STANDLEY TKEES AND SHRUBS OF MEXICO. 977 



Spines not elongate or, if so, not flexuous. 

 Tubercles terete or nearly so. 



Spines all radial 33. N. karwinskiana. 



Spines both radial and central. 

 Radial spines numerous, 12 or more. 



Central spines i-eddish, not much longer than the radials. 



Outer perianth segments ciliate 34. N, standleyi. 



Outer perianth segments setose 35. N. evermanniana. 



Central spines usually white except at tip, much elongate. 



Flowers yellow 36. N. parkinsonii. 



Flowers dark red 37. N. geminispina. 



Radial spines few, 5 to 9. 



Spines black when young 38. N. pyrrhocephala. 



Spines brownish or lighter 39. N. collinsii. 



Tubercles strongly angled. 



Spines both radial and central. 



Radial spines numerous 40. N. chinocephala. 



Radials spines few, bristle-like. 



Central spines 4 to 6 41. N. tenampensis. 



Central spines 2 42. iN. polygona. 



Spines all of one kind, few. 



Flowers yellow 43. N. confusa. 



Flowers rose-colored or white. 

 Flowers rose-colored. 



Plants globose; stigma lobes 4 or 5 44. N. villifera. 



Plants cylindric ; stigma lobes 8 45. N. polyedra. 



Flowers white 46. N. conzattii. 



CC. Tubercles not giving off milk when cut, the milk tubes developed, if 

 at all, only in the stem proper. 

 Central spines wanting. 



Spines subulate; areoles elliptic 47. N. napina. 



Spines acicular ; areoles circular. 



Spines numerous 48. N. lanata. 



Spines few (4 to 6). 



Spines 5 or 6, short, straight 49. N. kewensis. 



Spines 4, elongate, curved. 



Flowers large (2.5 cm. broad) 50. iN. subpolyedra. 



Flowers small. 



Spines long and weak 51. N. g'aleottii. 



Spines subulate 52. N. tetracantha. 



Central spines present. 



Central spines usually 2, sometimes solitary. 

 Radial spines 20 or more. 



Central spines stout and not very long. 



Plant round or nearly so at apex ; central spines often 1. 



53. N. elegans. 

 Plant strongly umbilicate; central spines always 2. 

 , 54. N. pseudoperbella. 



Central spines long 55. N. dealbata. 



Radial spines 20 or fewer. 



Radial spines white, bristle-like. 



