Spectrum　pooling,　which　allows　the　secondary　networks　to　utilize　the　available　spectrum　bands　from　different　licensed　networks,　is　one　of　the　most　promising　technologies　for　dynamic　spectrum　sharing　in　cognitive　radio　systems.　Most　previous　work　on　spectrum　pooling　concentrates　on　the　system　architecture　and　the　design　of　flexible　access　algorithms　and　schemes.　In　this　paper,　a　distributed　scheme　for　optimal　internetwork　spectrum　sharing　among　multiple　cognitive　radio　systems　is　proposed.　Besides,　the　spectrum　access　price　and　spectrum　efficiency　are　considered　as　the　design　criteria　in　the　proposed　scheme.　The　spectrum　sharing　problem　is　formulated　as　a　restless　bandits　system,　which　dramatically　reduces　the　computational　complexity　by　simply　allocating　the　new　available　band　to　the　secondary　network　with　the　lowest　index.　Furthermore,　extensive　simulation　results　illustrate　the　significant　performance　improvement　of　the　proposed　scheme　improves　compared　to　the　existing　scheme.