Section 85B. In any city or town having a police force serving in its park department and which has accepted this section, the appropriate retirement board, established under section twenty, or, if there is no such board in the case of a town, the selectmen, shall retire from active service and place upon the pension roll any permanent member of said police force found by it or them to be permanently incapacitated, mentally or physically, for useful service therein, by injuries received through no fault of his own in the actual performance of his duty. Said board or officer may also retire and place upon the pension roll any permanent member of said police force who has performed faithful service therein for not less than twenty years continuously, and is not less than sixty years old, if, in its or his judgment, such member is permanently incapacitated for further service therein. If a permanent member of such a police force was, prior to his appointment thereto, employed in said city or town as a regular member of its police department, the period of such last-mentioned employment shall be counted as a part of his continuous service as a permanent member of said police force. Every person so retired shall annually receive from the city or town as a pension a sum equal to one half the amount of the annual compensation received by him at his retirement. Said board or officer may, in case of emergency, call upon any person so pensioned for such temporary service as a member of such police force as he may be fitted to perform, and during such service he shall be paid the difference between the rate of full pay for such employment and the rate of pension received by him.