Section 18. Administration of civil service in jointly established agency

1.  Except where otherwise expressly provided by statute, when a  public
  agency  is  established  and  maintained  jointly  by  two or more civil
  divisions,  including  school  districts,  in  the  same   county,   the
  provisions  of this chapter shall be administered for such agency by the
  municipal commission of such county, unless the appointing authority  of
  such  agency  shall elect, within sixty days following the establishment
  thereof and with the approval of the civil service commission of a city,
  if there be one, participating in the establishment and  maintenance  of
  such  agency,  to  come  under  the  jurisdiction  of  the civil service
  commission of such city.