Each node has exactly 10 spines. The center and 3 lower-pointing ones are black. They are proportionally longer than the spines of most cacti, and perfectly straight.
Looking at these that are mentioned I can see definite similarities, I just wish I were good at identifying cacti. I have a couple books but they are rather general; I also have several bookmarked sites that are great but with so many genera and (to my eye) lots of similarities between them - not to say anything of varieties - it is so hard to pinpoint. This one is rather small yet but I am hopeful it will bloom next year and that would help.
