Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/114

28 Thus the ischia of a Dinosaurian are more bird-like than those of any existing reptile, but retain the reptilian union in a symphysis.

3. In all reptiles the pubis is inclined forwards as well as downwards towards the ventral median line. In all except the Crocodile it takes a considerable share in the formation of the acetabulum; and in all, except the Crocodile again, the ossified pubis unites directly with its fellow in the middle line. In the Crocodiles, the inner extremities of the pubes remain cartilaginous for a great extent, and consequently the ossified parts of the pubes remain widely apart in the dry skeleton.

Prof. Phillips has shown me what I believe to be fragments of the pubes of Megalosaurus in the Oxford Museum. If the determination is correct, they resembled those of the Ostrich in many respects. As they are detached, there is no certainty respecting their direction. The pubes of Compsognathus are, unfortunately, obscured by the femora. They seem to have been very slender; and they are directed forwards and downwards, like those of lizards. Some lizards, in fact, have pubes which, if the animal were fossilized in the same position as Compsognathus, would be very similar in form and direction.

Hypsilophodon, however, affords unequivocal evidences of a further step towards the bird. The pubes are not only as slender and elongated as in the most typical bird, but they are directed downwards and backwards parallel with the ischia, thus leaving only a very narrow and elongated obturator foramen, which is divided by the obturator process. I suspect that if only the pubis and the ischium of Hypsilophodon had been discovered, they would have been unhesitatingly referred to Aves.

Thus, as far as its pubis is concerned, Hypsilophodon affords an unmistakable transition between Reptilia and Aves. It remains to be seen how far the hypsilophodont modification extended among the Dinosauria. The remains of Compsognathus and of Stenopelyx lead me to suppose that it was by no means universal. In fact in this, as in many other respects, I have reason to think that the Dinosauria present us with serial modifications leading from the Parasuchian type of structure, on the one hand, to that of Birds on the other.

The evidence yielded by the distal end of the tibia and the astragalus has the same tendency.

In the splendid collection of Megalosaurian remains in the possession of Mr. James Parker, of Oxford, which I had the good fortune to see a few weeks ago, I recognized the astragalus of that reptile, which, as I had already divined from the structure of the tibia, is altogether like the corresponding bone in Poikilopleuron.

In another specimen the distal end of the tibia and the fibula were in place, and there was the impression of the ascending process of the astragalus, with a fragment of its. bony substance, exactly where it should be. With this complete knowledge of the