| | 1. Involucre bracts free to near base, ± stiff-bristly; perennial herb; central inflorescence axis straight |
| C. ciliaris |
| | 1' Involcure bracts fused to form a bur-like involucre, generally spiny; annual; central inflorescence axis wavy |
| |
| | 2. Lower involucre bracts whorled, ± flexible, upper bracts spiny |
| C. echinatus |
| | 2' Lower involucre bracts not clearly whorled, all bracts spiny |
| |
| | 3. Involucre bracts 8–40; longest bract < 5 mm |
| C. incertus |
| | 3' Involucre bracts > 50; longest bract > 5 mm |
| C. longispinus
|