Flowers solitary, axillary, bisexual. Stamens 5. Styles 2.
Fruiting perianth thickened, with a hollow base; wings usually 2–5, rarely 1, usually vertical. Seed horizontal, embryo annular.
| | Key to the species | |
| 1 | Fruiting perianth subglobular, sessile or almost so, without wings | Osteocarpum salsuginosum |
| Fruiting perianth stipitate on a hollow base or, if sessile, then 5 erect wings | 2 |
| 2 | Vertical wings absent | 3 |
| Vertical wings present
Back to 1 | 4 |
| 3 | Base of fruiting perianth swollen, broader than tube; radicular tubercle prominent; interpaline tubercules 1–4 | Osteocarpum scleropterum |
| Base of fruitng perianth cylindrical, equal to tube; radicular tubercle prominent; interpaline tubercle 1 or none
Back to 2 | Osteocarpum acropterum |
| 4 | Vertical wings 1–3 | 5 |
| Vertical wings 5
Back to 2 | Osteocarpum pentapterum |
| 5 | Vertical wings 1 or 2, obovate, not extending downward along the tube | 6 |
| Vertical wings 2 or 3, semicircular
Back to 4 | Osteocarpum dipterocarpum |
| 6 | Perianth base broad and swollen, wing solitary; tubercles 1–4 | Osteocarpum scleropterum |
| Perianth base cylindrical; wings 2, or if 1 then with a single tubercle
Back to 5 | Osteocarpum acropterum |
