Hi,
Im having a hard time identifying this mammillaria.
I think its a johnstonii, but after browsing more mammillaria's, I am not very sure.
I found this cacti in Tucumcari NM.
Maybe someone with a sharper eye can manage this one for me.
Thanks,
Brian
Here is the link: http://www.flickr.com/photos/92801931@N02/8705041555/