Ischium the Latin term derives from Greek ἰσχίον "iskhion" meaning "hip joint". The division of the "acetabulum" into "ischium" (ἰσχίον) and "ilium" (λαγών, "os lagonicum") is due to Galen, "De ossibus". Galen, however, omits mention of the "pubis" as a separate bone. The clade Dinosauria is divided into the Saurischia and Ornithischia based on hip structure, including importantly that of the ischium. In the majority of dinosaurs, the ischium extends down from the ilium and towards the tail of the animal. The acetabulum, which can be thought of as a "hip-socket", is a cup-shaped opening on each side of the pelvic
Hip bone pubic symphysis is present only in the ostrich, and the two hip bones are usually widely separated, making it easier to lay large eggs. In therapsids, the hip bone came to rotate counter-clockwise, relatives to its position in reptiles, so that the ilium moved forward, and the pubis and ischium moved to the rear. The same pattern is seen in all modern mammals, and the thyroid fenestra and obturator foramen have merged to form a single space. The ilium is typically narrow and triangular in mammals, but is much larger in ungulates and humans, in which it anchors powerful gluteal