870 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 



XVIII. LEUCOTRICHAE. 

 A single species 70. 0. leucotricha. 



XIX. ORBICULATAE. 



Hairs from the areoles of young plants long and white, long-peristent ; plants 



low 71. 0. orbiculata. 



Hairs from the areoles of young joints early deciduous; plants tall. 



72. O, pilifera. 



XX. FICUS-INDICAE. 

 Joints dull. 



Joints thin, up to 50 cm. long 73. 0. ficus-indica. 



Joints thick, 15 cm. long or less 74. 0. crassa. 



Joints ftlossy 75. 0. undulata. 



XXI. STREPTACANTHAE. 



Areoles close together, sunken 76. 0. spinulifera. 



Areoles not close together, not sunken. 



Spines acicular 77. O. lasiacantha. 



Spines subulate. 

 Areoles with 2 or more short reflexed hairs or bristles at the lower part 

 of the areole. 

 Spines strongly depressed ; areoles with several hairs. 



78. 0. hyptiacantha. 

 Spines not strongly depressed ; areoles with 1 or 2 hairs. 



Joints obovate . 79. O. streptacantha. 



Joints oblong 80. O. amyclaea. 



Areoles without reflexed hairs or bristles 81. O. megacantha. 



XXII. ROBUSTAE. 



Fruit deep red, 7 to 9 cm. in diameter 82. 0. robusta. 



Fruit greenish white, 4 to 5 cm. in diameter 83. O. guerrana. 



XXIII. STENOPETALAE. 



Spines dark; plants low, prostrate 84. 0. stenopetala. 



Spines white; plants erect. 



Joints narrow ; spines acicular 85. 0. glaucescens. 



Joints broader ; spines stouter 86. 0. grandis. 



XXIV. CHAFFEYANAE. 



A single species 87. O. chaffeyi. 



1. Opuntia mortolensis Britt. & Rose, Cactaceae 1: 47. 1919. 



Sonora ; described from cultivated plants. 



Slender, 60 cm. high or less, dull green, the ultimate twigs short, sometimes 

 only 2 cm. long, 4 to 5 mm. thick, scarcely tuberculate ; young areoles with 

 numerous early deciduous weak white hairs and several brown glochids ; are- 

 oles of old branches with solitary acicular spines 3 to 5 cm. long, these with 

 tightly fitting brown sheaths. 



