The Supreme Court yesterday agreed to decide whether a fish called the snail darter is an endangered species that Congress intended to preserve at the cost of blocking completion of a $127.5 million dam.

In a 3-to-0 ruling last Jan. 31, the Sixth U.S. Circuit Court of Appeals held that the Tennessee Valley Authority cannot complete the Tellico Dam and Reservoir Project on the Little Tennessee River because completion would jeopardize the only known habitat of the darter, a drab little dark-striped, bottom-dwelling member of the perch family that feeds on fresh-water snails.

And, the appeals court said, jeopardizing the snail darter would violate the Endangered Species Act of 1973, which requires the government to assure that federal programs neither imperil such species nor modify their habitats.

"Although this legal controversy may well enjoy a modicum of notoriety because it appears to pit the survival of an obscure fish against completion of a . . . reservoir, the principles of law controlling (the case) are neither complex nor revolutionary," the judges said.

They held that the grounds given by TVA for exempting Tellico from compliance were inadequate and added, "Conscientious enforcement of the act requires that it be taken to its logical extreme."

In a separate opinion, one of the judges, Wade H. McCree, now Solicitor General of the United States, wrote that the project can't be exemped just because it was started in 1967 - years before Congress passed the endangered species law - or because the House Approporiations Committee was aware that the snail darter would be threatened by completion of Tellico when Congress approved a $29 million additional appropriation for it in 1975.

TVA intended Tellico to be a multi-purpose, water-resource and regional development project primarily for the benefit of three counties.The dam would create a 33-mile-long, 16,000-acre reservoir, including 2,100 acres of Little Tennessee riverbed.

Almost from the start, environmentalists and others fought the project, partly with the claim that it would do more harm, overall, than good.

The opponents included the National Resources Defense Council and the Eastern Band of Cherokee Indians, which said that the land to be submerged included the town of Echota, the Cherokees' Jerusalem.

In 1973, when construction was half-completed, a University of Tennessee ichtyologist discovered the snail darter in a stretch of the Little Tennessee marked for impoundment.

Congress enacted the endangered species law four months later. In November, 1975, when the project was about 75 per cent finished, the Secretary of the Interior granted a petition about 90 perch species found in Tennessee - among the endangered species.

A month later, President Ford signed the $29 million Tellico appropriations bill. The environmentalists then sued for an injunction to halt construction. A federal judge ruled against them, saying that applying a 1973 law to a 1967 project would produce "an unreasonable result."

Reversing him, the appeal court said that the degree of completion on the dam "cannot be translated into a workable standard for judicial review. Whether a dam is 50 per cent or 90 per cent completed is irrelevant in calculating the social and scientific costs attributable to the disappearance of a unique form of life."

TVA argued that it was the express will of Congress that Tellico "be completed as promptly as possible in the public interest."

But the Sixth Circuit, noting that the TVA claim was based merely on "pronouncements of two congressional appropriations subcommittees," said that to let such advisory opinions shape its interpretation of the law would be to allow a "patent usurpation" of the province reserved by the Constitution to the courts.

The Supreme Court took other actions:

STATE EMPLOYEES

In December, 1974, The Oregon State Penitentiary dismissed tenured corrections officer Jerry Hammer, a disabled Vietnam veteran with a work-related back injury and more absences from the job than any of his peers. But the Oregon Supreme Court ruled a year ago that Hammer was owed back pay because he hadn't been notified of the dismissal and hadn't had a chance to reply to charges

Yesterday, the Supreme Court reversed 5 to 4, citing a decision handed down earlier this year. The dissenters termed the precedent nearly irrelevant - "wide of the mark" - and the reversal "cavalier."

Airline Baggage Checks

In December, 1975, a bomb in checked baggage exploded at Laguardia Airport in New York City, killing 11 and injuring more than 50. The Federal Aviation Administration then issued requlations under which airlines could X-ray checked bags.

Because Jeffrey S. Freeland of Dayton. Ohio, fitted a FAA profile, Delta Airlines insisted on X-raying his Florida-bound bag at Greater Cincinnati Airport in April, 1976, and found in it a Browining automatic pistol. Yesterday, the court let stand his conviction for knowingly delivering a firearm in a common carrier for interstate shipments.