| | 1. Stem clearly jointed; small barbed bristles present in areoles; seed white, bone-like |
| OPUNTIA |
| | 1' Stem not clearly jointed; barbed bristles 0; seed black or brown |
| |
| | 2. Stem ribs 0 or inconspicuous, tubercles prominent |
| |
| | 3. Tubercle longitudinally grooved on top (indented in X-section); central spine not hooked |
| ESCOBARIA (no key) |
| | 3' Tubercle round in X-section (not grooved); some central spine of areole hooked |
| MAMMILLARIA |
| | 2' Stem ribs prominent, tubercles 0 to prominent |
| |
| | 4. Plant > 3 m; stem > 30 cm diam, generally branching above 1.5 m; flower creamy white |
| CARNEGIEA (no key) |
| | 4' Plant < 3 m; stem < 30 cm diam, branching near ground or unbranched; flower yellow to red or magenta |
| |
| | 5. Stems length generally > 8 × width; empty fruit long-persistent |
| BERGEROCACTUS (no key) |
| | 5' Stems length generally < 8 × width; fruit not long-persistent |
| |
| | 6. Ovary and young fruit spiny, glabrous; stem soft-fleshy; branches generally few–many |
| ECHINOCEREUS |
| | 6' Ovary and young fruit either spineless or woolly; stem firm-fleshy; branches generally 0 (if present, then larger spines with ring-like ridges) |
| |
| | 7. Fruit and stem tip densely woolly; bracts sharp-tapered |
| ECHINOCACTUS (no key) |
| | 7' Fruit and stem tip not woolly; bracts wide, obtuse to acute |
| |
| | 8. Stem > 15 cm diam; seed pitted |
| FEROCACTUS |
| | 8' Stem < 15 cm diam; seed smooth or weakly tubercled |
| SCLEROCACTUS
|