A teenager was seriously injured while riding his dirt bike in Egg Harbor township Friday night, township police said.The teenage boy, whose name and age were not given by police, was operating his dirt bike on the grass portion next to the roadway in the 100 block of Cherry Drive when he struck a manhole cover and began to lose control, police said. He was thrown from his dirt bike and forcefully hit the grass.The teen sustained serious injuries and was airlifted to Cooper University Hospital in Camden by SouthSTAR Air Medical Services, police said.