I think this plant may be misidentified. It looks like an Oreocereus species rather than Cephalocereus, because the number of ribs is much lower than the 20-30 one would expect for the latter, there are long spines poking through the wool, and the growth habit is much less upright than I'm used to seeing for the Mexican "old man" cactus. Do you have any background info that might help sort this out, Lin?
Unfortunately I don't know what exactly to call it. I'm going to call upon a couple cactiphiles to see if they have opinions here -- paging
@skopjecollection and
@mcvansoest -- to help figure this out. If they don't have any better ideas then maybe we could file the image under the genus entry for Oreocereus.
Please let me know what you think. A group photo here for comparison of a few "old man" cacti:
