He doesn't run his mouth like a Maserati, or rent entire hotels for his followers. But like Muhammad Ali, Ronnie Evans is a physical man past his physical prime. It's wits and finesse these days, not skill or speed. Like Ali, one of Ronnie Evans' favorite speeches begins, "You should seed me when I was young."

And very much the way Ali did last month at 36, Ronnie Evans at 35 is gearing up for the greatest challenge of his career this weekend. Can a mild-mannered restaurateur win the national oyster-shucking championship for the fourth time? Down here at the mouth of the Potomac River, the smart money is shouting yes.

The test comes Sunday, at the St. Mary's County Fairgrounds in nearby Leonardtown. "I can see it now," said Evans the other day, as he nervously jabbed a pack of Winstons with a pencil. "The judge will say up with your arms. The arsters will all be laid out there. Then the gun will go off, and . . .

"Well, I'm not supposed to win the damn thing. But I just got a feeling."

Well he might. For this is the Ronnie Evans who astounded oysterdom in 1971 by shucking a dozen of the little devils in 55 seconds to win his first crown. The man who finished third in the world championship this summer at Galway, Ireland ("Got beat by a Swede and an Irishman. The Irishman was right smart fast"). The only man ever to win the shucking title who doesn't work as a shucker full time. The only man to win three times ('71, '72 and '77).

And a man who is banking on experience this time around.

The scoring system, you see, rewards beauty as well as briskness. Many people here and around nearby Chesapeake Bay can open oyster shells quickly with an oyster knife. But the judges deduct points if the oysters themselves look dirty, bloody or full of holes. Beautiful, whole oysters are the kind Ronnie Evans produces.

"It was my dad (Robert) and mom (Agnes) who taught me," Evans was saying, as he sat at the bar of his Clipper Restaurant here. "It was all because we were in the restaurant business. Guy comes in, asks for a dozen to go, sure, you do 'em fast. But you do 'em right, too."

Ronnie Evans has spent most of his life around oysters. The oldest of 10 children, he dropped out of school in the ninth grade at his father's urging. "The restaurant was hurting. Dad told me to get my tail down there and help him," he recalls.

THe help consisted largely of going oystering. Most dawns, young Ronnie would be out in the river or bay with a 16-foot-long pair of tongs, raking the beds for oysters. "Some days I made $100, some days $10. Most days it was $10," he said. But every day, his skill with a knife sharpened.

It was aboard ship that Evans learned Rule One of shucking: Never pick up an oyster the way yoy would a potato.

"You kind of slap your hand on top like this," he said, pounding a pa w on top of his Winstons. "Then you slip the knife in and free up the oyster," he said, poking at the pack with the pencil like a lockpicker. "Only then are you really shucking."

And only then are you sure you aren't about to slash your hand. "I've seen guys with knives all the way through their hands," Evans said. "Me? Well, I've nicked myself a few times. That's one thing about being an old shucker. I didn't used to worry about cutting my hands. Now, it hurts."

So would losing this year.

"The thing I pride myself on," he said, "is I can beat most of them for a couple of dozen or so. Them boys who do it for a living, they could beat me over a couple of hours or so." But the national championship is not decided over a couple of hours. It is decided over two dozen of the Chesapeake Bay's finest - and no more.

Still, Golden Thompson and Cornelius Mackerel promise to be formidable competitors. Thompson and Mackeral, who are coworkers at an oyster-packing plant in Charles County, Md., have each won the national title once. They will be gunning for Evans especially hard this year since the 1978 title carries a $500 prize and a free trip to the 1979 world shuck-off in Ireland.

If that isn't enough, Willie Moran, the right smart fast 1978 world champion, will fly from Ireland to Leonardtown Sunday, courtesy of the local Rotary Club, which sponsors the nationals. And he isn't coming just to kiss Miss Oyster.

To add to Evans' woes, one of this year's 35 contestants will be Evans' 12-year-old-son. Evans says his son is still in the up-and-coming category, but in a contest that usually lasts less than two minutes, "you never know."

So the battle lines are drawn, and as Oyster Day nears, Ronnie Evans' Winstons are being smoked fast and furious.

"My reflexes are slow. I know they are," Evans moans. "Plus I don't never get no practice. I run a restaurant now. I haven't opened an oyster since Ireland last month.

"I know a lot of the boys are gonna bet on me. And if I lose, I'll have to invite them down to the restaurant for a beer. A free beer."

But if the worries of an old oysterman sound familiar, so does the shucking of an old shucker.

A man named Ali was a pretty fair pre-fight psychologist in his day. Ronnie Evans is cut from the same mold, But he may not be as good. For as Evans finishes ruing his weaknesses and ruminating about his upcoming test, his eyes start glinting and he says:

"I can't, wait. When you get up there, it's just you and them arsters."

What he means is that Title Number Four should be just around the bend.