XVII] THE COMPARISON DF RELATED FORMS 1069 



Before we leave the group of reptiles we may glance at the very 

 strangely modified skull of Pteranodon, one of the extinct 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 down- 

 wards 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 i£ we 

 inscribe the latter in Cartesian coordinates (Fig. 531, a), and refer 



Fig. 531. a, skull of, Dimorphodon; b, skull of Pteranodon. 



our Pteranodon to a system of oblique coordinates (6), in which the 

 two coordinate systems of parallel lines become each a pencil of 

 diverging rays, we make manifest a correspondence 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 coordinate method as a means of comparing one 

 known structure with another. But it is obvious, as I have said, 

 that it may also be employed for drawing hypothetical structures, 

 on the assumption that they have varied from a known form in 

 some definite way. And this process may be especially useful, and 

 will be most obviously legitimate, when we apply it to the particular 

 case of representing intermediate stages between two forms which 



