Section 7. All in-lots, valued by said commissioners at more than one hundred dollars per lot, shall be rated number one; all valued at less than one hundred and more than seventy dollars per lot, shall be rated number two; all valued at more than forty and less than seventy dollars per lot, shall be rated number three; and all valued at forty or less than forty dollars per lot, shall be rated number four; and in making said valuation the buildings erected thereon shall in like manner be excluded.