The acetabulum for the head of the femur is imperforate, being a simple oval basin, as in Chelonian reptiles and the higher Mammals. It never shows the mark of the ligamentous attachment to the head of the femur, which is seen in Mammals. In Birds the acetabulum is perforated, as in many of the fossils named Dinosaurs, and in Monotremata. [Illustration: FIG. 29. COMPARISON OF THE LEFT SIDE OF THE PELVIS IN A BIRD AND A PTERODACTYLE] Secondly, the ilium is elongated, and extends quite as much in front of the acetabulum as behind it. The bone is not very deep in this front process. Among existing animals this relation of the bone is nearer to birds than to any other type, since birds alone have the ilium extended from the acetabulum in both directions. The form of the Pterodactyle ilium is usually that of the embryo bird, and its slender processes compare in relative length better with those of the unhatched fowl and Apteryx of New Zealand than with the plate-like form in adult birds. In mammals the ilium is directed forward, and even in the Cape ant-eater Orycteropus there is only an inappreciable production of the bone backward behind the acetabulum. Among reptiles the general position of the acetabulum is at the forward termination of the ilium, though the Crocodile has some extension of the bone in both directions, without forming distinct anterior and posterior processes. This anterior and posterior extension of the ilium is seen in the Theriodont reptiles of Russia and of South Africa, as well as in Dinosaurs. [Illustration: FIG. 30. LEFT PELVIC BONES WITH PREPUBIC BONE IN _PTERODACTYLUS LONGIROSTRIS_] Thirdly, in all pterodactyles the ischium and pubis are more or less completely blended into a sheet of bone, unbroken by perforation, though there is usually a minute vascular foramen; or the lower border may be notched between the ischium and the pubis, as in some of the Solenhofen species, and the pubis does not reach the median line of the body. But in Dimorphodon the pelvic sheet of bone is unbroken by any notch or perforation. The notch between the ischium and pubis is well marked in _Pterodactylus longirostris_, and better marked in _Pterodactylus dubius_, _Cycnorhamphus Fraasii_, and Rhamphorhynchus. The fossil animals which appear to come nearest to the Pterodactyles in the structure of the pelvis are Theriodonts from the Permian rocks of Russia. The type known as Rhopalodon has the ilium less prolonged front and back, and is much deeper than in any Pterodactyle; but the acetabulum is imperforate, and the ischium and pubis are not always completely separated from each other by suture. In the pelvis referred to the Theriodont Deuterosaurus there is some approximation to the pelvis of Rhamphorhynchus and of _Pterodactylus dubius_ in the depth of the division between the pubis and ischium. [Illustration: FIG. 31 PELVIS AND PREPUBIC BONES OF RHAMPHORHYNCHUS