Coryphantha retusa SB544 , Mitla, Oxaca, Mexico.
Dull-green cactus thickly covered with tawny wool and rounded, flattened above (retuse) tubercles.
Spines pectinated pale-yellow/white with darker tips later turning grey.
 

Description: Generally not offsetting, rarely clumping when old.
Stem: Depressed spherical 5-10 cm in diameter, 3-8 cm tall, with narrowed base and very woolly apex. Epidermis dark green or greyish-green.
Tubercles: Conical rounded, flattened above (retuse), unequally swollen, up to 8 mm tall with abundant wool in the axil and furrow when young, parastichy arrangement: 8-13 (13-21).
Areoles: Oblong 4 x 2 mm wide.
Roots: Fibrous.
Central spines: Absent. (very rarely has 1 central spine )
Radial spines: 6 – 12 ( to 15) pectinated, 3-4 of them on each side, strong subulate, flattened, curved to the body and slightly downward, radiating the lowest on straight downwards, the 3-5 in the upper part of the areoles thinner needle like. All 14-20 mm long, pale-yellow/white with darker tips later turning grey.
Flowers: Bell shaped, central, up to 4 cm cream or yellow, perianth segment lanceolate,oblong, acute, filaments reddish, anthers yellow, stigma and lobes whitish-yellow.
Fruits: Olive green, juicy, oblong 30 x 9 mm.

Scientific name:  Coryphantha retusa (Pfeiff.) Britt. & Rose.
In: Cactaceae 438,1923 

Origin:  Mexico (Oaxaca, Puebla)

Habitat:  Open grassland on lava soil.

Conservation status: Listed in CITES appendix 2.

Etymology: The species epithet "retusa" comes from Latin "retusus", past participle of "retundere", to beat back : "re-" + "tundere" to beat for the retuse tubrcles tip [ rounded or obtuse with a central shallow indentation ]

Synonyms:

• Melocactus mammillariaeformis Salm Dick
  In: Allg. Gartenz. 4:192,1836
• Mammillaria cephalophora Salm-Dick
  In: Cact. Hort. Dyck 1849:137 850
• Echinocactus cephalophorus (Salm-Dick) Poselger
  In: Alg. Gartenz.21: 102,1853
• Cactus cephalophorus (Salm-Dick) Kuntze
  In: Rev. Gen.Pl.1:260, 1891
• Coryphantha malleospina Bravo
  In: An. Inst. Biol. Mex. 25:526, 1954
• Coryphantha retusa var. melleospina (Bravo) Bravo
  In: Cact. Suc. Mex.27:17, 1982
• Cactus retusus (Pfeiff.) Kuntze
• Mammillaria retusa Pfeiff. 1837

Cultivation: In culture C. retusa is without problems, but it grows slowly.  It does best in slight shade, but does well in full sun, too. It is sensitive to over-watering (rot prone), and needs a very porous soil with good drainage.
It flowers quite early, but needs about 8-12 years to reach its typical, definite aspect.  It tolerates fairly low temperatures as long as it is kept dry (hardy to -5° C or less, for short periods of time).

Propagation: Propagation is by seed.