The pilot departed on a 1 hour 50 minute cross country flight with about 30 gallons of fuel in each wing tank (about 27 gallons usable fuel per side). The flight was uneventful until the pilot started a descent from 8,000 feet to 3,000 feet; the engine then made "two pops" and "quit." The pilot said the engine didn’t run rough; "it just stopped." He made several attempts to re-start the engine but was unsuccessful. The pilot declared an emergency and landed in a field. Upon landing, the nose gear dug into the dirt and separated from the airplane. The right wing rear spar fractured during the landing; however, both wing fuel tanks were undamaged. About 1 quart of fuel was drained from the left wing tank, and the right wing tank contained about 30 gallons of fuel. The pilot could not recall where the fuel selector was positioned at the time of the power loss. Examination of the engine and fuel system revealed no mechanical anomalies that would have precluded normal operation. It is likely that the pilot was feeding fuel to the engine from the left wing tank and did not switch to the right wing tank before the engine lost power or during the attempted restart.