The 10 distinct short teeth, by which the cylindrical, usually slightly curved capsule opens at its apex, are a very characteristic feature of this genus and should allow confusion with no other. Except in the very earliest part of the long flowering season, some capsules are likely to be ripe on a plant (from the first flowers of a cyme to bloom). A tiny pentagonal "plug" to which the styles are attached falls free from the apex of the capsule. As Rabeler & Cusick (1994) note, many of these species are very early flowering and easily overlooked.

1. Leaves, stems, and sepals ± densely white-tomentose.

C. tomentosum

1. Leaves, stems, and sepals with mostly straight or irregular hairs, not white, sometimes sticky.

2. Axils of mid-cauline leaves mostly with short sterile leafy tufts; leaves linear to narrowly oblong or lanceolate; corolla showy, exceeding the calyx by 2-5 mm or more; anthers ca. 0.7-1.1 mm long.

3. Widest leaves 1.5-3.5 (-4) mm wide; pubescence on upper part of stem usually with some reflexed hairs.

C. arvense

3. Widest leaves 4-8 mm wide; pubescence on upper part of stem uniformly spreading.

C. velutinum

2. Axils usually without leafy tufts, or leaves narrowly to broadly ovate to elliptic; corolla shorter than the calyx or barely exceeding it; anthers less than 0.7 mm long.

4. Uppermost bracts of the inflorescence completely herbaceous, glandular-puberulent even at the tip; pedicels often ± strongly arched ("hooked") beneath the calyx.

5. Pedicels mostly longer than the calyx; hairs of sepals not extending beyond tip of sepals.

6. Longest pedicels distinctly less than twice as long as the capsules, mostly equaling or shorter than capsules.

C. brachypodum

6. Longest pedicels twice as long as the capsules or longer.

C. nutans

5. Pedicels mostly shorter than the calyx; hairs of sepals extending beyond tip of sepals.

C. glomeratum

4. Uppermost bracts (often partly hidden in a congested inflorescence) with narrow to broad scarious margins at the tip, glabrous there or sometimes with a few stiff non-glandular hairs; pedicels slightly if at all curved.

7. Sepals lacking stalked glands, or very sparsely glandular; petals cleft 0.8-1.2 (-1.4) mm; longer capsules (7-) 8-10 mm long, stamens usually 10.

C. fontanum

7. Sepals densely stalked-glandular; petals very shallowly notched (less than ca. 0.6 mm); longer capsules 4.5-8.5 mm long; stamens 5.

8. Lowermost bracts of the inflorescence with inconspicuous scarious tips up to 0.6 mm long; seeds prominently tuberculate.

C. pumilum

8. Lowermost bracts of inflorescence with conspicuous scarious tips ca. 1-1.5 mm long; seeds with low, rounded tubercles or merely roughened.

C. semidecandrum