“ In support of their motion to quash the indictment because of consistent exclusion of Negroes from grand jury service, petitioners, both Negroes, offered testimony of the county tax supervisor showing that the tax records of the county, on which Negro and white persons are listed separately and from which the names of jurors are derived, revealed 12,250 white persons and 4,819 Negroes in the county, with 5,583 white men and 2,499 Negro men listed for poll tax. ”