The series alternates between cubes of natural numbers and their squares: 1³, 1², 2³, 2², 3³, 3², 4³, 4²... The pattern is (1,1), (8,4), (27,9), (64,16)... where each pair is (n³, n²). The missing term is 4³ = 64, which comes before 16 (which is 4²).