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
Bone A bird's beak is primarily made of bone as projections of the mandibles which are covered in keratin. A deer's antlers are composed of bone which is an unusual example of bone being outside the skin of the animal once the velvet is shed. The extinct predatory fish "Dunkleosteus" had sharp edges of hard exposed bone along its jaws. Many animals possess an exoskeleton that is not made of bone. These include insects and crustaceans. The proportion of cortical bone that is 80% in the human skeleton may be much lower in other animals, especially in marine mammals and marine