tertĭus-dĕcĭmus (-dĕcŭmus), a, um, num. ord. adj., the thirteenth: dies, Cels. 3, 4 med.: mense, Col. 6, 36, 2; Plin. 8, 44, 69, § 171: tertiadecuma legio, Tac. A. 1, 37.