Mammilaria prolifera, they're prized also for the show off of their fruits, since there's little else to show off apart from a nice impressive tuft.
let me be a bit sentimental here. this was sent to me by the awesome dude woof woof... from a place far- far away of this world... so for me, it will be always be woofwoof's mammi prolifera, which was seed grown...
peace
PS: mammilaria identification thread here
http://www.shaman-australis.com/forum/index.php?showtopic=41160&hl=mammilaria
seems to have some of my own mammilarias
PS2: some times , in mammilaria of cultivation especially, specimen can rot from the excess of fruits... so it is regularely suggested for mammilaria growers to remove the fruits as much as they can - not for this species though...
ps3: fruit seems more roundish, or eggplant shaped than usual