Yup - those are the horizontal Mighty Mounts. I have a set of four of those, and a set of four vertical mounts to be listed on eBay.
These mounts (or Yakima's big, ugly, and prone-to-theft universal mounts) are needed to attach a ski/bike rack, fairing, or other accessory to the cross-bars.
I'm all up IN those! Lemme know if you'll sell the lot "Pre-Ebay"...