Commerce Bancshares Corp., the holding company for Commerce Bank & Trust Co., announced Monday that it has completed its acquisition of the Boston-based Mercantile Capital Corp. and its subsidiary, Mercantile Bank & Trust Co.

The $26.5 million all-cash transaction was announced earlier this year, the parent of Worcester-based Commerce Bank said.

Mercantile Bank’s three banking offices, located in the Brighton, Fenway, and South End neighborhoods of Boston, opened Monday as Commerce Bank offices, the company said.

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The combined organization has assets of $1.6 billion and 16 retail banking offices serving people and businesses across Central and Eastern Massachusetts.

The deal comes at a time when the ranks of tiny banks have been thinning across the country