ARLINGTON, Texas -- The Cowboys remain ahead of the St. Louis Rams, 20-6, after a scoreless third quarter.

Some thoughts:

* Good hard hit by safety Mana Silva, who smashed wide receiver Brian Quick near the Rams' sideline. Silva's hit to the chest was clean. When you talk about who makes one of the final roster spots on the 53, check out Silva. He made an open field-tackle on a third-down screen pass. He also has an interception this preseason.

* Mario Butler sacked Kellen Clemens on a cornerback blitz for a 7-yard loss.

*Quarterback Stephen McGee is trying to make sure the Cowboys don't start the season with just two quarterbacks. His first pass went for 26 yards to James Hanna on a rollout. His second pass would have been a completion had Danny Coale not fallen down. When no receiver was open, McGee scrambled downfield for a first down with an 11-yard run.

* Backup offensive linemen were responsible for some penalties. Ronald Leary was flagged for a hold and Jermey Parnell was called for a a false start. Leary most likely will make the Cowboys, possibly forcing the team to release Derrick Dockery, who is also getting second-team snaps. But Parnell can't afford these mistakes. The Cowboys don't know who the swing tackle is and Parnell has a chance to win that spot. Pat McQuistan has played all three spots along the offensive line this summer, including center.

* Jason Garrett's nephew, Harry Flaherty, checked in with 4:18 to play in the third quarter. On his second snap, he lined up in the slot and was wide open, but McGee completed a 12-yard pass to Andre Holmes.

* Interesting to see defensive linemen Marcus Spears and Sean Lissemore with some late third-quarter snaps again. The Cowboys could go deep at this position, maybe having as many as eight linemen on the active roster. Spears lost a starting spot last year to Jason Hatcher and might be fighting for a gig. But it seems he's played pretty well this summer.

* Orie Lemon did a good job of reading a run play to force a tackle for loss and also tackle a running back in the open field. He's another one of these players who needs to make an impression in the final two preseason games to make the 53.