A new species of lion tamarin, a squirrel-sized monkey with a lion-like mane, has been discovered on a small, forested island off the coast of Brazil, according to two young Brazilian biologists.

The find, announced yesterday by the National Museum in Rio de Janeiro and a private conservation group here, was described by primate researchers as a significant and surprising discovery.

"The thing that is amazing is to find a new primate species in an area that is so developed. It shows how little we still know about the tropics and how much we stand to lose before we even know what's there," said Russell Mittermeier, president of Conservation International, in a telephone interview from Costa Rica.

Mittermeier, a primate specialist, called the find "one of the most amazing primatological discoveries in this century" and compared it to finding a new animal species in the suburbs of Los Angeles, because the island on which the monkey was found is close to Sao Paulo, one of the world's largest cities.

The new species was found by Maria Lucia Lorini and Vanessa Guerra Persson of the Capao da Imbuia Natural History Museum in the Brazilian state of Parana. They described the new species based on a well-preserved skin they obtained from a local fisherman and from a month of field observations of the monkeys. The researchers said no photographs exist.

They said the monkey, about the size of a big squirrel, has golden-colored fur over much of its body except that the tail, forearms and face are black.

Even though the new species was formally reported and described in the scientific literature, disputes still can arise over whether the animal is a new and distinct species, a subspecies or a hybrid. Such disputes often simmer for years.

"If it's true, if it's real, I find it extraordinarily exciting from a scientific point of view and extraordinarily challenging from the standpoint of conservation biology," said Ben Beck, associate director of biological programs at the National Zoo.

The Brazilian scientists estimate that population of the monkeys could be as few as several dozen, according to Mittermeier, whose group plans to fund a survey of the island to assess the population.

The discovery of a new species, named Leontopithecus caissara or the black-faced lion tamarin, brings to four the members of the lion tamarin genus. The best-known member is the golden lion tamarin, which is being bred in zoos. The other monkeys in the genus are the golden-headed lion tamarin and the black lion tamarin.