Suddenly, Sheaths : Fashion: So much for suits. New York designers dote on the dress, especially the body-hugger with a matching coat, in spring collections.

So far during this week of spring fashion shows, there is just one item worth talking about: the dress.

New York designers have all rediscovered it at once--possibly because it is the one garment most likely to sell well. For the past several years, designers have put their more creative energies into suits. Now, suddenly, dresses are a cause. Most often they fit close to the body and are worn with a matching coat or jacket for an ensemble look.

Carolina Herrera built her entire collection around dresses, among them a melon-colored pique sheath with gold dots woven into the fabric for day, and a sequin-striped dinner dress with white satin collar for a young and sporty evening look. Her white silk-crepe pantsuits with salmon- or aqua-sequined bustiers were her best alternative to dresses.

Herrera accessorized the collection with her first line of costume jewelry, filled with baroque pearl necklaces and jeweled flower earrings.

Noticeably absent from the front row of Herrera’s show was Ivana Trump, president of the Plaza hotel, where many of the shows are held. Trump’s father died last weekend. Her usual entrances at the shows stir things up. Photographers mob her, journalists crane their necks to see her latest hairdo and clothes. Her absence had a quieting effect.

From Bill Blass, one of the best dresses for spring was a layered style with a pale pink floral print over black lace.

Another strong contender was his double dress, a sheer black lace short sheath wrapped by a long black silk toga.

His champagne-colored lace evening pants, as narrow as leggings and worn with a gold evening jacket, were a definite antidote to a closetful of dresses.

For day, Blass showed tight white shifts with big pastel polka dots that looked like rich informal beachwear.

Lately Blass has been a strong proponent of plaid suits. For spring, he showed bright, watercolor floral-print suits instead. Through most of the show, models wore the same sand-colored suede high-heel pumps by Manolo Blahnik. Blass even put them with sequined dinner suits, and they looked right.

Ronaldus Shamask has been on his own for several seasons, since he and his longtime partner, Murray Moss, split up. Shamask has always been considered the arty, creative member of the team. And his spring collection reconfirms this.

He has proved he is inventive. Now it’s time to get real.

His asymmetric dresses in handkerchief-weight fabrics, some with silicone dots and silver chains for shoulder straps, were just too flimsy and flyaway. He did better with a series of narrowly cut pants in iridescent silk taffeta, worn with apron-like tops that fit close to the body and extend over the hips.

Perry Ellis designer Marc Jacobs used gingham checks, garden prints, mattress ticking and other fabrics popular with 20th-Century American interior decorators, such as Sister Parish, Elsie de Wolfe, and Billy Baldwin. In fact, he named segments of his collection after each of them.

Gingham-checked dresses were topped by very short, wide coats in mattress ticking. A floor-length floral-print dress was worn open from the waist to show off pastel leggings. A silver sun dress was topped by a full-cut, see-through, rhinestone-studded raincoat for a sporty-glamour effect.

Jacobs is staying within the All-American tradition that Ellis established. His own fascination with American ‘60s rich fashion, epitomized by the Jackie Kennedy look, works some of the time. When it doesn’t, it’s because Jacobs has gone too close to the matronly extreme.

Sheer and see-through looks were part of many spring collections. Jacobs and others put sheer blouses over bare breasts as often as over a camisole. But buyers were unsure the look would sell.

“I’m seeing a lot of pretty clothes that are not relevant,” said Jessica Mitchell, a fashion director for Saks Fifth Avenue. “How many women are really going to wear a sheer top?”

Rebecca Moses showed one of the safest collections of the week. There were touches of fantasy, such as her pale blue-sequined Capri pants. But she toned them down with a classic white linen shirt.

Her strapless dress in subtle green plaid glimmered with even more subtle beading.

For variations on her many cotton and silk shifts, she showed a shirttail tunic over shorts that barely peak out beneath.

Like so many others for spring, this was a collection of ensembles. Dresses were seldom shown alone. They had a coat or jacket to go with them.

Charlotte Neuville, who did not hold a show this season, invited the press and buyers to her showroom to review her new collection. She said she would contribute some of the money saved toward the care of AIDS patients.

Neuville’s white vinyl trench coat, with wide belt and patch pockets, doubled as a dress. A wool crepe chemise had large pearl buttons for day. An evening version in white silk had pastel rhinestone buttons.

One of the best ideas in the collection was a suit with zip-front jacket in a mix of pink, yellow and green houndstooth checks.

Here and elsewhere, skirts have been very short. And so far this week, the sheath has been the No. 1 dress shape.