Section 16. A petition for the assessment of damages under section fourteen may be filed within three years after the right to such damages has vested; but any person, including every mortgagee of record, whose property has been taken or injured, and who has not received notice under section eight or otherwise of the proceedings whereby he is entitled to damages at least sixty days before the expiration of such three years, may file such petition within six months after the taking possession of his property or the receipt by him of actual notice of the taking, whichever first occurs, or, if his property has not been taken, within six months after he first suffers actual injury in his property.