IT'S SATURDAY, 65 million B.C. A small band of intrepid explorers huddle in the drippy mist of the Mesozoic seas, fingers tight around shovels and plastic bags.

Paleontologist Dr. Peter M. Kranz guides the expedition, shouting instructions above the din of prehistoric winds.

The gleaming Addison Road Metro station across the street seems only a vision in a time warp. Are these folks in search of the Blue Line to Bedrock?

Hardly.

They are families on the trail of the dinosaurs that flourished for 100 million years in what is now the Washington metropolitan area. Their search is is one of a series of dinosaur and fossil hunts Kranz will lead this spring and summer for a price that won't eat up an entertainment budget -- children 12 and younger are free, and adult prices range from $10 to $15.

Several organizations are offering the Kranz-led excursions this season, including Capital Classes, the Prince George's County Department of Parks and Recreation and the City of Rockville. Kranz, a teacher and environmental consultant, will also arrange private group tours. He leads a caravan to the actual deposits, but you need your own transportation.

Although new dinosaur finds are relatively rare, no one goes home empty-handed, because the sites contain other fossils. Each trip varies, depending on the number of participants. And there's always the possibility of a significant discovery -- in 1982, Kranz and a student stumbled across a woolly mammoth skeleton in Landover.

Today, several dozen assembled kids and parents (mostly denim-clad dads) are pondering the challenge at hand. Four-year-old Michael Curran of Key School in D.C. looks as if he's up to it. At last count, Michael could identify 95 different kinds of dinosaurs. "What's this, Michael?" asks dad Bill, opening a book to a picture of a horned Green Thing.

"Styracosaurus," says the tot.

During the middle of the 19th century, the most important dinosaur-hunting area in North America was the curved belt -- called "Dinosaur Alley" -- between D.C. and Baltimore, parallel to U.S. 1. "The bones are still here; they haven't gone away," Kranz tells his audience. "The problem is, nobody is looking for them."

Most fossil sites are nondescript spots, and this one, a vacant lot near Addison Road and Central Avenue, is typical. Tidbits of wisdom are passed on to the eager hunters. First, they are warned of the misconception that they must dig for fossils. Kranz explains that nature exhumes fossils through rainfall and erosion. "You can bring your shovels and dig and have fun, but you're not going to find anything that way."

Tiny kids scatter with shovels to dig and have fun. More sophisticated adults and older children yearn to do the same, but wander around instead, toeing at the silvery dirt. Then, a shout from four-year-old Kenny Milcetich of Aspen Hill: "Daddy, I found something!" He is clutching an oyster shell with a series of ridges. Similar shells materialize all over the place.

"These are 'Exogyra' shells," says Kranz, "fossils from marine beds of the Upper Cretaceous Age."

Oh, yeah? And just how do we know the neighbors didn't have an oyster roast here last year?

Undaunted, the leader holds up a shell. "A good question. But no modern oyster makes a shell that looks like this."

A bulldozer operator once assumed that there had been a seafood restaurant on the site. "I told him that it must have been a long time ago, because these shells are 70 million years old," Kranz reports.

Next, the crowd is directed to walk a couple of blocks east to a hilly construction area. Here, the clay is mixed with a black dirt characteristic of what is called the Severn Formation, a local group of fossil beds. The kids are taught to drop the large black lumps to break them open. Inside are perfect spirals of snail shells 65 to 75 million years old, some so fragile that they crumble to the touch.

Dinosaurs were land-living creatures, but land environments are notoriously bad for fossils. Some scientists think that iron ore tends to preserve the bones, although no one knows why. Many dinosaur remains came to light during the 1800s, when sedimentary iron ores were mined with picks and shovels. Today's miners drive their own cars to the next deposit -- the site of an old brick foundry at the north gate of the National Arboretum, where iron ore is much in evidence.

Dinosaur bones are porous and contain holes, so the hunters look for those characteristics, and for anything that is red or black, since red iron-stained rocks have historically been a key to dinosaur finds.

Shiny blackened fossil wood is unearthed, as is pyritized wood (much heavier) from a tree that grew 110 million years ago.

Treasure bags are getting heavy, and the expedition is winding up. By now, parents have put aside any notions that they are merely accompanying their little ones. "I think it's nice having the excuse of a child to have things explained to, so the adults can listen, too," confides Andy Gallant of Rockville, as he tucks five-year-old son Michael into the car. No one seemed to notice the day's steady drizzle.

One, more recent sunny morning, another group of adventurers embark on a search for the Ruling Reptile in Landover, just off the Beltway. In a drainage ditch, eight-year-old Anna Smith of Silver Spring bags a passel of sharks' teeth to show her friends. Before the crew can find a coveted crocodile tooth, Kranz sets off down the middle of a cold stream. Seven gleeful kids and some reluctant adults don't miss a beat in pursuit.

Later, on a bank of soft grass, the brave explorers pause to reflect on the unaccustomed thrill of scientific discovery.

"That's why this work is important, as far as I'm concerned," says Kranz. "Everyday-people should feel that science belongs to them." DINOSAUR DOINGS

The following family events, all led by Kranz, are sure to cure a case of fossil fever. In addition, you can also arrange private tours by contacting Dr. Peter M. Kranz, 614 G Street NE, Washington, D.C. 20003. Phone 543-1165 or 547-3326 (messages).

CHILDREN'S DINOSAUR HUNTS -- Sunday and July 12, 10 a.m. to 5 p.m. Sponsored by the City of Rockville. Class meets at Dinosaur Hall of the Museum of Natural History on the Mall; bring lunch, beverage, old clothes, digging tools (optional). Kids under 12 free. Ages 12 and up, $10 Rockville resident, $12.50 nonresident. Call ahead for reservations, 424-8000, ext.340.

FAMILY FOSSIL HUNTS -- Sponsored by Capital Classes (brochures are distributed in all area public libraries). Bring lunch, old clothes, boots or sneakers, and brickmason's hammer (no clawhammers). Kids under 12 free; no experience necessary. Call Herb Frederickson for reservations or information, 840-2056. The outings:

Saturday 17, June 29 & July 26: $15; dinosaurs and other fossils from Chesapeake Bay beaches, including whales, dolphins, sharks, clams, snails, oysters, vertebrates and other modern (100 million years or younger) fossils; about 75 miles roundtrip.

May 31 through June 1: $25 plus motel; overnight trip to Pennsylvania coalbeds near Harrisburg and Easton (about 450 miles roundtrip). Search for trilobites, fossil ferns, clubmosses and other plants.

This Saturday, plus June 21 & July 20: $15; older Paleozoic fossils near Front Royal (about 250 miles roundtrip). Search for extinct or ancient forms of brachiopods, trilobites, dinosaur footprints, fossil fish.

June 14 & July 19: $15 plus boat rental; canoe trip down a nearby Maryland river to inaccessible shorelines for untouched marine fossils. You must be able to swim. Pre-trip meetings June 9 & July 14 at Kranz' home.

June 28 & July 13: $15; Dinosaur hunting in D.C. Call ahead for all trips.

FAMILY DINOSAUR HUNTS & WORKSHOPS -- Sponsored by Prince George's County Department of Parks and Recreation; dates to be decided. Call Chick Katz for information and cost, 249-9220.

FREE DINOSAUR LECTURE -- By Kranz, August 23, 2 p.m. Beltsville Library, 4319 Sellman Road, Beltsville, Maryland. Call 937-0294 for reservations.