PlantFiles: Living Rock Cactus, Seven Stars, Chaute, Chautle, Peyote Cimarron, Tsuwin
Ariocarpus retusus

Other valid synonyms are:
Mammillaria prismatica, Anhalonium pulvilligerum, Anhalonium elongatum, Ariocarpus pulvilligeris

Here are other forms/varieties or tenative subspecies still under scrutiny:
Ariocarpus retusus subsp. scapharostroides
Ariocarpus retusus subsp. horacekii [syn. A. elongatus subsp. horacekii]
Ariocarpus retusus subsp. jarmilae
Ariocarpus retusus subsp. panarottoi

This species and all subspecies are on the endangered/protected species list.
The 'retusus' subspecies has a gray-green or blue-green epidermis and the tubercles are not curved sharply and not sharply pointed in relation to the other subspecies. It's flowers are white or pinkish-white. This is the most common of the subspecies.
The 'trigonus' subspecies usually has a yellow-green epidermis and the tubercles are more pointed and incurved. It does not have areole pads at the tubercle tips like 'retusus'. The flowers are yellow or cream colored.
The 'furfuraceus' variety has a darker green-brown-gray epidermis and more obese tubercles.

This plant requires a minimum temperature of 50F/10C.

This is probably the most commonly kept species in this genus, and old specimens can be incredible to look at. It is a very slow growing and odd looking cactus (looks more like a succulent- no spines) and old specimens can be very expensive. It is not a plant that likes a lot of water, though if watered during the growing season, it will take it OK and maybe even make flowers. Watering it too much will easily rot it, though. I have had one of these for 8 years, and I'm not sure it's much larger than it was when I got it. ONe of those plants passed down from generation to generation.