Except for its more squat shape, it reminds me of my little mystery Mamm. seen here:
http://cactiguide.com/forum/viewtopic.p ... ty#p264932" onclick="window.open(this.href);return false; . Probably not the same, though if that's a very small plant...
Mine didn't have anything in the axils when it was younger, but later grew bristles and finally a little bit of wool there. This could be a similar case.
I do think it's a Mammillaria, but too young to say which species. I'd still like to know how big it is, too.