21 (20) . Perigynia at most 2 mm. wide (22) 



21. Perigynia more than 2 mm. wide (28) 



22(21). Bracts conspicuously exceeding the head but not leaflike 



19. C. athrostachya. 



22. Bracts wanting or setaceous (23) 



23(22). Perigynia barely distended over the achene, thin and scalelike (24) 



23. Perigynia obviously distended over achene, firm (26) 



24(23). Sheaths loose and loosely ribbed, veined ventrally with a hyaline summit; 

 wings of perigynia abruptly narrowed above the base (Tex. and 

 Okla.) 20. C. tribuloides. 



24. Sheaths close, ventrally hyaline; wings of perigynia extending continuously 



to the base (Ariz. & N.M.) (25) 



25(24). Perigynia 3.5-5 mm. long; beak of perigynium slender and terete, not 

 serrulate (or only slightly) at the usually dark-colored tip; spikes 

 distinguishable but aggregated into an ovoid or suborbicular trun- 

 cate-based head 21. C microptera. 



25. Perigynia 4-7 mm. long; beak of perigynium flattened and serrulate to the 



pale tip; spikes aggregated into an oblong or linear-oblong head 



22. C. scoparia. 



26(23). Perigynia ovate, broadest below the middle; spikes often clavate at base.. 

 23. C. festiicacea. 



26. Perigynia obovate to suborbicular or elliptic to rhombic, broadest at or above 



the middle (27) 



27(26). Inflorescence 6-10 mm. thick; spikes ovoid, each with 30 to 50 perigynia 



which are 2.8-3.2 (-3.5) mm. long and 1.6-2.2 mm. broad 



24. C. alholutescens. 



27. Inflorescence 12-15 mm. thick; spikes narrowly ovoid, each with 55 to 80 



perigynia which are (3-) 3.5-4.2 (-4.5) mm. long and 1.7-2.5 

 mm. broad 25. C. Longii. 



28(21). Scales acute to blunt, without awn-tips (29) 



28. Scales awn-tipped; body of perigynium broadest near the summit 



26. C. alata. 



29(28). Perigynia thin and scalelike, barely distended over the achene, lanceolate 

 to narrowly ovate, 1.2-2.6 mm. wide 22. C. scoparia. 



29. Perigynia firmer and thicker, usually well-distended over the achene (30) 



30(29). Beak of perigynium short and broad, gradually tapering into the firm 

 broadly elliptic to rhombic body of the perigynium which is 3-4.5 

 mm. long and 1.7-2.5 mm wide 25. C Longii. 



30. Beak of perigynium elongate and narrow above, more abruptly difTerentiated 



from the obovate or suborbicular body of the perigynium which is 

 3.5-7.7 mm. long (31) 



31 (30). Perigynia pale-green to dull-brown. 2.8-3.5 mm. long, 1.6-2.2 mm. wide, 



the obovate to suborbicular bodies broadest above the middle 



24. C. alholutescens. 



31. Perigynia stramineous or greenish, 3.5-8.5 mm. long, 2.3-6 mm. wide, the 



bodies broadest below the middle (32) 



32(31). Perigynia strongly nerved on ventral surface (33) 



32. Perigynia ventrally essentially nerveless or nerves few (34) 



33(32). Perigynia 15 to 30 per spike, broadly ovate, with transverse wrinkles 

 between the veins on the ventral side; perigynia 2.5-3.2 mm. wide.... 

 27. C. hyalina. 



492 



