§ 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.