From the face, it looks like a raccoon. From the side, it looks more like a dog. From the chromosomes, however, it looks like nothing that biologists have ever seen before in a mammal.

Raccoon dogs, as the little-known animals are called, violate one of the rules of genetics that had been presumed to apply to vertebrates, or animals with bones. The rule is that each species carries its genes on a specific number of chromosomes and that every cell in an individual contains this same number. Normal humans, for example, have 46 chromosomes that come in 23 pairs, each member of a pair coming from one parent.

Raccoon dogs vary in chromosome number from 38 to 56. The chromosome number even varies from cell to cell within individual animals.

"This is definitely contrary to the usual expectation," said Doris Wurster-Hill, a Dartmouth College biologist who recently returned from field studies in Japan, where the odd animals live wild and are called tanukis. They are also native to eastern Siberia.

The chromosomes that account for the differences are of an unusual type, called B chromosomes, that are unpaired and previously known only in certain insects and other invertebrates. It is not known whether the B chromosomes carry functional genes or are excess genetic baggage.

In the 1920s some Siberian raccoon dogs were taken to Finland and farmed as sources of fur. Tanuki coats sell in New York for as much as $5,000. Escaped tanukis have established themselves in Europe and have spread as far as Switzerland.

Wurster-Hill's field studies, carried out with Oscar Ward of the University of Arizona, suggest that the animals are chiefly nocturnal and that, like raccoons, they prowl riverbanks and lakesides for nuts, berries and fish and also raid garbage cans. From this and certain aspects of their appearance, they would seem to be members of the raccoon family. Other aspects of appearance, however, have placed them officially in the dog family. The new studies, aimed at a surer classificiation, turned up the finding that in winter they hibernate like members of the bear family.