Massachusetts General Laws Part I. Administration of the Government (Ch. 1-182) Ch. 59, § 34

A mortgagor or mortgagee of real estate may bring in to the assessors of the town where it lies, within the time prescribed by the notice under section twenty-nine, a sworn statement of the amount secured thereon or on each separate parcel thereof, with the name and residence of every holder of an interest therein as mortgagor or mortgagee. If such property is situated in two or more places, or if a recorded mortgage includes two or more estates or parts of an estate as security for one sum, such statement shall include an estimate of the interest of the mortgagee in each estate or part thereof. The assessors shall, from such statement or otherwise, ascertain the proportionate interests of the mortgagor or mortgagee respectively in said estates, and shall assess the same accordingly. If, in any year, such statement is not brought in, the tax for that year on such real estate shall not be invalid merely for the reason that the interest of the mortgagee therein has not been assessed to him.