Section 72. If a deed or other instrument is filed in order to transfer registered land in trust, or upon any equitable condition or limitation expressed therein, or to create or declare a trust or other equitable interest in such land without transfer, the particulars of the trust, condition, limitation or other equitable interest shall not be entered on the certificate; but a memorandum thereof shall be entered by the words ''in trust'', or ''upon condition'', or other apt words, and by a reference by number to the instrument authorizing or creating the same. The assistant recorder shall note upon the original instrument creating or declaring the trust or other equitable interest a reference by number to the certificate of title to which it relates, and to the volume and page in the registration book in which it is registered. If the instrument creating or declaring a trust or other equitable interest is already recorded in the registry of deeds or of probate, a certified copy may be filed by the assistant recorder and registered.