Ischium

The ischium ( /ˈɪskiəm/) forms the lower and back part of the hip bone (os coxae).Ornithischian pelvic structure (left side)Saurischian pelvic structure (left side).Right hip bone. External surface.Right hip bone. Internal surface.Plan of ossification of the hip bone.The Obturator externus.Pelvis The ischium ( /ˈɪskiəm/) forms the lower and back part of the hip bone (os coxae). Situated below the ilium and behind the pubis, it is one of these three bones whose fusion creates the hip. The superior portion of this bone forms approximately one third of the acetabulum.