Key to the species of Deamia

1. Stems 0.6–2 cm in diameter, the ribs (6–)7 or 8, acute or rounded; flowers longistylous .................................................................2

- Stems 2.5–15 cm in diameter, the ribs 3–10, acute; flowers longistylous or brevistylous.................................................................3

2. Spines up to 1.4 cm long; stem ribs rounded; flowers 8–10 cm long ............................................................................ Deamia funis

- Spines up to 2.6 cm long; stem ribs acute; flowers (23–) 27–30 cm long ........................................................... Deamia montalvoae

3. Flowers 6–9 cm long, brevistylous with the stamens 0.4–0.5 cm long and the style 2–2.1 cm long, the stigma positioned well below the level of anthers; stems 2.5–3.5 cm in diameter ............................................................................................ Deamia chontalensis

- Flowers 22–28 cm long, longistylous with the stamens 9–20 cm long and the style 22–24 cm long, the stigma positioned among or just beyond the anthers; stems 3–15 cm in diameter.............................................................................................. Deamia testudo