THE TRILOBITES are coming! Lined up single file in the Learning Center at the Museum of Natural History, these paleozooic fossils are a collector's delight -- especially for one Thomas Johnson of Morrow, Ohio. This display is the cream of Johnson's 800-trilobite collection.

As far as trilobites go -- which is as far as you can throw them, because they're extinct and just found in rocks -- this is "one of the best personal collections in the world," says Frederick Collier, collections manager of the Paleobiology Department at the Museum.

At age 34, Johnson has retired from the rat race and joined the trilobite trials. He travels the country foraging in quarries for his prey and selling them to museums, schools and other collectors.

He found his first fossil at the age of four. But he probably didn't know what it was, or how it got the name trilobite, for the three lobes that run the length of its back. This marine animal possesses a mess of legs as well, which some of the trilobites used for swimming and others for burrowing and others for crawling slowly on the bottom of a shallow sea.

Arthropods all, they look a little like your garden variety of sowbug, and they were capable of rolling themselves up like their distant cousin to protect a soft underside. Like lobsters, another relation, they molted their chitinous covering; some trilobite fossils are actually molts rather than complete animals.

While they haven't been around for the past 250 million years, in their heyday trilobites ranged in length from a quarter-inch to 21/2 feet. There were times during their 340 million years on this planet that they were quite abundant; witness some of the shale on display here that is embedded with small swarms of them.

And they can still be found in the Appalachians, for example, despite the fact that collectors covet them.

"These things," notes Collier, "are second to dinosaurs in popularity."

TRILOBITES: THE THOMAS T. JOHNSON COLLECTION -- Opening Saturday in the Learning Center at the Natural History Museum, through 1986.