Radials 7 or 8: tubercles 40 to 50 mm. long.

longimamma (36).

Radials 10 to 12: tubercles 12 to 15 mm. long.

gummiferus (8).

++++ Radials numerous (16 to 60), capillary or bristle-like.

Radials 15 to 30, slender but rigid (bristly).

acanthophlegmus(2), densispinus (4), bispinus (14), rhodanthus (26), sulphureospinus (27), palmeri (29), pringlei (32).

Radials 30 to 60 or more, mostly capillary.

tetrancistrus* (22), capillaris (28), texa.n.u.s* (31), spaerotrichus (33).

***** Central spines more than one and but one of them hooked.

Radials 10 to 15.

goodrichii* (16), setispinus (24).

Radials 15 to 30.

pondii (17), grahami* (19), bocasa.n.u.s (20).

Radials 30 to 60.

tetrancistrus (22).

****** Central spines more than one, and more then one of them hooked.

Radials 8 to 12.

wrightii* (15).

Radials 30 to 60.

tetrancistrus (22).

II. Tubercles with a more or less prominent groove.

* Central spines none.

+ Radials whitish and rigid, oppressed (pectinate) and interwoven with adjacent cl.u.s.ters.

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-dyckia.n.u.s (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-mexica.n.u.s* (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-mexica.n.u.s* (59). arizonicus* (60), macromeris* (64).