| | 1. Inflorescences in dense axillary cyme-like clusters; stem erect — sparsely hairy |
| C. nutans |
| | 1' Inflorescences 1 per node, sometimes crowded on lateral branches; stem prostrate to ascending (sometimes erect) |
| |
| | 2. Involucre, fruit hairy; stem, leaves generally hairy or becoming glabrous |
| |
| |
| | 4. Involucre urn-shaped; staminate flowers < 12 — Sonoran Desert |
| C. arizonica |
| | 4' Involucre ± bell-shaped; staminate flowers > 15 |
| |
| | 5. Seed 3-angled, transversely 4–5-ridged, ridges rounded |
| C. pediculifera |
| | 5' Seed 4-angled, smooth to ± wrinkled |
| |
| | 6. Stem and leaf hairs short, straight |
| 2 C. polycarpa |
| | 6' Stem and leaf hairs tomentose or long, appressed, dense |
| |
| C. melanadenia |
| | 7' Involucre glands green to yellow |
| C. vallis-mortae |
| |
| | 8. Staminate flowers > 30; fruit > 2 mm; n Sacramento Valley |
| C. ocellata subsp. rattanii |
| | 8' Staminate flowers < 20; fruit < 2 mm; s California |
| |
| | 9. Gland appendage deeply 3–5-lobed; involucre urn-shaped |
| C. setiloba |
| | 9' Gland appendage entire, shallowly scalloped, or 0; involucre bell-shaped or obconic |
| |
| | 10. Gland appendage 0; seed smooth to slightly wrinkled |
| 2 C. micromera |
| | 10' Gland appendage present; seed wrinkled or ridged |
| |
| | 11. Fruit > 1.5 mm; seed > 1 mm, wrinkled; gland appendage narrower than gland |
| C. serpyllifolia subsp. hirtula |
| | 11' Fruit < 1.5 mm; seed generally < 1 mm, transversely ridged or wrinkled; gland appendage = to or wider than gland |
| |
| | 12. Seeds transversely wrinkled; fruit evenly strigose; appendage = to gland width |
| C. maculata |
| | 12' Seeds transversely ridged; fruit hairy on edges only; appendage wider than gland |
| C. prostrata |
| | 2' Involucre, fruit, stem, leaves glabrous (sometimes hairy) |
| |
| | 13. Stipules fused into wide, membranous scale |
| |
| | 14. Perennial; staminate flowers > 12 |
| C. albomarginata |
| | 14' Annual; staminate flowers < 10 |
| C. serpens |
| | 13' Stipules separate or fused below |
| |
| |
| | 16. Glands round, appendage 0; leaves < 5 mm |
| C. parishii |
| | 16' Glands elliptic or oblong, appendage present; leaves > 3 mm |
| |
| | 17. Fruit > 2 mm; stipules separate; seed > 2 mm |
| C. fendleri |
| | 17' Fruit < 2 mm; lower stipules fused; seed < 2 mm |
| 2 C. polycarpa |
| |
| | 18. Leaves linear, base symmetric; stem ascending to erect |
| |
| | 19. Glands 1–4, oval, appendage narrower than gland; fruit and seed > 1.5 mm; seed smooth |
| C. parryi |
| | 19' Glands ± 4, round, appendage wider than gland or 0; fruit and seed < 1.5 mm; seed transversely ridged |
| C. revoluta |
| | 18' Leaves lanceolate to round, base asymmetric; stem prostrate to decumbent |
| |
| | 20. Leaf margin toothed, at least toward tip |
| |
| | 21. Style entire; gland appendages deeply 3–5-lobed — vernal pools |
| C. hooveri |
| | 21' Style divided; gland appendages entire or shallowly lobed |
| |
| | 22. Plant hairy; seed transversely ridged |
| 2 C. abramsiana |
| | 22' Plant glabrous; seed smooth, wrinkled, or transversely ridged |
| |
| | 23. Seed transversely ridged; appendage wider than gland |
| C. glyptosperma |
| | 23' Seed smooth to wrinkled; appendage narrower than gland |
| C. serpyllifolia subsp. serpyllifolia |
| |
| | 24. Seed flattened top to bottom, surface smooth; gland ovate |
| C. platysperma |
| | 24' Seed 3–4-angled, surface smooth, wrinkled, or transversely ridged; glands disc-like or elliptic |
| |
| | 25. Gland appendage present; seed transversely ridged |
| 2 C. abramsiana |
| | 25' Gland appendage 0; seed smooth or wrinkled |
| |
| | 26. Fruit < 2 mm; seed < 1 mm; staminate flowers < 10 |
| 2 C. micromera |
| | 26' Fruit > 2 mm; seed > 1 mm; staminate flowers > 30 |
| C. ocellata |
| | 27. Leaf > 10 mm, lanceolate to ovate; Mojave Desert |
| subsp. arenicola |
| | 27' Leaf < 10 mm, ovate, sickle-shaped; California Floristic Province |
| subsp. ocellata
|