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.