A federal court jury decided today that the Wampanoag Indians of Mashpee do not meet the legal definition of a tribe, thus apparently crippling their federal suit claiming title to about 13,000 acres of valuable Cape Cod land.

The jury in the arduous 2 1/2-month-old case determined today that the Wampanoags were not in American Indian tribe in 1970, when Congress passed the Non-Intercourse Act to protect tribal lands, and that while they might have been a tribe after that date, they have not been a tribe for over a century and thus they cannot claim property as ancestrally theirs.

The Wampanoags had first to prove they were a tribe rather than a scattered group of Indians before pursuing a second trial in the same case to determine ownership of the land in the small island town.

U.S. District Court Judge Walter J. Skinner will hear arguments from both sides on Jan. 20 before reaching a final decision on whether to dismiss the case and the jury. But today's jury findings appeared to cripple the case. An appeal is considered likely.

While attorneys for the defendants - the town, developers and property owners - praised the decision as a "clear victory," lawyers for the Indians were calling the jury's verdict "inconclusive" because the jury did not provide uniform answers to all the judge's questions about the tribal status on six key dates in the history of the Wampanoags.

The jury agreed after 2 1/2 days of deliberations that the Wampanoags were only a tribe in 1834, when the state established an Indian district in the area now known as Mashpee, and in 1842, when the state granted the Indians permission to sell and transfer their land.

The legal definition of a tribe, established in a 1901 U.S. Supreme Court decision, is "A body of individuals of the same or similar race united in a community under one leadership or government and inhabiting a single though sometimes ill-defined territory."

"We've been shafted," said Russell Peters, president of the Wampanoag Tribal Council, Inc., which was established in 1974, two years before the suit was filed.

"But what else can you expect from an all-white jury that has no knowledge or capacity to understand the feelings for Indian customs, heritage or identity," Peters said outside the courtroom after the decision was announced. "We're dealing with 200 years of injustice perpetrated against the American Indians, and this decision is just a continuation cloaked in the halls of justice."

The 42-day trial - the first Indian suit to come before a jury since the emergence of Indian ethnic awareness in recent years spurred a rash of Indian claims on millions of acres of land along the East Coast - included more than 5,000 pages of testimony from 52 witnesses, among them, anthropologists, historians, experts on Indian affairs, government officials and several town residents.

"It's a conclusive verdict," said Allan van Gestel, an attorney for the defense who is also involved in land claim suits in New York and Long Island. "The jury found there is no tribe at the present time, and if there is no tribe then there's no one to bring a suit."

Attorneys for both sides agree the final outcome of the Mashpee case will have a major psychological impact on Indian land claim suits currently under litigation in many of the original 13 colonies.

However, the attorneys maintain that this case will have little legal impact on other suits because it hinged on the question of tribal identity rather than addressing other legal questions.

"The significance of this case lies in its psychlogical guide to white attitudes about the Indian presence in this country," said Lawrence Shubow, a counsel for the Indians.

"This decision is a guide to how a random sample of people, the injury, feels about the survival of the American Indian," he said. "It says we want them to become extinct except on festive occasions, television or in the movies."

A final decision favoring the town against the Indians, the attorneys agree, could help stiffen resistance to other potential and pending Indian land claim suits by similarly calling for a decision on tribal status before the question of land ownership can be considered.

"In the very least it's going to make these cases take a whole lot longer to resolve," said Indians' attorney Barry Margolin, an employee of the Native American Rights Fund, the major financial and legal force behind many of the Indian land claim suits on the East Coast.

The Mashpee Indians and many other tribes seeking reclamation of ancestral tribal lands have based their cases on the Non-Intercourse Act - which the Indians describe as a paternalistic gesture by Congress to protect Indian property from more sophisticated, exploitive whites and a protective measure against inciting the Indians to war - which gives Congress the sole power to allow the sale or transfer of tribal lands.

"This has been a great victory for our attorneys, but it is not a social victory for the town," said Mashpee Selectmen chairman Kevin O'Connell. "This decision does not eradicate the problems in our community. Somewhere along the line Congress will have to step in to save those poor souls in Mashpee who have economically been flushed down the drain."

The residents' property deeds have been placed in legal limbo, with ownership uncertain until the case is finally resolved. The suit filed by the Wampanoags on Aug. 26, 1976, created economic shock waves in the town, halting ail construction and real estate transactions and suspending the granting of new mortgage loans by banks.

The town is seeking financial aid from the state, because town officials say unemployment and business bankruptcies brought on by the suit have resulted in a dramatic decrease in local property tax payments and exorbitant legal fees have eaten into the town's budget.