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. [Illustration: FIG. 32. THE PELVIC BONES OF AN ALLIGATOR SEEN FROM BELOW The bones in front are here regarded as prepubic, but are commonly named pubic] This median union of the prepubic bones is a difference from those mammals like the Ornithorhynchus and Echidna, which approach nearest to the Reptilia. In them the prepubic bones have a long attachment to the front margin of the pubis, and extend their points forward without any tendency for the anterior extremities to approximate or unite. The marsupial mammals have the same character, keeping the marsupial bones completely distinct from each other at their free extremities. The only existing animals in which an approximation is found to the prepubic bones in Pterodactyles are Crocodiles, in bones which most writers term the pubic bones. This resemblance, without showing any strong affinity with the Crocodilia, indicates that Crocodiles have more in common with the fossil flying animals than any other group of existing reptiles; for other reptiles all want prepubic bones, or bones in front of the pubic region. THE HIND LIMB