Another security based on the price of bitcoin, the digital currency that has soared in value and volatility this year, began trading on the Chicago Mercantile Exchange on Sunday. The CME Group, which owns the exchange, opened up bitcoin futures for trading at 6 p.m. EST on Sunday. The futures contract that expires in January opened higher at $20,650, then declined steadily. The futures were trading at $18,775 at 9:00 p.m. EST, down $725. The CME futures, like the ones that CME competitor the Cboe started trading last week, do not involve actual bitcoin. The CME's futures will track an index of bitcoin prices pulled from several private exchanges. The Cboe's futures track the price of bitcoin prices on the particular private exchange known as Gemini. Each contract sold on…