Mammillaria sheldonii fa inaiae

Mesa Garden seeds, 1999 - Bahia San Carlos

That's interesting, Ento - some straight spines, some curved but not fully hooked. Quite a variable species it seems!

The first plant have only straight spines, while the second have straight and curved spines. The two plants are from the same sowing

flowers

I do like all the sheldonii / swinglei / iniae / gueldermanniana / guirocobensis plants, their flowers are so good, and vary in colour from dark to light.

MG 900 from mesa catalogue, is a forma of mammillaria sheldonii.
pilbeam in his mammillaria - the cactus file handbook cita...
referred to mammillaria sheldonii.....
referred or forma?

I think it is a matter of personal choice. Many botanists don't use forma, whereas often horticulturalists and we amateurs will, usually because it adds something to our knowledge of the plant. "forma" is often used to refer to the place where this form of the plant is found, and sometimes it is used to refer to the previous name by which a plant has been known.
So we see names like M. bombycina fma. Colomas, and also M. decipiens fma. Lourdes. But we can also see M. heyderi fma. parrasensis, of of course M. sheldonii fma. swinglei.
Personally, I try to keep my naming somewhat consistent, and will use the fma. mainly to recognise a location, where the plant in question does not have a field number.
For various reasons, I tend to follow Hunt (and so Pilbeam) in species defintions, but do want to recognise earlier names, because for me they add information to the name. As a result, I will use M. bombycina fmal. Colomos, but I would write M. sheldonii 'swinglei'.
In some cases, I also use the field number, so I have plants with names like M. magnimamma Repp. 1139 'saxicola'.

The one thing you can be sure of is that if Pilbeam uses "referred" then he means that it is synonomous with the species name to which it is referred. MG900 is only a catalogue number, but it adds ot the information about the plant, and if it is a distinct form of a species, I can see no reason why you shouldn't use fma. MG900 after M. sheldonii.

I had a quick look at the ICBN, and it seems perfectly "legal" to use forma in any of the ways I have suggested.

my sheldonii fa inaiae...

Nice photos of a nice plant, Maurillo.
M. sheldonii "iniae" is quite variable. In April, I visited Bahia San Carlos, and saw both straight spined plants and hooked spined plants, and some which had strongly curved spines, but not quite a hook. Some forms of M. sheldonii hardly seem to have a central spine at all.
This variability exists in other species in the Ancistracanthae - for example M. fraileana, M. grahamii and M. albicans - and they make for a nice group of plants. You can understand why different names were given to some of them - guirocobensis; gueldermanniana; iniae and so on.

from carlo zanovello collection, a mammillaria sheldonii "inaiae" with hooked central spines and another one with straight central spines.
from baja san carlos.