Cal. Pub. Util. Code § 27281

If the city to which any territory included in any district has been annexed already provides transit service in the territory annexed, or if any territory in any district is annexed to a city, not a part of any district as originally established pursuant to this part, and not annexed thereto pursuant to this part, the city council or other governing body of the city may petition the district for the exclusion of such territory from the district.