9. As shown in the figure, points $P_{1}, P_{2}, \cdots, P_{10}$ are either the vertices or the midpoints of the edges of a tetrahedron. The number of quadruplets of points $\left(P_{1}, P_{i}, P_{j}, P_{k}\right)(1<i<j<k \leqslant 10)$ that lie on the same plane is $\qquad$.


A tetrahedron can form $C_{5}^{3}$ $=10$ four-point groups on each side containing $P_{1}$; each edge containing $P_{1}$ and the midpoint of the opposite edge can form 1 four-point group.

So, the number of four-point groups $\left(P_{1}, P_{i}, P_{j}, P_{k}\right)$ in the same plane is $3 C_{5}^{3}+3=33$.