With a plastic encased snail darter in his hand, Attorney General Griffin B. Bell urged the Supreme Court yesterday to stop worrying about the tiny, three-inch fish and permit a $120 million dam project to operate in eastern Tennessee.

Bell, making his debut in oral argument before the nation's highest tribunal, noted that the dam itself was virtually completed and that the celebrated, near-extinct snail darter had been given new life in a different habitat

On the other side, attorney zygmunt J.B. Plater of Detroit, representing conservationists, declared that permitting the dam to go into operation could "wipe the snail darter off the face of the earth." He defended the usefulness of the fish as "an indicator of water quality."

[Plater said the conflict is not between the fish and dam, but between the Interior Department and the Tennessee Valley Authority (TVA), the only federal agency that has "persistently declined to abide by the law," that commands consultation with the Interior. Such consulations have resolved hundreds of conflicts, he said.]

[Plater also said that the bulk of the $120 million went not for the dam, but for land that will be more valuable if used for agriculture and tourism than if flooded by closing the dam.]

At issue is a ruling last year by a federal appeals court that stopped work on the Tellico Dam project to prevent violation of a federal law protecting "endangered species."

The TVA began constructing the dam shortly after it was authorized by Congress in 1966. In 1973, the snail darter, a previously unknown member of the perch family, was discovered inhabiting part of the Little Tennessee River that would be inundated by the dam reservoir.

Later, Congress enacted the Endangered Species Act, forbidding federal projects that jeopardized those species. In 1975, the snail darter was added to the endangered list.

The appellate court ruling, bringing the dam project to a halt, caused a split among federal agencies. The Interior Department supported the ruling and TVA challenged it.

In yesterday's arguments, Bell said the TVA should be allowed to operate the dam, warning that under the appellate court's decision, any federal project could be shut down upon the discovery of an endangered species.

Hundreds of snail darters, he told the court, were being transplanted to another river in the area in an attempt to insure their survival.

Justice Lewis F. Powell Jr. asked Plater what purpose the snall darter served. "It was utilitarian purpose," replied Plater, pointing out that it seemed to thrive only in unpolluted waters. "It's an indicator of water quality."

Powell seemed dismayed by the failure of the federal agencies to resolve their differences in the case.

"This is a not-so-friendly question," Powell said to Bell. "It's not easy for us to resolve issues of vast importance to the country when two federal agencies can't even agree. Why wasn't this resolved in the Cabinet?

Bell replied that it should have been - but wasn't.