600 MISC. PUBLICATION 4 2 3, U. S. DEPT. OF AGRICULTURE 



3. Stems several to many; ribs 13 to 21, narrow and acute, partially obscured 

 by the spines; flowers yellow (4). 



4. Seeds tessellate; spines glabrate 2. E. xeranthemoides. 



4. Seeds papillose; spines densely puberulent 3. E. polycephalus. 



2. Hypanthium and fruit glabrous, the scales orbicular-ovate, obtuse, finely 



ciliate; spines densely puberulent; stem normally solitary, 0.5 to 3 m. 



high; seeds reticulate and also somewhat tuberculate; flowers mainly 



yellow, but the perianth segments usually with a differently colored 



central stripe (5) . 



5. Central spines usually brown or gray and hooked; stem thick, barrellike, 



the circumference approximating the length; inner perianth segments 



with a broad purplish central stripe (6). 



6. Bristlelike spines wanting; spines variable, often not hooked, not rarely 



as thick as wide 4. E. covillei. 



6. Bristlelike spines borne on the margin of the areole; spines hooked, 



flattened 5. E. wislizeni. 



5. Central spines pink, red, or rarely straw-colored; stem of mature plants 

 thinner, not barrellike; bristlelike spines normally borne on the mar- 

 gins of the areoles; inner perianth segments with a narrow reddish 

 central stripe, or entirely yellow (7) . 



7. Central spines commonly deflexed or ascending, usually not much 



twisted, rarely hooked 6. E. lecontei. 



7. Central spines porrect, commonly twisted and hooked. 



7. E. ACANTHODES. 



1. Fruit thin-walled, dry, dehiscent by longitudinal fissure or rarely by a basal 

 orifice, persisting only a few weeks; plant small, rarely 50 cm. high; stem 

 solitary; spines not annulate, nearly always glabrous; seeds prevailingly 

 papillose or tuberculate (8) . 

 8. Ribs obsolete, the distinct tubercles arranged in spiral rows; spines not 

 hooked (9). 

 9. Central spines terete, rigid, straight, at first nearly black; perianth seg- 

 ments conspicuously fringed; hypanthium scales fringed; lower tu- 

 bercles spiniferous; flowers yellowish 12. E. sileri. 



9. Central spines strongly flattened and grooved, flexible, curved-ascending, 



chartaceous, ashy white; perianth segments entire; hypanthium scales 

 entire or very sparsely lacerate-toothed; lower tubercles unarmed; 



flowers white ; seeds tessellate .13. E. papyracanthus. 



8. Ribs evident; hvpanthium scales broad, erose or ciliate, in fruit chartaceous 

 (10). 



10. One or more of the lower central spines strongly hooked, the upper central 



erect, not hooked, often whitened, subulate, and strongly flattened; 



flowers greenish yellow or magenta 11. E. whipplei. 



10. None of the spines hooked or flattened, all uniformly pink or red (11). 

 11. Central spines 4 to 9, all alike, straight or slightly curved; flowers yellow 



or magenta 8. E. johnsoni. 



11. Central spines 1 to 4 (if more than 1, then dissimilar), straight; flowers 

 pink (12). 

 12. Central spines 1 or 2, rarely several, the upper one long and erect, the 

 lower porrect and commonly much shorter. 



9. E. ERECTOCENTRUS. 



12. Central spines about 4, one very short and porrect, the upper ones 

 appressed and otherwise like the radials__ 10. E. intertexttjs. 



1. Echinocactus horizonthalonius Lemaire, Cact. Gen. Nov. Sp. 19. 



1839. 

 Silver Bell, Pima County, 2,600 feet, May and June. Western 

 Texas, southern New Mexico, Arizona, and Mexico. 



2. Echinocactus xeranthemoides (Engelm.) Rydb., Fl. Rocky Mount. 



579. 1917. 



Echinocactus polycephalus var. xeranthemoides Engelm. ex 

 Coult., Contrib. U. S. Natl. Herbarium 3: 358. 1896. 



Coconino and Mohave Counties. Southern Utah and Arizona. 



