The assistant district attorney for the Eighteenth Judicial District of Louisiana shall receive as compensation for his services, in addition to the annual salary paid by the state, an annual salary of four thousand fifty dollars, payable monthly. The additional salary shall be paid by the parishes comprising the Eighteenth Judicial District apportioned as follows:
(1) By the parish of Iberville, the sum of one thousand three hundred fifty dollars per annum.
(2) By the parish of Pointe Coupee, the sum of one thousand three hundred fifty dollars per annum.
(3) By the parish of West Baton Rouge, the sum of one thousand three hundred fifty dollars per annum.