Here is Escobaria deserti. It has flowers that are brownish like in the picture or dirty-green, 15 to 20mm broad. Plants are smaller than chlorantha, but spines on larger plants look similar. Juvenal plants look completely different though, Escobaria deserti forming nice white balls with tight cover of shots radial spines.
This species is very rare seen in collections. It has fairly modest look, even when in bloom, and pretty tricky to grow. Escobaria usually means 'easy', but here it is not the case. The plant not much in fashion with collectors, and many never even heard of it, so there is not much incentive to struggle with it. When you will proudly show your success to colleges the best you can count on is a polite not-understanding.