with related types. To this subject we shall presently return.
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
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