Section 12. Stockholders entitled to vote may vote in person or by proxy. No proxy dated more than six months before the date of the meeting named therein shall be valid, and no proxy shall be valid after the final adjournment of such meeting. A proxy with respect to stock held in the name of two or more persons shall be valid if executed by any one of them unless at or prior to the exercise of the proxy such corporation receives a specific written notice to the contrary from any one of them. A proxy purporting to be executed by or on behalf of a stockholder shall be deemed valid unless challenged at or prior to its exercise, and the burden of proving invalidity shall rest on the challenger. Except as otherwise provided in the articles of organization or by-laws of the corporation, special meetings of the stockholders may be called pursuant to the provisions of section thirty-four of chapter one hundred and fifty-six B.