Section 220. Apportionment on formation of new towns

Whenever any board of
  supervisors shall form a new town within  its  respective  county,  from
  parts  of  other  towns,  or  town which shall have bonded to aid in the
  construction of any railroad under any act  authorizing  the  same,  and
  such  bonds, or any part thereof, shall remain unpaid, or when any board
  of supervisors shall change the line of any town which shall have bonded
  to aid in the construction of any  railroad  in  this  state,  and  such
  bonds,  or any part thereof, shall remain unpaid, the new town so formed
  and the part taken from a town and added to another  town  shall  pay  a
  proportionate  share  of  such bonds as shall remain unpaid, which share
  shall be ascertained from the assessed valuation of such town  or  towns
  as  contained  in  the  last equalized valuation of the assessment-roll,
  made prior to the formation of such town or the change of any such  town
  line.