xviil THE COMPARISON OF RELATED FORMS 



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 Cryptodeidus. a, young; 6, adult. 



In Fig. 385, a, b, I have drawn the shoulder-girdle of Crypto- 

 deidus, 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 



Fig. 386. Shoulder-girdle of Ichthyosaurus. 



drawn the shoulder-girdle of an Ichthyosaur, referring it to 

 Cryptodeidus as a standard of comparison. The interclavicle, 

 which is present in Ichthyosaurus, is minute and hidden in Crypto- 

 deidus ; but the numerous other differences between the two 



48—2 



