That would be some type of Ariocarpus, I'm thinking maybe Ariocarpus fissuratus.
What a great plant to see it in habitat! They have become harder to find due to over collecting. I believe that they are protected now.
Alicia :~)

It is an Ariocarpus fissuratus. A cactus believe it or not despite the lack of spines, and one of the more endangered species. Difficult to spot anywhere in the wild, you can imagine how well hidden it would be without the flowers. It is only found the Texas in the Big Bend area and along the Rio Grande, then more widly in northern Mexico.

false peyote