A dirt bike rider was seriously injured Sunday in a crash at the Means Dry Lake bed in Johnson Valley. About 4:30 p.m., a man identified only as being in his 30s, was riding his dirt bike in the sand dunes with several other people when he lost control and was thrown from the bike. Rescue personnel said the man was wearing a helmet, but received serious injuries and was flown by Mercy Air to Arrowhead Regional Hospital. The California Highway Patrol is investigating.