68 PROCEEDINGS OF THE AMERICAN ACADEMY. 



lower surfaces ; margins entire or closely serrulate, slightly revolute, with 

 5 to 9 pairs of setae mostly below the middle : heads solitary or in small 

 terminal or axillary clusters : involucre oblong, 5 to 7 mm. high, 3 or 4 

 mm. broad, 9-2o-flowered ; bracts 5, oblong, blunt, with scarious margins 

 and prominent thickened keels : pales sparingly serrulate, 2 to 2^ mm. 

 long : akenes spreading-hirsute or glabrate, 3|- to 4 mm. long. — Ic. iv. 

 12. t. 324 ; Less. Linnaja, vi. 714 ; DC. Prodr. v. 100. Chthonia pros- 

 trata^ Cass. Diet. Sci. Nat. ix. 173. — A common and polymorphous 

 species, the typical form growing from the Southwestern United States 

 to Yucatan and Guatemala. Arizona, near Camp Grant (Rothrock, 

 no. 722), sandy plains near Mexican boundary (Pringle) ; New Mexico 

 (C. Wright, nos. 245, 1123); Sonora (Thurber, no. 971), Alamos 

 (Palmer, 1890, no. Go9) ; Durango (Palmer, 1896, no. 512) ; San Luis 

 PoTOSi (Parry & Palmer, no. 361); Oaxaca (Nelson, no. 1693); 

 Yucatan (Schott, no. 533, Gaumer, nos. 788, 1092) ; Guatemala, 

 Santa Rosa (Heyde & Lux in John Donnell Smith's Exsic, no. 3366^), 

 Canchon (Heyde & Lux, no. 6154). A puzzling form with narrow 

 heads and leaves setiferous to the tip has been collected in Guatemala 

 (Heyde & Lux, no. 4232). 



Var. cylindrica. Leaves generally rounded at the tips, rarely 

 mucronate ; seta3 fewer, 2 to 5 pairs near the base : heads generally soli- 

 tary (in one specimen clustered) : involucre longer and narrower, 8 or 

 9 mm. high, 2 or 3 mm. broad ; bracts 3, thin, plane or a little condupli- 

 cate, finely canaliculate, the mid-vein slightly or scarcely thickened into 

 a keel : akene and pappus a third longer than in the type. — Southwest- 

 ern United States and adjacent Mexico. Arizona (Palmer, 1867, 

 no. 128), Apache Pass (Lemmon, no. 619) ; Nevt Mexico (Palmer) ; 

 CoAHUiLA, Monclova (Palmer, 1880, no. 640) ; Sonora, Guaymas 

 (Palmer, 1887, no. 145). 



Var. urceolata. Ferruginous-pubescent on the stems, midribs, and 

 involucres : leaves large, the larger 3|- cm. long, 6 mm. wide, with 8 to 

 12 pairs of setag : involucre urceolate, 9 or 10 mm. high; the 5 thick 

 bracts with broad rounded pubescent keels. — Chihuahua, Hacienda 

 San Jose (Palmer, 1885, no. 53). 



= = Pappus a row of linear-attenuate setulose pales, with a minute exterior setu- 

 lose crown at the base ; pales the same number both in the disk and in the ray. 



P. Schaffneri, Schz. Bip. Very low and slender, 5 cm. or so high : 

 stems puberulent: leaves linear-attenuate, cuspidate, Ij to 2\ cm. long, 

 1 to 1 1 mm. wide, margins entire or minutely serrulate, becoming entire : 



