STANDLEY TREES AND SHRUBS OF MEXICO. 869 



XII. PHAEACANTHAE. 



Plants small and creeping 53. O. rastrera. 



Plants more or less bushy. 

 Joints thin ; spines, when present, very long and confined to the upper and 

 middle areoles. 



Spines dark brown, stout, rigid 46. O. macrocentra. 



Spines pale brown, flexible or subulate 47. 0. gosseliniana. 



Joints thick ; spines not confined to the upper and middle areoles. 



Joints relatively small, seldom over 15 cm. broad; plants relatively low. 



Plaats erect, 2 meters high or less _ 48. 0. azurea. 



Plants bushy, rarely over 1 meter high 49. 0. phaeacantha. 



Joints relatively large, mostly over 15 cm. broad ; plants relatively tall. 



Spines clear brown nearly throughout 50. 0. occidentalis. 



Spines nearly white above or throughout. 



Spines with dark brown bases 51. O. engelmannii. 



Spines whitish throughout 52. 0. discata.^ 



XIII. ELATIORES. 

 A single species in Mexico 54. 0. fuliginosa. 



XIV. SCHEERIANAE. 

 A single species 55. 0. scheeri. 



XV. DILLENIANAB. 



Spines nearly setaceous, most of them reflexed 56. 0. chlorotica. 



Spines, when present, acicular to subulate. 



Spines mostly stout, commonly flattened 57. 0. dillenii. 



Spines acicular to subulate, terete or slightly flattened at base. 

 Spines long. 



Areoles mostly 1.5 to 2 cm. apart 58. 0. tapona. 



Areoles mostly 2.5 to 4 cm. apart. 



Spines yellow or yellowish brown 59. O. lindheimeri. 



Spines pale yellow or whitish 60. 0. cantabrigiensis. 



Spines 1.5 cm. long or less, or becoming longer on old joints. 



61. 0. pyriformis, 

 XVI. MACDOUGALIANAE. 



Joints merely finely puberulent or glabrous ; spines 1.5 cm. long or less. 



62. O. durangensis. 

 Joints distinctly pubescent ; spines 2 to 3 cm. long. 

 Petals red. 



Style shorter than the petals 63. 0. atropes. 



Style as long as the petals 64. O. aflinis. 



Petals yellow; 



Spines acicular, at first yellow, soon white 65. 0. macdougaliana. 



Spines subulate. 



Petals retuse; areoles of ovary many, approximate 66. 0. velutina. 



Petals mucronate; areoles of ovary few, distant 67. 0. wilcoxii. 



XVII. TOMENTOSAE. 



Joints narrowly obovate 68. 0. tomentosa. 



Joints broadly obovate 69. 0. guilanchi. ' 



