After the date of establishment of the retirement system, each member shall be credited annually with a retirement annuity of an amount equal to one and one-half percent of the salary or wage earned by him or her during the then current fiscal year not in excess of five thousand dollars, except that (1) for each member who retires on or after August 31, 1969, such credit shall be an amount equal to one and sixty-five hundredths percent of such salary or wage not in excess of five thousand dollars and (2) for each member who chose the new system in 1951 and who retires on or after August 31, 1976, for service from September 1, 1951, to August 31, 1955, such credit shall be two and four-tenths percent of such salary or wage not in excess of five thousand dollars, for service from September 1, 1955, to August 31, 1963, one and forty-four hundredths percent of such salary or wage not in excess of six thousand dollars, for service from September 1, 1963, to August 31, 1969, one and forty-four hundredths percent of such salary or wage up to the social security wage base, plus two and four-tenths percent of salary or wage in excess thereof, and for service after September 1, 1969, one and forty-four hundredths percent of the first seventy-eight hundred dollars of such salary or wage and two and four-tenths percent of the excess of such salary or wage over seventy-eight hundred dollars. With respect to service rendered prior to the date of establishment of the retirement system, each employee in service or on leave of absence on such date shall be entitled to an annuity on account of prior service. Such annuity shall be such percentage of the maximum annuity to which such member might have been entitled under the terms of a retirement plan previously in effect as the number of years of service under such plan bears to the total number of years for which credit for service might have been granted under such plan, except that no credit shall be given in excess of the maximum annuity provided under such preexisting plan. The number of years of prior service for which credit shall be given under this section shall be the number of years of service with which the employee is credited under such preexisting plan on May 21, 1951. The sum of these two annuities shall constitute the retirement allowance to which the member shall be entitled to be paid beginning on his or her retirement date. Such annuity shall be paid in twelve equal monthly installments unless the amount thereof is less than ten dollars per month in which event payments shall be made quarterly or semiannually.