9(8). Leaf sheaths close; blades firm; culms slender and firm; perigynia firm, flat 

 or merely convex on inner surface (10) 



9. Leaf sheaths loose; blades soft to firm; culms soft, flattened under pressure; 



perigynia spongy or corky at base, thin and soft, more or less 

 inflated (13) 



10(9). Inner nerveless ventral band of leaf sheath not cross-puckered; perigynia 



broadly compressed, obovoid or obpyramidal, abruptly beaked 



7. C. decomposita. 



10. Inner nerveless ventral band of leaf sheath cross-puckered and/ or red-dotted; 



perigynia flat on inner face (11) 



11(10). Leaves flat, exceeding the culm; perigynia ascending, 1.7-3 mm. long; 

 scales, ovate, the 3-nerved center green, terminating in a long 

 rough awn 8. C. vulpinoidea. 



11. Leaves thickish, flat or channeled, usually not exceeding the culm; perigynia 



ascending or spreading, 3.5-4 mm. long; scales acute or cuspidate 

 (12) 



12(11). Perigynia 1.6-1.8 mm. wide, almost black at maturity; achenes 1.5 mm. 

 long (Ariz.) 9. C. alma. 



12. Perigynia 2 mm. wide, light-green or brown; achene 2 mm. long (Okla.) 



10. C. fissa. 



13(9). Bases of perigynia disklike; beak 2 to 3 times length of body 



11. C. crus-corvi. 



13. Bases of perigynia not disklike; beak 1 to 2 times length of body (14) 



14(13). Leaf sheaths cross-puckered ventrally, without band at the orifice 



12. C. stipata. 



14. Leaf sheaths smooth ventrally, with band at the orifice.... 13. C. laevivaginata. 



15(13). Perigynia with rounded to very narrow margins or edges, without def- 

 inite winged margins, thickened or corky at the base (16) 



15. Perigynia with thin or winged margins, mostly with concave inner faces, not 



spongy or corky at the base (20) 



16(15). Perigynia with rounded margins, ascending or merely spreading-ascend- 

 ing, of soft or membranaceous texture (17) 



16. Perigynia with thin but scarcely winged margins, ascending to horizontally 



divergent or reflexed in maturity, firm, very spongy at base (19) 



17(16). Perigynia ovoid-oblong, 1.8-3 mm. long, more or less nerved on both 

 surfaces; beak very short or obsolete 5. C. canescens. 



17. Perigynia ovate-lanceolate or lanceolate, 3.5-4.5 mm. long; beak half the 



length of the body (18) 



18(17). Perigynia shallowly bidentate, 3.5-4 mm. long; spikes ovoid or oblong, 

 the lateral pistillate 14. C. leptopoda. 



18. Perigynia deeply bidentate, 4-4.5 mm. long; spikes linear-oblong, all gyne- 



candrous 15. C Bolanderi. 



19(16). Beak of perigynia only minutely notched; perigynia 1-2 mm. wide, 

 nerveless or essentially so; heads 1-3 cm. long, of subglobose spikes 

 about 4 mm. in diameter 16. C. interior. 



19. Beak of perigynia sharply bidentate at tip; perigynia 2-2.5 mm. wide, strongly 



nerved on both surfaces; heads 2-6 cm. long, of 3 to 6 echinate 

 spikes mostly 7-12 mm. long and 6-8 mm thick 17. C. atlantica. 



20(15). Spikes 1.5-2.5 cm. long, long-cylindric; perigynia 7-10 mm. long, thin 

 and scalelike 18. C. muskingumensis. 



20. Spikes less than 1.5 cm. long; perigynia less than 7.7 mm. long (21 ) 



491 



