Upon retirement for ordinary disability a member shall receive a service retirement allowance if he has attained age sixty, otherwise he shall receive an ordinary disability retirement allowance which shall consist of:
(1) An annuity which shall be the actuarial equivalent of his accumulated contributions at the time of retirement; and
(2) A pension which together with his annuity shall make a total retirement allowance equal to ninety percent of one-seventieth of his average final compensation multiplied by the number of years of creditable service, if such retirement allowance exceeds one-fourth of his average final compensation; otherwise a pension which together with his annuity shall provide a total retirement allowance equal to one-fourth of his average final compensation; provided, however, that no such allowance shall exceed ninety percent of one-seventieth of his average final compensation multiplied by the number of years which would be creditable to him were his service to continue until the attainment of age sixty.