EARTH CITY, Mo. — Entering the 2014 season, the St. Louis Rams wanted to be a team that runs the ball well and plays good defense.

The defense finally came around boldly in Sunday’s 13-10 win over the San Francisco 49ers.

But the running game is still trying to find its identity. Halfway through the season, the Rams have rushed for 807 yards, an average of 100.9 per game, which ranks 29th in the NFL.

In their eight games, three different running backs have started games, and the leading rusher with a mere 257 yards is Zac Stacy, who didn’t even get on the field in the game against the 49ers.

Rookie Tre Mason was inactive for the first four games of the season, and since getting playing time, has rushed for 222 yards, second on the team, and he made his first start against San Francisco.

Stacy started the first five games and Benny Cunningham the next two. There have been four rushing touchdowns — two by Cunningham and one each for Stacy and Mason. Cunningham has 173 yards rushing. In addition to not playing against the 49ers, Stacy was on the field for just one snap two weeks earlier against Seattle.

Mason had 19 carries for 65 yards Sunday, including a 21-yard run. The Rams have had three runs of 20 or more yards this season, and Mason has all three: 28, 24 and 21.

Cunningham has emerged as the pass-catching back with 20 receptions for 167 yards and a touchdown.

Still, despite Mason’s success, coach Jeff Fisher refused to state publicly that Mason is the starter, insisting that the philosophy remains running back by committee.

Asked if Mason is the starter, Fisher said, “No, we’re going to continue with the same thing. I can’t tell you who’s starting this week. Zac has handled things professionally. Unfortunately, he didn’t get to play. He will get to play. He will get his reps. We’ve not lost faith or confidence in him, but it’s hard to spread the ball around.

“We felt going into this game that Tre, some of the things he was doing and some of the ways that we were trying to attack the defense, gave us the best chance.”

Fisher said of Mason’s play against the 49ers, “He missed a couple holes, but he ran real hard. He overcame some things. His ball security’s good, pass protection is good. He needed get out in the routes. I thought he played pretty well.”

Asked if the decision on who starts is based on practice performance, Fisher said, “It’s not based on practice. It’s based on scheme and approach and those kinds of things. We’ve got good backs, just can’t play them all.”

Finally, asked if the starter will be selected on a week-to-week basis, Fisher said, “Yeah. You guys come out to Arizona and watch the first play to figure out who’s going to start.”