4 plants to identification

I have some cacti to identify. Maybe someone knows?

1.


2.


3.


4.

1. Mam. monancistracantha ?
2. Mam. elongata
3. Mam. mammillaris
4. Mam. backbergiana var. ernestii ?

Kind regards Jonathan

I think so too, Jonathan. M. monancistracantha is synonomous, apparently, with M. nana. Just for info. Both can be used.

Thanks a lot for your help! And what about number 3? I asked also on the Cactiguide web and i saw the answer that my plant is m. simplex. In the section "Mammillaria A to Z" says that m. mammillaris and m. simplex are synonyms and the m. nivosa is different plant. While llifle says that m. nivosa and m. simplex are synonims but m. mammillaris is different plant. You guys know the explanation? Is there a mistake there and maybe all 3 names are synonyms of the same plant?

No, M. mammillarias and M. nivosa are very different parts, though they both come from the same region (Caribbean, Venezuela ). M simplex is a synonym of M. mammillaria, though at one stage M. nivosa was described as M. simplex v. flavescens. Since M. simplex was never a validly published name it is best forgotten. I suspect that llifle has picked up this last occurrence.