Ariocarpus trigonus, a typical plant in the October flowering period. 

Plant yellowish-green, rising slightly above ground level, globose, rounded on top, 5-25cm high, 4-30cm in diam.; tubercles divergent, erect, not crowded or basally compressed, acute at the apices, flattened and smooth adaxially, not fissured, incurved strongly, 3-8cm long, 1-2.5cm broad, usually twice as long as wide; spine-bearing portions of the areoles absent at the tips of the tubercles; flowers 3-5cm in diam., 2.5-4cm long; outer perianth parts pale yellow or cream-colored occasionally with reddish mid-ribs, 1.5-3cm long, 3-4mm broad, mucronate  but occasionally rounded  apically;  inner perianth  parts  yellow or cream  colored, 1.4-3cm  long, 4- 8mm broad; pistil occasionally exserted above the stamens; style 1-2cm long; stigmas 6-10, 3-5.5mm long; fruit whitish or greenish, 7-20mm long, 5-10mm in diam. 

Distribution (Map)

On the eastern slopes of the Sierra Madre Oriental from north of Monterrey, between Montemorelos and Linares, down the Jaumauve Valley to south of Jaumauve in the states of Neuvo Leon and Tamaulipas. It occurs at altitudes of 500-1200m on limestone scree. A confused situation exists in the vicinity of Aramberri  were A. trigonus appears to intergrade with A. retusus (See the Aramberri Enigma). Anderson (Cact. & Succ.J. (1999), 71, 4, 180-190) has reported that the habitat north of Monterrey has been destroyed by a combination of urban sprawl and agriculture and that large areas of the Jaumauve Valley habitats have been made into agricultural fields, some growing Aloe vera!.

Ariocarpus trigonus horacekii, a seedling plant of this smaller form

Ariocarpus trigonus 'elongatus', a form with longer thinner tubercles.

Comments

Ariocarpus trigonus is one of the largest species of the genus and is often found growing in colonies of considerable size. The tubercle size and form show considerable variation, from long and thin to short and broad, but the yellow flower is remarkably consistent given a habitat length of over 400km.The variation in tubercle sizes has resulted in the erection of several varietal names, these are of no botanical value, representing no more than minor phenotypic differences. Ariocarpus trigonus is an easy plant to grow and flower in a sandy-clay compost with plenty of coarse grit and limestone added.