C. nigra: stem 1-2 feet high; leaves scabrous, lanceolate, sinuate-dentate, the upper entire or nearly so, stem-clasping; heads usually not radiant; florets all fertile, purple or rarely white; involucre globular, nearly black externally, the appendages of the outermost scales smaller and narrower than the others, those of the innermost row roundish, dark brown, membranous, jagged, all contracted just below the appendage. Knapweed. - Pastures and roadsides. Fl. June to September.
** Involucral scales lanceolate, their upper half with a scarious fringed decurrent margin.
C. Scabiosa: stem 2-3 feet high, rough, furrowed; leaves hispid, pinnatifid, roughish, the segments lobed, with callous points, the lobes of the upper ones entire; heads on long, naked stalks, solitary; involucres rather woolly, the scales pale, blunt, with dark acute membranous pectinated decurrent appendages; florets purple, the outer row radiant. Great Knapweed. - Fields and hedges. Fl. July to September.
*** Involucral scales horny at the end, with palmate spines.
C. solstitialis: annual; stem1-2 feet high, branched, spreading, winged with the decurrent bases of the linear-lanceolate entire hoary leaves, the root-leaves lyrate; heads terminal, solitary; involucral scales woolly, palmately-spinous, the central spine of the intermediate scales very long, needle-shaped; florets yellow. - Yellow Star-thistle. - Cornfields and waste places near the sea. Fl. July to September.