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.
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.
FIG. 31 PELVIS AND PREPUBIC BONES OF RHAMPHORHYNCHUS
On the left-hand side the two prepubic bones are separate. On the right-hand they are united into a transverse bar which overlaps the front of pelvis seen from the under side
There are three modifications of the Ornithosaurian pelvis. First, the type of Rhamphorhynchus, in which the pubis and ischium are inclined somewhat backward, and in which the two prepubic bones are triangular, and are often united together to form a transverse bow in front of the pubic region.
Secondly, there is the ordinary form of pelvis in which the pubis and ischium usually unite with each other down their length, as in Dimorphodon, but sometimes, as in Pterodactylus dubius, divide immediately below the acetabulum. All these types possess the paddle-shaped prepubic bones, which are never united in the median line.
Thirdly, there is the cretaceous form indicated by Ornithocheirus and Ornithostoma, in which the posterior half of the ilium is modified in a singular way, since it is more elevated towards the sacrum than the anterior half, suggesting the contour of the upper border of the ilium in a lizard. Without being reptilian—the anterior prolongation of the bone makes that impossible—it suggests the lizards. This type also possesses prepubic bones. They appear, according to Professor Williston, to be more like the paddle-shaped bones of Pterodactylus than like the angular bones in Rhamphorhynchus. The prepubic bones are united in the median line as in Rhamphorhynchus. But their median union in that genus favours the conclusion that the bones were united in the median line in all species, though they are only co-ossified in these two families.