756 



THE THEORY OF TRANSFORMATIONS 



[CH. 



forms, chief among which is the great elongation in Ichthyosaurus 

 of the two clavicles, are all seen by our diagrams to be part and 

 parcel of one general and systematic deformation. 



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

 very strangely modified skull of Pferanodon, 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 



Fig. 387. a, Skull of Dimorpliodon. b. Skull of Pteranodon. 



inclined downwards and forwards. The whole skull has a con- 

 figuration 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 co- 

 ordinates (Fig. 387, a), and refer our Pteranodon to a system of 

 oblique co-ordinates (6), in which the two co-ordinate 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 co-ordinate method as a means of comparing one 

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



