On a corner lot, with a triangular area, no wall or fence or other structure shall be erected of a height in excess of three (3) feet, and no hedge, shrub or other growth shall be maintained at a height in excess of three (3) feet, except that trees whose branches are trimmed away to a height of at least ten (10) feet above the curb level shall be permitted. Such triangular area shall be determined by the intersecting center lines of the streets and a diagonal connecting points on each street center line seventy-five (75) feet from the intersection of such street center lines.