Cal. Gov't Code § 31721

(a) A member may be retired for disability upon the application of the member, the head of the office or department in which he is or was last employed, the board or its agents, or any other person on his behalf, except that an employer may not separate because of disability a member otherwise eligible to retire for disability but shall apply for disability retirement of any eligible member believed to be disabled, unless the member waives the right to retire for disability and elects to withdraw contributions or to permit contributions to remain in the fund with rights to service retirement as provided in Article 9 (commencing with Section 31700).
(b) When a member appeals from a separation for disability, disputing the employer’s assertion or assumption that he is not eligible for disability retirement, the official, entity other than the board, or court to whom appealed shall transfer the proceedings to the board for determination of the eligibility and of disability if so eligible.
The appointing authority shall have the burden of proving disability. Thereafter, the appellant shall have the burden of proving job causation.
This subdivision shall not be operative in any county until such time as the board of supervisors shall, by resolution adopted by a majority vote, make the provisions applicable in that county.