A SEA CHANGE has overtaken the National Museum of Natural History. Where dusty seashells long had languished in semi-darkness, a feast of fossils rich and strange celebrates the dawn and development of life in our primordial seas.

Visitors don't enter the hall so much as plunge into it. The roar of breaking waves drowns out the chatter of children in the rotunda, and all eyes are transfixed by a huge and incredibly detailed reef, 11 feet high and covered with more than 100,000 models of ancient organisms. Each is cast from a mold made from an actual fossil, and each was placed by hand by Terry Chase, who makes a career of creating such replicas in his Ozark Mountains studio.

From the reef one drifts with the tide -- simulated by lights shining through surging water -- past the armored trilobites that flourished for 130 million years. A fiber-optics video lens scans passersby and shows them as they might have been seen by these many-eyed first lords of the sea, which vanished in the first Great Dying, 230 million years ago, when 70 to 90 percent of earth's living creatures disappeared during climatic disturbances.

Ammonites big as millstones come and go in their season, along with the ancestors of whales and the descendants of creatures that crawled out onto the land only to return to the sea. Here arise the crocodiles and the sharks, the whole great pageant detailed in a 120-foot mural by Ely Kish, who's spent the last 16 years creating painterly but scientifically precise prehistoric panoramas for a dozen museums.

Four years and $4 million in the making, the new permanent hall offers reassurance that the Smithsonian still knows how to do it right. The 4,500-square-foot area is wide enough to accommodate the inevitable crowds, and subtly shaped so that those who want to linger and study the more than 2,000 fossils can do so without being trampled.

The fossils are beautiful -- or fearsome -- in themselves, and the enjoyment is doubled by the delectable text, written by Sue Voss, who doesn't seem to know that scientific exposition is supposed to be dull. She writes irresistibly about "the shell game," explaining the functions of features whose beauty is utterly incidental, and "screwballs" that wandered off into evolutionary dead ends, not to mention barnacles that "stand on their heads and kick food into their mouths." Every museum text-writer in town should study her style.

Even those who rush right through the hall can't escape a vivid impression of the way the life has ebbed and flowed in our seas for 670 million years. This exhibit is just exactly what Mr. Smithson had in mind when he founded this "institution for the increase and diffusion of knowledge;" it's the sort of thing that turns children toward science.

LIFE IN THE ANCIENT SEAS -- Permanent exhibit at the National Museum of Natural History. Open 10 to 5:30 daily. Metro: Federal Triangle.