The Bank of New York Company and Fleet Financial Group Inc. have held discussions within the last month about a possible merger, an investment banker familiar with the talks says. The banker said the talks were "very preliminary." He declined to say whether they were continuing. A merger of the two banks would create the nation's seventh-largest bank holding company, with $93.5 billion in assets and branches from Maine to New Jersey. Officials of the Bank of New York, based in New York, and Fleet, based in Providence, R.I., declined to comment. "It is a matter of bank policy not to comment on market speculation," said Rhonda Barnat, a spokeswoman at the Bank of New York. The Bank of New York's stock rose $1.75 yesterday on the New York Stock Exchange to $53.75. and Fleet was up 12.5 cents, to $39.75. Merger Interest Expressed