Rory, my Great Dane has been eating out of the litterbox!?

Rory, my beloved Great Dane has been eating chunks out of the ltterbox, but the odd thing is...only out of Rory (the cat's box)...I saw the big mutt chewing and crunching, but couldn't figure out from where....So..I watched him closely. He sneaks over covertly, and grabs big pieces of hardened poop and eats them. It is so gross! How do I stop this annoying and troubling behavior? I think he is jealous of Rory, the cat.