Well, that door remains open.
I have been updating a few Mammillaria entries in the database and have tagged a number of plants with cryptocarpic fruit. This is by no means a complete list (and please suggest additions or make changes as you see fit)...
https://garden.org/plants/sear...
In case that term is new for you, it refers to fruit which matures within the stem, rather than being extruded from it, usually in a club-shaped form, like other Mammillarias. This reproductive feature is characteristic of the Longiflorae, a handful of Mammillarias which tend to be small, with tuberous roots and big flowers. There's a very informative blurb on the subject here (see bottom right):
http://www.cactus-art.biz/sche...
There are some interesting consequences of cryptocarpic fruit related to seed collection and germination.
Some of the members of this group have very attractive spines (like hernandezii). The database has some good flower pictures for a bunch of these plants, which are generally reputed to be difficult (rot-prone and demanding about strong light) in cultivation.
