Ribs thick at base, triangular in cross-section.
Ribs 10 to 14 1. E. coptonogonus
Ribs about 35 2. E. hastatus
Ribs always numerous, very thin even at base.
Ribs 100 or more 3. E. multicostatus
Ribs 25 to 55.
Some or all of radial spines acicular or setaceous. Radial spines all acicular, white, straight. Flowers greenish yellow.
Central spines terete 4. E. wippermannii
Central spines narrow, but flattened 5. E. heteracanthus
Flowers not greenish yellow.
Central spines 4 6. E. albatus
Central spines 3.
Central spines annulate; apex of plant not depressed 7. E. lloydii
Central spines not annulate; apex of plant umbilicate 8. E. zacatecasensis
Upper radial spines subulate, some flattened. Spines yellow or white.
Spines only 5 or 6 9. E. lamellosus
Spines 8 to 11 10. E. grandicornis
Central and upper spines brownish 11. E. arrigens
None of the spines acicular.
Perianth-segments rather short.
All spines appressed against plant 12. E. violaciflorus
Some spines erect or porrect. Ribs about 25.
Four upper spines much elongated 13. E. obvallatus
Spines all somewhat similar.
Spines only 5 14. E. pentacanthus
Spines 10 or more 15. E. crispatus
Ribs 30 or more.
Radial spines white 16. E. dichroacanthus
Radial spines brown.
Flowers purplish 17. E. anfractuosus
Upper and flattened spines 3, rather short, red 18. E. tricuspidatus
Upper and flattened spines usually 1, rarely 2 19. E. phyllacanthus
Perianth-segments much elongated and widely spreading or recurved 20. E. lancifer c • ^ 1 Í2I. E. gladiatus
Species not grouped
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