The pelvic diaphragm is composed of the levator ani and the coccygeus muscle.

http://en.wikipedia.org/wiki/Pelvis

The pelvic diaphragm is composed of the levator ani and the coccygeus muscle. These arise between the symphysis and the ischial spine and converge on the coccyx and the anococcygeal ligament which spans between the tip of the coccyx and the anal hiatus.