If you're going to beat up your cabbie over a fare, you might want to make sure not to leave any ID cards behind

Arlington Police report arresting a pair of alleged knuckleheads they say pummeled a cab driver when he insisted they pay their fare after a ride from Alewife station late Saturday.

Christopher LaFrance, 25, of Arlington, and Jacob Shaps, 24, of Boston, were arrested on charges of assault and battery, unarmed robbery and intimidation of a witness after police tracked down LaFrance's residence based on the ID cards he left behind in the cab's back seat, police say.

According to police, the cab driver picked the duo up at Alewife around 11:30 p.m. and drove them to Arlington, telling them the ride would be about $20.

When they got to Lowell Street and Park Avenue Extension, about 3 1/2 miles from the station, police say, one of the two paid with a credit card - but only for $2.

When the driver told the two men that they still owed him money, they began to argue. When the driver threatened to call the police, one of the men allegedly started punching the driver. The driver then reached for his cell phone, but the suspect allegedly grabbed it and threw it out the window of the cab onto the pavement.

The driver declined medical attention, police say.

Innocent, etc.