flying reptiles, or Pterosauria. In this very curious skull the region of the jaws, or beak, is greatly elongated and pointed; the occipital bone is drawn out into an enormous backwardly-directed crest; the posterior part of the lower jaw is similarly produced backwards; the orbit is small; and the
quadrate bone is strongly inclined downwards and forwards. The whole skull has a configuration which stands, apparently, in the strongest possible contrast to that of a more normal Ornithosaurian such as Dimorphodon. But if we inscribe the latter in Cartesian coordinates (Fig. 387, a), and refer our Pteranodon to a system of oblique co-ordinates (b), in which the two co-ordinate systems of parallel lines become each a pencil of diverging rays, we make manifest a corôÙreôÙsponôÙdence which extends uniformly throughout all parts of these very different-looking skulls.
We have dealt so far, and for the most part we shall continue to deal, with our co-ordinate method as a means