Section 6D. The clerk of the courts for the county of Norfolk may designate, subject to removal by the court or by the clerk, one of the assistant clerks for said county, appointed under section five, to perform, under the direction of said clerk, the duties of clerk pertaining to the criminal proceedings in said court, who shall receive from said county, in addition to the salary paid to him as an assistant clerk under section five, a sum equivalent to ten per cent of the salary of the clerk.