrower than long, entire to long-hairy. Ovules 2 per loculus. Stigma papillose to hairy. Seeds 2.1–2.65 mm long"	22
22	"Young stems 4-angled and ridged or winged on each angle, glabrous or rarely sparsely hairy"	23
22	"Young stems more terete, hairy"	24
23	"Young stem ridges incurved and often flattened over surface of stem. Leaf margins with a narrow scarious border. Occurring mainly on the coastal plain in the Spearwood soil type"	"Hypocalymma sp. Nambung (R. Spjut & R. Smith s.n. 22/09/1992)"
23	"Young stem ridges usually patent. Leaf margins mostly lacking a narrow scarious border. Occurring inland of the Spearwood soil type?"	"Hypocalymma sp. Dandaragan (C.A. Gardner 9014)"
24	"Leaves (2–)3–9 mm wide"	"Hypocalymma xanthopetalum"
24	"Leaves 0.6–1.5 mm wide"	"Hypocalymma gardneri"
25	"Style base terminal on ovary but sometimes apparently inset between 3 wing-like ridges on the summit of the ovary"	26
25	"Style base inserted in a depression that reaches down to the level of the placentas"	27
26	"Leaves thick, with serrulate-denticulate margins. Petals white or pink. Flowering predominantly in autumn"	"Hypocalymma serrulatum"
26	"Leaves flat, entire or sparsely ciliolate. Petals yellow. Flowering predominantly in spring"	"Hypocalymma linifolium"
27	"Leaves flat, 1.5–6 mm wide, obtuse, not or scarcely pointed, with numerous oil glands each surrounded by a circle of papillae"	"Hypocalymma tetrapterum"
27	"Leaves usually thick and 0.4–1.3 mm wide, rarely flat and up to 3.5 mm wide, acute or with an apical point 0.1–0.7 mm long, if flat then with non-papillate oil glands"	28
28	"Young stems/leaves? densely papillose with prominent star-like oil glands"	"Hypocalymma tenuatum"
28	"Young stems glabrous, not papillose"	29
29	"Slender single-stemmed or multi-branched shrubs 0.1–1 m high, with leaves 6–20 mm long and very dense inflorescences. Young flowering stems with loose papery epidermis obscuring their shape. Petals cream to bright yellow"	30
29	"Stoutly single-stemmed or lignotuberous shrubs 0.4–2.5 m high, with leaves (12–)15–50 mm long and usually with obvious gaps between the axils of the inflorescences. Young flowering stems 4-angled. Petals white or pink"	31
30	"Petals 2.3–4 mm long, usually yellow. Stamens 20–50 in a full circle"	"Hypocalymma ericifolium"
30	"Petals 1.5–2.3 mm long, usually cream. Stamens 12–20, often with gaps opposite the petals"	"Hypocalymma scariosum"
31	"Leaves long-linear, apex strongly recurved or uncinate, apical point c. 0.5 mm long. Occurring on granite outcrops inland"	"Hypocalymma uncinatum"
31	"Leaves subulate to long-linear, apex ± straight or incurved; usually associated with wetlands or on laterite and mainly near the coast, inland collections with apical point of leaves usually absent or up to 0.3 mm long"	"Hypocalymma angustifolium"