THE RENEWED, revitalized Baltimore keeps getting newer and more vital. But the city has not accomplished this at the expense of its old neighborhoods or architecture.

Out on Fells Point, a waterfront area just minutes from the bustle of Harborplace, renovation and restoration are continuing apace. But Bertha's Dining Room--named for a salvaged stained glass window that included the name "Bertha's" for no known reason--is already an established fixture.

The unusual appeal of Fells Point, with all its rough edges, is what initially attracted the proprietors of Bertha's, Laura and Tony Norris who are both professional musicians. It was back in 1972 when they conceived the notion of a bar featuring live chamber music, and the slightly battered structure on Broadway seemed just right.

As it turned out, some peculiarity of zoning precluded the live music. But the Norrises perservered and two years later, after considering and rejecting various possibilities, including the option of bringing in professional restaurateurs, and after experimenting with recipes themseleves, they undertook the addition of a restaurant on their own.

It was with a considerable sense of adventure and Tony Norris' honey-and-lemon broiled dish called Shrimp Maurice that they opened their dining room that first night. Laura Norris, then several months pregnant, recalls being unsure if anyone would show up. But by the end of the frantic night, they had run out of everything, the new dining room of 40 seats had been overrun.

This auspicious beginning--which surprised them because their only advertising had consisted of about 75 flyers distributed in the neighborhood--was set against an uncertainty over the future of Fells Point. Its revival was stymied; its continued existence as a neighborhood depended upon the resolution of routing plans for I-83. Although there has not been a final decision, a compromise sparing Fells Point was reached in 1977 and the area's resurgence began with gusto.

Meanwhile, mussels, at least for the aficionadas, had become what Bertha's is all about . . . plump, delectable mussels.

How the Norrises became virtuosos of the mussel is a tale that began with Tony Norris' paella. At first, the bivalves, when the Norrises could get them, were a pleasant and traditional addition to the Spanish recipe. But soon the Norrises had contracted with another restaurant for part of the total mussels ordered and the quest began in earnest. Currently, Bertha's goes through a ton of mussels a week (shipments arrive Tuesdays through Saturdays about 5 a.m.; the cleaning is an awesome task). The menu features mussels from pure and simple to imaginatively sauced in six variations. Of course, they still turn up in the paella and other dishes.

However, true fans muscle in on Mussels in Melted Butter, or in White Wine and Spanish Sauce, or with Garlic Butter and Capers, with Creamed Mussel Sauce, with Summer or Winter Sauces, or with Lancaster Cream. Frequently, these fans manage to wipe out Bertha's hoard completely.

So a word to the devoted: if your heart's set on mussels, call first--especially on Mondays when the supply depends on appetites of the preceeding Saturday and Sunday, and during heat waves when the finicky shellfish sometimes become no-shows.

Loyal fanatics objected to the usual method of cleaning the sand and grit out of the bivalves by submerging them in a cornmeal mush mixture. They felt that the mussels lost their unique character and individuality, and Bertha's stopped using the technique. When someone encounters sandy mussels, the Norrises recommend swishing them in the leftover broth. Whether the mussels will be sandy depends on where they are harvested: Those off jettys in deeper water tend to be cleaner than those picked up along the shoreline.

It's a bit of a mystery why mussels are not more generally appreciated. They are inexpensive (compared with other shellfish, they constitute a bargain), versatile and delicious. Some folks, no doubt, are put off by the care and cleaning of the smallish purple-to-black shellfish. But others have never even sampled the distinctive taste of mussels because they are a rarity in supermarket seafood departments. Certainly they are misunderstood, but it's not for lack of effort on the part of the Norrises.

Tony Norris dreamed up the memorable Bertha's bumper sticker: EAT BERTHA'S MUSSELS. The response has been incredible. The stickers have been carried off not only on bumpers, but on bicycles and wheelchairs; they have been spotted in Cairo (that's Egypt, not Illinois); they have been reported in Europe and in London. They are currently on their way to Africa with some enthusiasts who happen to be in the Foreign Service.

Much of the Norrises adventure has been serendipitous. The idea of offering an English tea on Wednesdays emerged when four late-afternoon customers sought something light to eat. Laura Norris who came from England with her parents in 1962 says she remembered the batch of scones her mother, Jean McKinnon, had baked that morning and offered them as the solution. The diners were so pleased that they asked for an encore.

Expansion is currently under way. Soon there will be a second-floor tea room and a separate baking kitchen. McKinnon, in addition to making the scones and such for tea, also bakes the pies, cakes and tarts for Bertha's. Also, the zoning problems have been surmounted by a pilot project allowing live music in restaurants on Fells Point that derive at least 50 percent of their gross income from food. Now jazz is offered on Tuesdays, and authentic, traditional Irish, Scotch and English music and sea chanteys on Thursdays, Fridays and Saturdays.

Bertha's is cozy, woody and homey. There's a bulletin board by the side door with upcoming community and restaurant events, and past the tiny bar and small dining room is a hallway--with an unusual ceiling comprised of wine bottles bottom-side down--to the larger front dining room. The staff almost makes you feel like family.

One visit this summer--on a Monday with low supplies--was preceeded by a phone call. Yes, they had mussels. But when we arrived, alas, one solitary order remained. We ate it, lamenting. Then, in addition to a staff suggestion of where else we might find mussels, followed by a confirming phone call and detailed directions, the waitress brought a small sampling of other non-mussel dishes. The shrimp were done especially well. Congeniality overcame disappointment.

The Norrises have developed a unique business. There is no manager, no titles, job descriptions; in emergencies, multitalented staff members pitch in where needed. Laura Norris believes that it was their very lack of experience in the restaurant field that has led them to success. In some cases they simply did not know any better, and somehow it all worked. They just take each day as it comes, she says, and the fun comes when the results are not at all what you expected.