| | 1. Corolla rotate, yellowish green, cream-colored, or white; ovary and fruit ± round, prickles sparse to ± dense, ± stiff, unhooked |
| M. fabaceus |
| | 1' Corolla ± cup-shaped, white; ovary and fruit sometimes not round, prickles sparse to dense, stiff or flexible, hooked or unhooked |
| |
| | 2. Staminate flowers generally < 8 mm wide; herbage glaucous; ovary and fruit ± round, prickles sparse to dense, flexible, generally hooked; seeds 1–4, round |
| M. watsonii |
| | 2' Staminate flowers generally > 8 mm wide; herbage not glaucous; ovary and fruit ovate, oblong, or tapered to a beak, prickles generally dense, stiff or flexible, unhooked; seeds generally 3–24, disc-shaped, ovate, or oblong |
| |
| | 3. Mature fruit (and ovary) tapered to a beak, often striped dark green, prickles sparse to dense, flexible; seeds generally 3–6, disc-shaped, ± flat; Northwestern California, San Francisco Bay Area |
| M. oreganus |
| | 3' Mature fruit and ovary oblong or ovate, generally not tapered to a beak, not striped green, prickles ± dense, stiff; seeds generally > 6, ovate or oblong, sometimes flat at 1 end; Sierra Nevada Foothills, Southwestern California, Desert |
| |
| | 4. Corolla deeply cup-shaped; seeds very flat at 1 end; c&s Sierra Nevada Foothills, Tehachapi Mountain Area |
| M. horridus |
| | 4' Corolla shallowly cup-shaped; seeds not very flat at either end; Southwestern California, Sonoran Desert |
| M. macrocarpus |
| | 5. Staminate flowers 8–13 mm wide; seeds 15–20 mm; mainland |
| var. macrocarpus |
| | 5' Staminate flowers 14–30 mm wide; seeds 20–33 mm; Channel Islands |
| var. major
|