Peg Carpenter was presiding over a Smithsonian exhibit of particularly spectacular seashells when a young boy approached her.

"Lady," he asked, "do you paint these yourself?"

"Oh no," replied the Natural History Museum docent. "Little animals make these shells to cover their bodies, and Mother Nature creates the colors and designs."

The boy paused, looked back at the vibrant array of shells, crossed his arms and stared Carpenter straight in the eye.

"Sure lady," he said. "Sure."

After 23 years as a "serious shell nut," Carpenter still identifies with that sense of wonder at the "almost impossible magnificence" of shells.

"It's fantastic that these small, unobtrusive animals can create objects of such breathtaking beauty," says the conchologist ("a fancy word for what shell collectors do"). "A perfect shell is a work of art."

Indiana-born Carpenter, 62, first went into "shell shock" in 1949 when she visited her first beach. "We were on a small Florida island, and there wasn't another living soul around. A hurricane had washed up piles of shells, and my husband and I just went wild."

The Carpenters started reading about shells and trading with "shell pals" around the world. When her husband Walter landed a Treasury Department job in 1952, Peg Carpenter became friendly with the Smithsonian's curator of mollusks and helped found the National Capital Shell Club. Five years ago she started volunteering at the museum's Naturalist Center, a "behind-the-scenes" natural history library open to the public. The Carpenters recently built a special room in their Virginia home to house their extensive shell collection. The Compleat Conchologist

"There are two kinds of shell collectors," says Carpenter, "non-serious collectors who just pick up the discarded shell, and serious collectors who go for the live animals. . .which you do by wading or snorkeling or hiring a boat.

"From a purely scientific standpoint, you want to get the animal live in its natural habitat so you can note where you found it, how deep the water was, what was around it. From an aestheic standpoint, you get specimens that are prettier and more likely to be complete."

The Smithsonian, counters Naturalist Center manager Richard Efthim, "does not support the practice of collecting live animals (since) it could have a detrimental effect on the ecology. Novice collectors who aren't aware of what animals are classified as endangered species could do real harm. Most people are better off collecting the empty, dead shells."

Carpenter and other conchologists, however, contend that live-shell collecting can be ecologically sound if certain basic rules are respected.

"If you're looking for shells in coral reefs," she says, "you must be extremely careful not to damage the living reef. Anytime you turn over a rock, put it back the way you found it, or you could be destroying an animal's habitat.

"And don't collect everything in sight. Take what you need for yourself and to trade. Don't take a broken one--it's no good for your collection, but it can still reproduce. And never, never take a female laying eggs. If you see the egg mass outside the shell, leave it alone."

Serious shelling, she admits, requires "a touch of nuttiness. You wind up assuming some outrageous poses and getting yourself into some ridiculous predicaments--like wading in the ocean in winter--in search of the perfect shell."

To turn your beachcombing into a bonanza, here's some other advice from Carpenter: Brains and the Beach

Think like a mollusk. "If you lived in a shell, would you pick a beach with a violently pounding surf?"

Look for "more or less level places without a quick drop off," and for "protective geography" such as sandbars or offshore islands that break the force of the surf.

Remember timing is crucial. Collecting usually is best at low tide when otherwise inaccessible sand and rock are exposed. "Some species are night feeders, so bring along your flashlight."

Prime shell time for Carpenter "is when high tide has just gone out, so you can take full advantage of the low tide . . . After a storm is a dandy time to go shelling, since lots of things get washed up. Look for little pockets or tidal pools where interesting debris collects."

Experienced shell collectors pour over tide tables with the fervor of E.F. Hutton agents examining stock reports, checking for the especially low tides ("minus tides") that occur a few times each month or season. Shell Meccas

International conchologists' dreams: Australia, Hawaii, Philippines, South Africa, Japan and Sanibel Island, Fla. Pointers

Ideal stash bags are "the net bags onions or potatoes come in because they don't float."

To see the bottom better, bring a "water glass:" a wooden bucket or box with a glass bottom.

Since some mollusks bury in the sand and others look like rocks, keep a sharp eye out for trails, mounds, air holes and tiny siphons.

Save your back by turning over suspicious rocks with a stick. Dangers

Wear sneakers to protect feet from broken glass and rocks, and gloves if you're going to pick up "odd things."

Never reach into a hole--where urchins, eels and other "ickies" live.

Since water reflects burning rays, wear water-proof sunscreen or loose cotton clothing such as doctor's "scrubs" or pajamas.

To avoid arrest or fine, check to see if the area you're in prohibits or requires a license before collecting live animals. "Be sure you're not trespassing on a private beach." Preserving and Displaying

"You can clean most shells by soaking them in a solution of equal parts Clorox and water."

Carefully pry off barnacles with a knife and scrub away limey deposits with an old toothbrush.

To preserve the shine, paint on a solution of equal parts mineral oil and mineral spirits.

"Polishing shells just isn't cricket."

Classic conchologists don't mount shells, but keep them loose in drawers away from light. "Who wants to dust all those shells?" Investment Shill

Although some rare shells sell for thousands of dollars, "If you want to make money collecting, forget about shells. I got caught a few times spending lots of money on a rare shell, then they found new areas where they're available and the same shell sells for under $20.

"You can't second guess Mother Nature."