1	"Seeds smooth or minutely colliculate, 0.8–1.7 mm long. Leaves with margins recurved to revolute, discolorous with upper surface darker than undersurface. Occurring south of Perth. Stamens 10–50."	2
1	"Seeds shallowly to deeply reticulate-pitted, 1.3–2.65 mm long. Leaves (in most species) with the margins level or gently incurved to inrolled, concolorous or with upper surface paler than undersurface, the very few species with upper surface darker occurring north of Perth. Stamens 12–200."	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 (sect. Verticilla)."	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 (sect. Cardiomyrtus)."	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. (distribution unknown)"	"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, shed before fruit forms. Anthers very variable in size. Ovules 1 per loculus. (N of Walpole)"	"Hypocalymma verticillare"
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."	5
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."	6
5	"Young stems and peduncles glabrous; petals 4.5–6.5 mm long. (Eastern Stirling Range)"	"Hypocalymma myrtifolium"
5	"Young stems and peduncles with minute hairs. Petals 6–9 mm long. (Stirling Range)"	"Hypocalymma phillipsii"
6	"Petals deep pink-purple, 6–7 mm long. Ovary almost fully superior in flower. Seeds 1.6–1.7 mm long, minutely colliculate. (Stirling Range)"	"Hypocalymma speciosum"
6	"Petals white or pale pink, 1.2–4 mm long. Ovary c. half-inferior in flower. Seeds 0.8–1.3 mm long, smooth."	7
7	"Leaves cordate or depressed-cordate, 6–19 mm long. Petals 2.5–4 mm long. Stamens 25–45, opposite petals as well as sepals. Mature style 2.5–5 mm long. (Jarrahdale–Margaret River area–Cape Riche)"	"Hypocalymma cordifolium"
7	"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. (Whicher Range–Augusta–Denmark)"	"Hypocalymma minus"
8	"Flowers solitary in the axils. Petals bright pink, 6–15 mm long, 2–4 times longer than the stamens (sect. Grandiflora)."	9
8	"Flowers paired in all or many of the axils. Petals white, yellow or pink, 1.5–5.5 mm long, about as long as or shorter than the stamens (sect. Hypocalymma)."	13
9	"Leaves linear in outline, thick. Ovary 2-locular in all or most flowers."	10
9	"Leaves narrowly ovate to obovate or elliptic. Ovary 3 (4)-locular in all or most flowers."	11
10	"Young stems smooth to moderately rugose. Leaves with oil glands usually not very prominent; abaxial surface with 15–25 moderately close-packed oil glands in the 1 or 2 rows closest to (i.e. on either side of) the midvein. Stamens 35–55. (Ballidu–Lake Grace area)"	"Hypocalymma puniceum"
10	"Young stems moderately to very rugose. Leaves with oil glands usually very prominent; abaxial surface with 9–14 widely spaced oil glands in the 1 or 2 rows closest to (i.e. on either side of) the midvein. Stamens 75–130. (Kirwan–Manmanning)"	"Hypocalymma polyandrum"
11	"Leaves sessile, ovate, 4–7 mm wide. Petals 11–15 mm long. Ovules 7–11 per loculus. (Young River area)"	"Hypocalymma magnificum"
11	"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."	12
12	"Young stems smooth. Leaf margins gently incurved. Petals 8–10 mm long. (Lake King area)"	"Hypocalymma inopinatum"
12	"Young stems minutely tuberculate. Leaf margins strongly incurved. Petals 6–8 mm long. (Fitzgerald River National Park)"	"Hypocalymma melaleucoides"
13	"Ovary 2-locular in all or most flowers."	14
13	"Ovary 3-locular in all or most flowers."	20
14	"Leaves 50–70 mm long. Petals white or cream. Ovules 7–11 per loculus. (Murchison River area)"	"Hypocalymma longifolium"
14	"Leaves 2.5–32 mm long. Petals pale to bright pink. Ovules 1–4 per loculus."	15
15	"Ovules 1 per loculus."	16
15	"Ovules 2–4 (usually 3) per loculus."	17
16	"Shrub up to 1.6 m high; branchlets with widely spaced leaves. Largest leaves ± sessile, 20–32 mm long. Flowers borne at 3–13 consecutive nodes. (NW edge Stirling Ra.–Wellstead)"	"Hypocalymma elongatum"
16	"Shrub 0.3–1.2 m high; branchlets with densely clustered leaves. Largest leaves usually with a petiole 0.4–1.3 mm long and blade 5–15 mm long. Flowers borne at 1–3 (–5) consecutive nodes. (Ravensthorpe Ra.–Fitzgerald River National Park)"	"Hypocalymma jessicae"
17	"Leaves 15–30 mm long, 1.2–3 mm wide, distinctly wider than thick. Petals bright to deep pink, 4–5.5 mm long. (Moore River–Walpole)"	"Hypocalymma robustum"
17	"Leaves 2.5–25 mm long, 0.6–1 mm wide, very thick. Petals pale to bright pink, 2.7–4 mm long."	18
18	"Leaves usually densely arranged on the branchlets; petiole 0.4–1.3 mm long. Stamens 35–100. Seeds usually 1.6–2 mm long. (Ravensthorpe Ra.–Fitzgerald River National Park)"	"Hypocalymma jessicae"
18	"Leaves widely spaced on the branchlets; petiole ± absent or poorly defined, up to 0.5 mm long. Stamens 25–55. Seeds commonly 1.4–1.8 mm long."	19
19	"Young stems with star-like protruding oil glands. Leaves 2.5–9 (–13) mm long, with stellate protrusions like those on the stems. (Mount Barker area–Lake Grace–near Napier–Cape Arid National Park)"	"Hypocalymma asperum"
19	"Young stems ± smooth. Leaves 6–25 mm long, ± smooth. (Augusta–Hassell Beach)"	"Hypocalymma strictum"
20	"Ovules 2–5 per loculus. Flowers yellow and with style base continuous with the ridges on summit of ovary, i.e. terminal. Stamens 55–200. Stigma with long papillae (except in H. sylvestre), 0.15–0.5 mm diam. including the papillae."	21
20	"Ovules 1 in each loculus. Flowers white, pink or yellow, with style base slightly to deeply inset (rarely not inset in H. x linifolium) into ovary summit. Stamens 12–55. Stigma with low papillae or entire (except sometimes in H. tenuatum x H. xanthopetalum), 0.05–0.1 (–0.15) mm diam."	26
21	"Flowers 12–15 mm diam., densely arranged at 1–3 consecutive nodes. Ovules 3–5 in most loculi. Style 6–8 mm long; stigma with low papillae or smooth. (Chittering area)"	"Hypocalymma sylvestre"
21	"Flowers 5–11 mm diam., at up to 8 (–16) consecutive nodes and often fairly widely spaced. Ovules 2 per loculus. Style 3.5–5.5 mm long; stigma with elongated papillae."	22
22	"Leaves hairy on both surfaces, sometimes becoming scabrid on the adaxial surface. Flowers bright yellow, turning orange with age. Stamen filaments united for c. 1/3–1/2 their length. (Eneabba–Mount Peron area)"	"Hypocalymma hirsutum"
22	"Leaves glabrous or rarely scabrid on the surfaces. Flowers pale or lemon yellow, fading or turning brownish with age. Stamen filaments united for up to 1/4 of their length."	23
23	"Young stems 4-angled, glabrous."	24
23	"Young stems ± terete or irregular, hairy."	25
24	"Young stems with ridges incurved and often flattened over surface of stem. Leaf margins denticulate. Anthers 0.3–0.35 mm long. Occurring mainly in the Spearwood soil type. (Nambung National Park–Moore River State Forest)"	"Hypocalymma quadrangulare"
24	"Young stems with ridges projecting at right angles to the adjacent surface of the stem. Leaf margins entire on most specimens. Anthers c. 0.5 mm long. Occurring further inland in lateritic soils, (Badgingarra–Dandaragan area)"	"Hypocalymma lateriticola"
25	"Lax, widely spreading subshrub to 0.3 m high, with numerous very fine stems from the base. Leaves 0.4–1.5 mm wide. (Mount Lesueur area)"	"Hypocalymma gardneri"
25	"Low-growing shrub to 0.8 (–1.2) m high, with multiple stems from a lignotuber but with fewer stems, which are not as fine as in H. gardneri. Leaves 2–9 mm wide. (Mingenew–Muchea)"	"Hypocalymma xanthopetalum"
26	"Style base only slightly inset on ovary (although seated between prominent wings in H. serrulatum) or terminal."	27
26	"Style base inserted in a cylindrical depression."	29
27	"Leaves less than twice as wide as thick. Flowers opening predominantly in autumn and winter. Ovary 3-winged on summit. Fruits shed with seeds intact. (Badgingarra area)"	"Hypocalymma serrulatum"
27	"Leaves more than twice as wide as thick. Flowers opening predominantly in spring. Ovary 3-ridged on summit. Fruits dehiscent while attached to the plant, releasing seeds through 3 terminal valves."	28
28	"Stems minutely hairy. Leaves 1.2–2 mm wide. (Mount Lesueur National Park)"	"Hypocalymma tenuatum x H. xanthopetalum"
28	"Stems glabrous. Leaves 2–4 mm wide. (Badgingarra–Dandaragan area)"	"Hypocalymma x linifolium"
29	"Flowering stems densely papillose and/or narrowly winged. Leaves often with oil glands star-like or surrounded by a circle of papillae."	30
29	"Flowering stems not papillose and not winged Leaves with oil glands not as above."	31
30	"Leaves 0.5–1.3 mm wide; apical point 0.2–0.4 mm long. Petals 2–2.5 mm long. (Warradage–Mount Lesueur area)"	"Hypocalymma tenuatum"
30	"Leaves 2–7 mm wide; apical point absent or up to 0.2 mm long. Petals 3–4 mm long. (Eneabba–Badgingarra)"	"Hypocalymma tetrapterum"
31	"Young flowering stems with a loose papery epidermis obscuring their shape, bearing a very dense inflorescence. Leaves 6–20 mm long. Petals cream to bright yellow."	32
31	"Young flowering stems 4-angled, usually with obvious gaps between the nodes of the inflorescence. Leaves (8–) 15–50 mm long. Petals white or pink."	33
32	"Shrubs lignotuberous. Petals 2.3–3 mm long, usually yellow. Stamens 25–50 in a full circle. (near Harvey–Augusta–N of Walpole)"	"Hypocalymma ericifolium"
32	"Shrubs single-stemmed or multi-branched at base. Petals 1.5–2.3 mm long, usually cream. Stamens 12–20, often with gaps opposite the petals. (N of Scott River–Two Peoples Bay Nature Reserve)"	"Hypocalymma scariosum"
33	"Leaves with minutely crenulate margins; apical point 0.3–1.5 mm long, if less than 0.6 mm long then uncinate. Seeds 1.4–1.8 mm long."	34
33	"Leaves with entire margins, apex ± straight or incurved; apical point absent or up to 0.5 mm long. Seeds 1.8–2.3 mm long."	35
34	"Shrubs with a lignotuber. Leaves with apex ± straight but the apical point sometimes recurved; apical point 0.7–1.5 mm long. Associated with wetlands or watercourses, mainly near the coast, always well west of the range of H. uncinatum. (Hutt River–Denmark)"	"Hypocalymma balbakiae"
34	"Shrubs without a lignotuber. Leaves with apex strongly recurved or uncinate; apical point 0.3–0.5 mm long. Occurring on granite outcrops inland. (Merredin–Lake King)"	"Hypocalymma uncinatum"
35	"Shrubs single-stemmed or multi-branched at base, up to 3 m high at full maturity. Leaves (0.6–) 0.8–1 (–1.2) mm wide; about as wide as thick or slightly wider, the oil glands rather densely packed and often prominent. Stamens 20–50. (Watheroo National Park–Perth–near Ongerup area)"	"Hypocalymma suave"
35	"Shrubs with a lignotuber, up to c. 1.5 m high. Leaves 1–6 mm wide, the floral leaves several times wider than thick, the oil glands usually sparser and less prominent than in H. suave. Stamens 18–35."	36
36	"Leaves (especially the floral ones) 1–2.3 mm wide; blade broader in basal part than the upper third. (Badgingarra area–Albany area)"	"Hypocalymma angustifolium"
36	"Leaves 1.8–6 mm wide; blade similar in width in basal part and upper third. (Cataby–Muellering Brook area)"	"Hypocalymma x proliferum"