Conn. Gen. Stat. § 6-2: Boundaries on Long Island Sound.

The counties of New London, Middlesex, New Haven and Fairfield extend southerly to the southerly boundary line of the state as that boundary line is settled and defined by an agreement with the state of New York bearing date December 8, 1879, and published with the special acts of the General Assembly passed in the year 1880, and consented to and approved by the Congress of the United States by an Act approved February 26, 1881. The dividing line between any two of said counties which border on each other is and shall continue to be a line drawn due south from their original southeast and southwest corners to the said southerly boundary line of the state.