The many diverse forms of Dinosaurian reptiles, all of which manifest a strong family likeness underlying much superficial diversity, furnish us with plentiful material for comparison by the method of transformations. As an instance, I have figured the pelvic bones of Stegosaurus and of Camptosaurus (Fig. [384], a, b) to show that, when the former is taken as our Cartesian type, a slight curvature and an approximately logarithmic extension of the x-axis brings us easily to the configuration of the other. In the original specimen of Camptosaurus described by Marsh[658], the anterior portion of the iliac bone is missing; and in Marsh’s restoration this part of the bone is drawn as though it came somewhat abruptly to a sharp point. In my figure I {755} have completed this missing part of the bone in harmony with the general co-ordinate network which is suggested by our comparison of the two entire pelves; and I venture to think that the result is more natural in appearance, and more likely to be correct than was Marsh’s conjectural restoration. It would seem, in fact, that there is an obvious field for the employment of the method of co-ordinates in this task of reproducing missing portions of a structure to the proper scale and in harmony with related types. To this subject we shall presently return.
Fig. 385. Shoulder-girdle of Cryptocleidus. a, young; b, adult.
In Fig. [385], a, b, I have drawn the shoulder-girdle of Cryptocleidus, a Plesiosaurian reptile, half-grown in the one case and full-grown in the other. The change of form during growth in this region of the body is very considerable, and its nature is well brought out by the two co-ordinate systems. In Fig. [386] I have drawn the shoulder-girdle of an
Fig. 386. Shoulder-girdle of Ichthyosaurus.
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 {756} 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 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
Fig. 387. a, Skull of Dimorphodon. b, Skull of Pteranodon.