Mammillaria ekmannii is a name that was used for a plant which was found on a small Caribbean island, Navassa island, off the coast of Haiti. It is suggeste din Pilbeam that it is probably referrable to M. mammillaris, based upon a herbarium specimen at Kew.
I have never come across this species in cultivation, but recently I have been sent a photo which is on a website of the US Coastguard. Here is the photo:
This is the website: http://coastal.er.usgs.gov/navassa/biology/flora4.html
The original name was given in 1931, but clearly this photo is from later than that! Until it is recollected, we won't know exactly what it is. To me is looks like being somewhere between M. nivosa and M. mammillaris.
Anyone going sailing in the Caribbean? I don't think it is a recognised stop on a cruise liner........