Mammillaria beneckei is only of Mammillaria tat has the 'true yellow (c)' flowers, and one of only two cacti like that I now of - the other been Matucana aurieflora. Flowers are about 3.5 inches across and large for Mammillaria. But the most unusual feature of this plant is it's seeds. They are by fare the largest in the genus. On that base solely the plant was at some point segregated to a mono-specific genus Oehmea.
I had trouble growing this plant when still used the soil-based cactus substrate, but after switching to pure mineral base this plant become more like a greenhouse weed. It pops a lot, and hooked offsets catch my cloths, to be then dispensed to some nearby pot like a cuckoo egg, often with adventurous roots ready.
This plant is not at all common in culture, something that puzzles me. It s spread over large territory of Western Mexico, not rare there. It also fast growing and very easily offsetting. And the plant was first presented to collectors some 150 years ago.