Medical examination of a member in city-service for ordinary disability shall be made upon the application of the commissioner, or upon the application of such member or of a person acting in his or her behalf, stating that such member is physically or mentally incapacitated for the performance of duty and ought to be retired. If such medical examination shows that such member is physically or mentally incapacitated for the performance of duty and ought to be retired, the medical board shall so report and the board shall retire such member for ordinary disability not less than thirty nor more than ninety days after the execution and filing of application therefor with the pension fund.