"SHARK!" is a little show with a big mouth.

The Natural History Museum's version of "Jaws" opens Saturday in Fossil Hall, where the jaws of a prehistoric great white shark hang over visitors' heads and cast an ominous shadow on the wall.

Smithsonian scientists have used fossil teeth to reconstruct the prehistoric giant's jawline. An orthodontist's nightmare, the five-foot tall mouth is big enough for a small Jonah to stand in. Estimated length of this particular shark, that lurked about the North Carolina coast 41/2 million years ago, is 40 feet, twice the size of the largest recorded modern great white. The estimate is based on the dental records of more recently dead sharks of known length.

The Carcharodon megalodon, like the modern-day Carcharodon carcharias, was mean and hugry. On display here, whale and porpoise bones from the same period show cut marks made by the teeth of an attacking great white. "We have drawers and drawers of whalebones with gashes in them," says Robert Emry, Smithsonian curator of fossil mammals. Often, a piece of shark's tooth is embedded in them.

When you say sharks' teeth, you've said a mouthful. A great white could have as many as seven or eight rows of 48 teeth each. "They have this conveyor of teeth that keeps rolling out," says Emry. The teeth are in different stages of growth and when a tooth is lost, another replaces it.

The Smithsonian was able to assemble a full set of teeth, one to six inches long (plus a few rows of plastic copies) thanks to amateur paleontologist Peter Harmatuk. Four years ago, he donated to the museum more than 500 fossil shark teeth, collected mainly at a North Carolina phosphate strip mine.

"The last records we have of sharks this size are four million years ago," says Emry. "But it may not have become extinct," he suggests. "It may be the same kind of shark living today, but smaller."

Why did they get smaller?

"I don't know," says Emry. "Why did they ever get that big?"

SHARK! -- A new permanent exhibit at the Natural History Museum.