Hi

I got these from Home Depot, so all bets are off on the names. THe first is labelled Mamm columbiana - taken when I got it in march 2007. I later repotted the two plantes separately.

The second - the clumper is the same plant as above, it sister is similarly clumping. The small one is labelled yucatanensis.

I looke around a lot, Pilbeam web etc, and have a very hard time distinguishing. Yucatanensis seems to have hardier spines, sometimes redish. Hunt says terete v conic tubercles

Do you think either is labelled correctly?

If hard to see, they both have 6 csp - I see yucatanensis is supposed to have 4 or 5, but I have tried to not put that much emphasis on spine counting anymore, at least in the case of small discrpancies.

Yes, columbiana ssp columbiana is supposed to have 4-6 central spines and columbiana ssp yucatanensis only 4. One of the main differences, according to the description in Reppenhagen is that the tubercles of yucatanensis are a lot slimmer than in ssp columbiana, 4-5mm long vs 5-7mm, so that the plant looks like it has more densely packed tubercles, and consequently spinier than ssp columbiana which looks a bit more open and greener.

Now looking at your plants, I am not sure that there is a huge amount of difference, but that could be growing conditions, light etc.

Above is young plant of M.columbiana ssp yucatanesis ML 373 (chiapensis) for comparison.
In Cactaceae Systematics Initiatives, No 24, page 30, Hunt states that this subspecies is possibly Extinct in the Wild in Yucatan so you may want to look after those plants.