Whenever the formation of a library district is desired, a petition which may consist of any number of instruments, may be presented at a regular meeting of the board of supervisors of the county in which is located the largest proportionate value of the lands within the proposed district as shown by the last equalized county assessment roll. The petition shall specify whether the proposed library district shall be governed by a three-member board of library trustees or by a five-member board of library trustees. The board of supervisors to whom the petition is presented is designated in this chapter as the supervising board of supervisors.