William J. Shea, a vice chairman of BankBoston Corp. who helped engineer its merger with BayBanks Inc., resigned abruptly on Thursday.

Mr. Shea, 49, a five-year veteran who was also chief financial officer and treasurer, said he would remain at the bank for an unspecified period to ensure a smooth transition. But his CFO duties were immediately handed off to Susannah M. Swihart, 41, an executive vice president.

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