Petty officers
Black's Law Dictionary · Henry Campbell Black, M.A. · 1910
Black's Law Dictionary
Inferior officers in the naval service, of various ranks and kinds, corresponding to the non-commissioned officers in the army.
See U. S. v. Fuller, 160 U. S. 593 ,16 Sup. Ct. 386, 40 L. Ed. 549.
As to petty "Average," "Constable," and "Sessions," see those titles.