884 D'ARCY WENTWORTH THOMPSON ON 



its nature is well brought out by the two co-ordinate systems. In fig. 45 I have 

 drawn the shoulder-girdle of an Ichthyosaur, referring it to Cryptocleidus as a standard 

 of comparison. The interclavicle, which is present in Ichthyosaurus, is minute 

 and hidden in Cryptocleidus ; but the numerous other differences between the 

 two 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 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 



Fig. 46. — Skull of Dimorphodon. 



Fig. 47.— Skull of Pteranodon. 



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 co-ordinates (fig. 46), and refer our Pteranodon to a 

 system of oblique co-ordinates (fig. 47), in which the two co-ordinate systems of 

 parallel lines become each a pencil of diverging rays, we make manifest a corre- 

 spondence which extends uniformly throughout all parts of these very different- 

 looking skid Is. 



We have dealt so far, and for the most part we shall continue to deal, witli our 

 co ordinate 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 hypo 



