three bones that fuse to form the os coxae
Hip bone The hip bone (os coxa, innominate bone, pelvic bone[1] or coxal bone) is a large flat bone, constricted in the center and expanded above and below. In some vertebrates (including humans before puberty) it is composed of three parts: the ilium, ischium, and the pubis.
Hip bone The hip bone is formed by three parts: ilium, ischium, and pubis. At birth, these three components are separated by hyaline cartilage. They join each other in a Y-shaped portion of cartilage in the acetabulum. By the end of puberty the three regions will have fused together, and by the age of 25 they will have ossified. The two hip bones join each other at the pubic symphysis. Together with the sacrum and coccyx, the hip bones form the pelvis.[2]