* Central spines none.
+ Radials whitish and rigid, oppressed (pectinate) and
interwoven with adjacent clusters.
Depressed-globose and simple.
compactus (44).
Globose and simple.
radians* (45), corniferus (47).
Cespitose.
pectenoides (46), sulcatus* (49).
++ Radials more slender and spreading.
Radials 10 to 17.
missouriensis* (37), similis* (38), macromeris* (64).
Radials 30 to 50, capillary.
dasyacanthus* (51).
** Central spine solitary, not hooked.
+ Central spine porrect.
Radials 6 to 17.
missouriensis* (37), robustior* (39), scheerii* (40).
Radials 30 to 50, white and capillary.
dasyacanthus* (51).
++ Central spine curved downwards.
Radials 8 to 12.
sulcatus* (49).
Radials 12 to 26.
robustispinus (41), recurvatus (42), corniferus (47),
scolymoides* (48).
+++ Central spine erect: Mexican.
Radials 7 or 8: central 50 mm. long.
salm-dyckianus (43).
Radials 10 or 11: central 25 to 35 mm. long.
maculatus (52).
Radials 13 to 16.
compactus (44).
*** Central spine solitary and hooked.
brunneus (53).
**** Central spines more than one and none of them hooked.
Centrals 2: radials 6 to 20.
scheerii* (40), robustispinus (41), recurvatus (42),
scolymoides* (48).
Centrals 3: radials 6 to 40.
scheerii* (40), scolymoides* (48), echinus* (50),
conoideus (54), neo-mexicanus* (59), arizonicus* (60).
Centrals 4 or 5: radials 6 to 40.
scheerii* (40), scolymoides* (48), echinus* (50),
conoideus (54), tuberculosus* (56), viviparus* (57),
radiosus* (58), neo-mexicanus* (59). arizonicus* (60),
macromeris* (64).
Centrals 6 or 7: radials 12 to 40.
potsii* (55), tuberculosus*(56), viviparus* (57),
neo-mexicanus* (59), arizonicus* (60), chloranthus (62).
Centrals 8 to 14: radials 12 to 40 or more.
potsii* (55), tuberculosus* (56), viviparus* (57),
neo-mexicanus* (59), deserti* (61), chloranthus* (62),
alversoni* (63).