
These Strombocactus disciformis are rather small cacti grow in an interesting way, on vertical walls between crumbling and decomposing stone.
Between their size and color they are very tough to spot. Which is probably a good thing as like many other rare cacti there is a lot of pressure from extraction which can easily decimate a population. And when plants are limited to very specific places where they can grow, it makes them very vulnerable.

Here is a more broad view of the plants and one of their companions, Agave xylonacantha.